Subject: [PATCH] [v4] tty/powerpc: introduce the ePAPR embedded hypervisor byte channel driver

From: Timur Tabi <[email-masked]>
Date: Thu, 25 Aug 2011 10:32:25 -0500

The ePAPR embedded hypervisor specification provides an API for "byte
channels", which are serial-like virtual devices for sending and receiving
streams of bytes.  This driver provides Linux kernel support for byte
channels via three distinct interfaces:

1) An early-console (udbg) driver.  This provides early console output
through a byte channel.  The byte channel handle must be specified in a
Kconfig option.

2) A normal console driver.  Output is sent to the byte channel designated
for stdout in the device tree.  The console driver is for handling kernel
printk calls.

3) A tty driver, which is used to handle user-space input and output.  The
byte channel used for the console is designated as the default tty.

Signed-off-by: Timur Tabi <[email-masked]>
---
 arch/powerpc/include/asm/udbg.h |    1 +
 arch/powerpc/kernel/udbg.c      |    2 +
 drivers/tty/Kconfig             |   34 ++
 drivers/tty/Makefile            |    1 +
 drivers/tty/ehv_bytechan.c      |  888 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 926 insertions(+), 0 deletions(-)
 create mode 100644 drivers/tty/ehv_bytechan.c

diff --git a/arch/powerpc/include/asm/udbg.h b/arch/powerpc/include/asm/udbg.h
index 93e05d1..5354ae9 100644
--- a/arch/powerpc/include/asm/udbg.h
+++ b/arch/powerpc/include/asm/udbg.h
@@ -54,6 +54,7 @@ extern void __init udbg_init_40x_realmode(void);
 extern void __init udbg_init_cpm(void);
 extern void __init udbg_init_usbgecko(void);
 extern void __init udbg_init_wsp(void);
+extern void __init udbg_init_ehv_bc(void);
 
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_UDBG_H */
diff --git a/arch/powerpc/kernel/udbg.c b/arch/powerpc/kernel/udbg.c
index faa82c1..b4607a9 100644
--- a/arch/powerpc/kernel/udbg.c
+++ b/arch/powerpc/kernel/udbg.c
@@ -67,6 +67,8 @@ void __init udbg_early_init(void)
 	udbg_init_usbgecko();
 #elif defined(CONFIG_PPC_EARLY_DEBUG_WSP)
 	udbg_init_wsp();
+#elif defined(CONFIG_PPC_EARLY_DEBUG_EHV_BC)
+	udbg_init_ehv_bc();
 #endif
 
 #ifdef CONFIG_PPC_EARLY_DEBUG
diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index bd7cc05..535af0a 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -350,3 +350,37 @@ config TRACE_SINK
 
 	  If you select this option, you need to select
 	  "Trace data router for MIPI P1149.7 cJTAG standard".
+
+config PPC_EPAPR_HV_BYTECHAN
+	tristate "ePAPR hypervisor byte channel driver"
+	depends on FSL_SOC
+	help
+	  This driver creates /dev entries for each ePAPR hypervisor byte
+	  channel, thereby allowing applications to communicate with byte
+	  channels as if they were serial ports.
+
+config PPC_EARLY_DEBUG_EHV_BC
+	bool "Early console (udbg) support for ePAPR hypervisors"
+	depends on PPC_EPAPR_HV_BYTECHAN
+	help
+	  Select this option to enable early console (a.k.a. "udbg") support
+	  via an ePAPR byte channel.  You also need to choose the byte channel
+	  handle below.
+
+config PPC_EARLY_DEBUG_EHV_BC_HANDLE
+	int "Byte channel handle for early console (udbg)"
+	depends on PPC_EARLY_DEBUG_EHV_BC
+	default 0
+	help
+	  If you want early console (udbg) output through a byte channel,
+	  specify the handle of the byte channel to use.
+
+	  For this to work, the byte channel driver must be compiled
+	  in-kernel, not as a module.
+
+	  Note that only one early console driver can be enabled, so don't
+	  enable any others if you enable this one.
+
+	  If the number you specify is not a valid byte channel handle, then
+	  there simply will be no early console output.  This is true also
+	  if you don't boot under a hypervisor at all.
diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index ea89b0b..2953059 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -26,5 +26,6 @@ obj-$(CONFIG_ROCKETPORT)	+= rocket.o
 obj-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_SYNCLINKMP)	+= synclinkmp.o
 obj-$(CONFIG_SYNCLINK)		+= synclink.o
+obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o
 
 obj-y += ipwireless/
diff --git a/drivers/tty/ehv_bytechan.c b/drivers/tty/ehv_bytechan.c
new file mode 100644
index 0000000..e67f70b
--- /dev/null
+++ b/drivers/tty/ehv_bytechan.c
@@ -0,0 +1,888 @@
+/* ePAPR hypervisor byte channel device driver
+ *
+ * Copyright 2009-2011 Freescale Semiconductor, Inc.
+ *
+ * Author: Timur Tabi <[email-masked]>
+ *
+ * This file is licensed under the terms of the GNU General Public License
+ * version 2.  This program is licensed "as is" without any warranty of any
+ * kind, whether express or implied.
+ *
+ * This driver support three distinct interfaces, all of which are related to
+ * ePAPR hypervisor byte channels.
+ *
+ * 1) An early-console (udbg) driver.  This provides early console output
+ * through a byte channel.  The byte channel handle must be specified in a
+ * Kconfig option.
+ *
+ * 2) A normal console driver.  Output is sent to the byte channel designated
+ * for stdout in the device tree.  The console driver is for handling kernel
+ * printk calls.
+ *
+ * 3) A tty driver, which is used to handle user-space input and output.  The
+ * byte channel used for the console is designated as the default tty.
+ */
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/err.h>
+#include <linux/interrupt.h>
+#include <linux/fs.h>
+#include <linux/poll.h>
+#include <asm/epapr_hcalls.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/cdev.h>
+#include <linux/console.h>
+#include <linux/tty.h>
+#include <linux/tty_flip.h>
+#include <linux/circ_buf.h>
+#include <asm/udbg.h>
+
+/* The size of the transmit circular buffer.  This must be a power of two. */
+#define BUF_SIZE	2048
+
+/* Per-byte channel private data */
+struct ehv_bc_data {
+	struct device *dev;
+	struct tty_port port;
+	uint32_t handle;
+	unsigned int rx_irq;
+	unsigned int tx_irq;
+
+	spinlock_t lock;	/* lock for transmit buffer */
+	unsigned char buf[BUF_SIZE];	/* transmit circular buffer */
+	unsigned int head;	/* circular buffer head */
+	unsigned int tail;	/* circular buffer tail */
+
+	int tx_irq_enabled;	/* true == TX interrupt is enabled */
+};
+
+/* Array of byte channel objects */
+static struct ehv_bc_data *bcs;
+
+/* Byte channel handle for stdout (and stdin), taken from device tree */
+static unsigned int stdout_bc;
+
+/* Virtual IRQ for the byte channel handle for stdin, taken from device tree */
+static unsigned int stdout_irq;
+
+/**************************** SUPPORT FUNCTIONS ****************************/
+
+/*
+ * Enable the transmit interrupt
+ *
+ * Unlike a serial device, byte channels have no mechanism for disabling their
+ * own receive or transmit interrupts.  To emulate that feature, we toggle
+ * the IRQ in the kernel.
+ *
+ * We cannot just blindly call enable_irq() or disable_irq(), because these
+ * calls are reference counted.  This means that we cannot call enable_irq()
+ * if interrupts are already enabled.  This can happen in two situations:
+ *
+ * 1. The tty layer makes two back-to-back calls to ehv_bc_tty_write()
+ * 2. A transmit interrupt occurs while executing ehv_bc_tx_dequeue()
+ *
+ * To work around this, we keep a flag to tell us if the IRQ is enabled or not.
+ */
+static void enable_tx_interrupt(struct ehv_bc_data *bc)
+{
+	if (!bc->tx_irq_enabled) {
+		enable_irq(bc->tx_irq);
+		bc->tx_irq_enabled = 1;
+	}
+}
+
+static void disable_tx_interrupt(struct ehv_bc_data *bc)
+{
+	if (bc->tx_irq_enabled) {
+		disable_irq_nosync(bc->tx_irq);
+		bc->tx_irq_enabled = 0;
+	}
+}
+
+/*
+ * find the byte channel handle to use for the console
+ *
+ * The byte channel to be used for the console is specified via a "stdout"
+ * property in the /chosen node.
+ *
+ * For compatible with legacy device trees, we also look for a "stdout" alias.
+ */
+static int find_console_handle(void)
+{
+	struct device_node *np, *np2;
+	const char *sprop = NULL;
+	const uint32_t *iprop;
+
+	np = of_find_node_by_path("/chosen");
+	if (np)
+		sprop = of_get_property(np, "stdout-path", NULL);
+
+	if (!np || !sprop) {
+		of_node_put(np);
+		np = of_find_node_by_name(NULL, "aliases");
+		if (np)
+			sprop = of_get_property(np, "stdout", NULL);
+	}
+
+	if (!sprop) {
+		of_node_put(np);
+		return 0;
+	}
+
+	/* We don't care what the aliased node is actually called.  We only
+	 * care if it's compatible with "epapr,hv-byte-channel", because that
+	 * indicates that it's a byte channel node.  We use a temporary
+	 * variable, 'np2', because we can't release 'np' until we're done with
+	 * 'sprop'.
+	 */
+	np2 = of_find_node_by_path(sprop);
+	of_node_put(np);
+	np = np2;
+	if (!np) {
+		pr_warning("ehv-bc: stdout node '%s' does not exist\n", sprop);
+		return 0;
+	}
+
+	/* Is it a byte channel? */
+	if (!of_device_is_compatible(np, "epapr,hv-byte-channel")) {
+		of_node_put(np);
+		return 0;
+	}
+
+	stdout_irq = irq_of_parse_and_map(np, 0);
+	if (stdout_irq == NO_IRQ) {
+		pr_err("ehv-bc: no 'interrupts' property in %s node\n", sprop);
+		of_node_put(np);
+		return 0;
+	}
+
+	/*
+	 * The 'hv-handle' property contains the handle for this byte channel.
+	 */
+	iprop = of_get_property(np, "hv-handle", NULL);
+	if (!iprop) {
+		pr_err("ehv-bc: no 'hv-handle' property in %s node\n",
+		       np->name);
+		of_node_put(np);
+		return 0;
+	}
+	stdout_bc = be32_to_cpu(*iprop);
+
+	of_node_put(np);
+	return 1;
+}
+
+/*************************** EARLY CONSOLE DRIVER ***************************/
+
+#ifdef CONFIG_PPC_EARLY_DEBUG_EHV_BC
+
+/*
+ * send a byte to a byte channel, wait if necessary
+ *
+ * This function sends a byte to a byte channel, and it waits and
+ * retries if the byte channel is full.  It returns if the character
+ * has been sent, or if some error has occurred.
+ *
+ */
+static void byte_channel_spin_send(const char data)
+{
+	int ret, count;
+
+	do {
+		count = 1;
+		ret = ev_byte_channel_send(CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE,
+					   &count, &data);
+	} while (ret == EV_EAGAIN);
+}
+
+/*
+ * The udbg subsystem calls this function to display a single character.
+ * We convert CR to a CR/LF.
+ */
+static void ehv_bc_udbg_putc(char c)
+{
+	if (c == '\n')
+		byte_channel_spin_send('\r');
+
+	byte_channel_spin_send(c);
+}
+
+/*
+ * early console initialization
+ *
+ * PowerPC kernels support an early printk console, also known as udbg.
+ * This function must be called via the ppc_md.init_early function pointer.
+ * At this point, the device tree has been unflattened, so we can obtain the
+ * byte channel handle for stdout.
+ *
+ * We only support displaying of characters (putc).  We do not support
+ * keyboard input.
+ */
+void __init udbg_init_ehv_bc(void)
+{
+	unsigned int rx_count, tx_count;
+	unsigned int ret;
+
+	/* Check if we're running as a guest of a hypervisor */
+	if (!(mfmsr() & MSR_GS))
+		return;
+
+	/* Verify the byte channel handle */
+	ret = ev_byte_channel_poll(CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE,
+				   &rx_count, &tx_count);
+	if (ret)
+		return;
+
+	udbg_putc = ehv_bc_udbg_putc;
+	register_early_udbg_console();
+
+	udbg_printf("ehv-bc: early console using byte channel handle %u\n",
+		    CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE);
+}
+
+#endif
+
+/****************************** CONSOLE DRIVER ******************************/
+
+static struct tty_driver *ehv_bc_driver;
+
+/*
+ * Byte channel console sending worker function.
+ *
+ * For consoles, if the output buffer is full, we should just spin until it
+ * clears.
+ */
+static int ehv_bc_console_byte_channel_send(unsigned int handle, const char *s,
+			     unsigned int count)
+{
+	unsigned int len;
+	int ret = 0;
+
+	while (count) {
+		len = min_t(unsigned int, count, EV_BYTE_CHANNEL_MAX_BYTES);
+		do {
+			ret = ev_byte_channel_send(handle, &len, s);
+		} while (ret == EV_EAGAIN);
+		count -= len;
+		s += len;
+	}
+
+	return ret;
+}
+
+/*
+ * write a string to the console
+ *
+ * This function gets called to write a string from the kernel, typically from
+ * a printk().  This function spins until all data is written.
+ *
+ * We copy the data to a temporary buffer because we need to insert a \r in
+ * front of every \n.  It's more efficient to copy the data to the buffer than
+ * it is to make multiple hcalls for each character or each newline.
+ */
+static void ehv_bc_console_write(struct console *co, const char *s,
+				 unsigned int count)
+{
+	unsigned int handle = (unsigned int)co->data;
+	char s2[EV_BYTE_CHANNEL_MAX_BYTES];
+	unsigned int i, j = 0;
+	char c;
+
+	for (i = 0; i < count; i++) {
+		c = *s++;
+
+		if (c == '\n')
+			s2[j++] = '\r';
+
+		s2[j++] = c;
+		if (j >= (EV_BYTE_CHANNEL_MAX_BYTES - 1)) {
+			if (ehv_bc_console_byte_channel_send(handle, s2, j))
+				return;
+			j = 0;
+		}
+	}
+
+	if (j)
+		ehv_bc_console_byte_channel_send(handle, s2, j);
+}
+
+/*
+ * When /dev/console is opened, the kernel iterates the console list looking
+ * for one with ->device and then calls that method. On success, it expects
+ * the passed-in int* to contain the minor number to use.
+ */
+static struct tty_driver *ehv_bc_console_device(struct console *co, int *index)
+{
+	*index = co->index;
+
+	return ehv_bc_driver;
+}
+
+static struct console ehv_bc_console = {
+	.name		= "ttyEHV",
+	.write		= ehv_bc_console_write,
+	.device		= ehv_bc_console_device,
+	.flags		= CON_PRINTBUFFER | CON_ENABLED,
+};
+
+/*
+ * Console initialization
+ *
+ * This is the first function that is called after the device tree is
+ * available, so here is where we determine the byte channel handle and IRQ for
+ * stdout/stdin, even though that information is used by the tty and character
+ * drivers.
+ */
+static int __init ehv_bc_console_init(void)
+{
+	if (!find_console_handle()) {
+		pr_debug("ehv-bc: stdout is not a byte channel\n");
+		return -ENODEV;
+	}
+
+#ifdef CONFIG_PPC_EARLY_DEBUG_EHV_BC
+	/* Print a friendly warning if the user chose the wrong byte channel
+	 * handle for udbg.
+	 */
+	if (stdout_bc != CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE)
+		pr_warning("ehv-bc: udbg handle %u is not the stdout handle\n",
+			   CONFIG_PPC_EARLY_DEBUG_EHV_BC_HANDLE);
+#endif
+
+	ehv_bc_console.data = (void *)stdout_bc;
+
+	/* add_preferred_console() must be called before register_console(),
+	   otherwise it won't work.  However, we don't want to enumerate all the
+	   byte channels here, either, since we only care about one. */
+
+	add_preferred_console(ehv_bc_console.name, ehv_bc_console.index, NULL);
+	register_console(&ehv_bc_console);
+
+	pr_info("ehv-bc: registered console driver for byte channel %u\n",
+		stdout_bc);
+
+	return 0;
+}
+console_initcall(ehv_bc_console_init);
+
+/******************************** TTY DRIVER ********************************/
+
+/*
+ * byte channel receive interupt handler
+ *
+ * This ISR is called whenever data is available on a byte channel.
+ */
+static irqreturn_t ehv_bc_tty_rx_isr(int irq, void *data)
+{
+	struct ehv_bc_data *bc = data;
+	struct tty_struct *ttys = tty_port_tty_get(&bc->port);
+	unsigned int rx_count, tx_count, len;
+	int count;
+	char buffer[EV_BYTE_CHANNEL_MAX_BYTES];
+	int ret;
+
+	/* ttys could be NULL during a hangup */
+	if (!ttys)
+		return IRQ_HANDLED;
+
+	/* Find out how much data needs to be read, and then ask the TTY layer
+	 * if it can handle that much.  We want to ensure that every byte we
+	 * read from the byte channel will be accepted by the TTY layer.
+	 */
+	ev_byte_channel_poll(bc->handle, &rx_count, &tx_count);
+	count = tty_buffer_request_room(ttys, rx_count);
+
+	/* 'count' is the maximum amount of data the TTY layer can accept at
+	 * this time.  However, during testing, I was never able to get 'count'
+	 * to be less than 'rx_count'.  I'm not sure whether I'm calling it
+	 * correctly.
+	 */
+
+	while (count > 0) {
+		len = min_t(unsigned int, count, sizeof(buffer));
+
+		/* Read some data from the byte channel.  This function will
+		 * never return more than EV_BYTE_CHANNEL_MAX_BYTES bytes.
+		 */
+		ev_byte_channel_receive(bc->handle, &len, buffer);
+
+		/* 'len' is now the amount of data that's been received. 'len'
+		 * can't be zero, and most likely it's equal to one.
+		 */
+
+		/* Pass the received data to the tty layer. */
+		ret = tty_insert_flip_string(ttys, buffer, len);
+
+		/* 'ret' is the number of bytes that the TTY layer accepted.
+		 * If it's not equal to 'len', then it means the buffer is
+		 * full, which should never happen.  If it does happen, we can
+		 * exit gracefully, but we drop the last 'len - ret' characters
+		 * that we read from the byte channel.
+		 */
+		if (ret != len)
+			break;
+
+		count -= len;
+	}
+
+	/* Tell the tty layer that we're done. */
+	tty_flip_buffer_push(ttys);
+
+	tty_kref_put(ttys);
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * dequeue the transmit buffer to the hypervisor
+ *
+ * This function, which can be called in interrupt context, dequeues as much
+ * data as possible from the transmit buffer to the byte channel.
+ */
+static void ehv_bc_tx_dequeue(struct ehv_bc_data *bc)
+{
+	unsigned int count;
+	unsigned int len, ret;
+	unsigned long flags;
+
+	do {
+		spin_lock_irqsave(&bc->lock, flags);
+		len = min_t(unsigned int,
+			    CIRC_CNT_TO_END(bc->head, bc->tail, BUF_SIZE),
+			    EV_BYTE_CHANNEL_MAX_BYTES);
+
+		ret = ev_byte_channel_send(bc->handle, &len, bc->buf + bc->tail);
+
+		/* 'len' is valid only if the return code is 0 or EV_EAGAIN */
+		if (!ret || (ret == EV_EAGAIN))
+			bc->tail = (bc->tail + len) & (BUF_SIZE - 1);
+
+		count = CIRC_CNT(bc->head, bc->tail, BUF_SIZE);
+		spin_unlock_irqrestore(&bc->lock, flags);
+	} while (count && !ret);
+
+	spin_lock_irqsave(&bc->lock, flags);
+	if (CIRC_CNT(bc->head, bc->tail, BUF_SIZE))
+		/*
+		 * If we haven't emptied the buffer, then enable the TX IRQ.
+		 * We'll get an interrupt when there's more room in the
+		 * hypervisor's output buffer.
+		 */
+		enable_tx_interrupt(bc);
+	else
+		disable_tx_interrupt(bc);
+	spin_unlock_irqrestore(&bc->lock, flags);
+}
+
+/*
+ * byte channel transmit interupt handler
+ *
+ * This ISR is called whenever space becomes available for transmitting
+ * characters on a byte channel.
+ */
+static irqreturn_t ehv_bc_tty_tx_isr(int irq, void *data)
+{
+	struct ehv_bc_data *bc = data;
+	struct tty_struct *ttys = tty_port_tty_get(&bc->port);
+
+	ehv_bc_tx_dequeue(bc);
+	if (ttys) {
+		tty_wakeup(ttys);
+		tty_kref_put(ttys);
+	}
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * This function is called when the tty layer has data for us send.  We store
+ * the data first in a circular buffer, and then dequeue as much of that data
+ * as possible.
+ *
+ * We don't need to worry about whether there is enough room in the buffer for
+ * all the data.  The purpose of ehv_bc_tty_write_room() is to tell the tty
+ * layer how much data it can safely send to us.  We guarantee that
+ * ehv_bc_tty_write_room() will never lie, so the tty layer will never send us
+ * too much data.
+ */
+static int ehv_bc_tty_write(struct tty_struct *ttys, const unsigned char *s,
+			    int count)
+{
+	struct ehv_bc_data *bc = ttys->driver_data;
+	unsigned long flags;
+	unsigned int len;
+	unsigned int written = 0;
+
+	while (1) {
+		spin_lock_irqsave(&bc->lock, flags);
+		len = CIRC_SPACE_TO_END(bc->head, bc->tail, BUF_SIZE);
+		if (count < len)
+			len = count;
+		if (len) {
+			memcpy(bc->buf + bc->head, s, len);
+			bc->head = (bc->head + len) & (BUF_SIZE - 1);
+		}
+		spin_unlock_irqrestore(&bc->lock, flags);
+		if (!len)
+			break;
+
+		s += len;
+		count -= len;
+		written += len;
+	}
+
+	ehv_bc_tx_dequeue(bc);
+
+	return written;
+}
+
+/*
+ * This function can be called multiple times for a given tty_struct, which is
+ * why we initialize bc->ttys in ehv_bc_tty_port_activate() instead.
+ *
+ * The tty layer will still call this function even if the device was not
+ * registered (i.e. tty_register_device() was not called).  This happens
+ * because tty_register_device() is optional and some legacy drivers don't
+ * use it.  So we need to check for that.
+ */
+static int ehv_bc_tty_open(struct tty_struct *ttys, struct file *filp)
+{
+	struct ehv_bc_data *bc = &bcs[ttys->index];
+
+	if (!bc->dev)
+		return -ENODEV;
+
+	return tty_port_open(&bc->port, ttys, filp);
+}
+
+/*
+ * Amazingly, if ehv_bc_tty_open() returns an error code, the tty layer will
+ * still call this function to close the tty device.  So we can't assume that
+ * the tty port has been initialized.
+ */
+static void ehv_bc_tty_close(struct tty_struct *ttys, struct file *filp)
+{
+	struct ehv_bc_data *bc = &bcs[ttys->index];
+
+	if (bc->dev)
+		tty_port_close(&bc->port, ttys, filp);
+}
+
+/*
+ * Return the amount of space in the output buffer
+ *
+ * This is actually a contract between the driver and the tty layer outlining
+ * how much write room the driver can guarantee will be sent OR BUFFERED.  This
+ * driver MUST honor the return value.
+ */
+static int ehv_bc_tty_write_room(struct tty_struct *ttys)
+{
+	struct ehv_bc_data *bc = ttys->driver_data;
+	unsigned long flags;
+	int count;
+
+	spin_lock_irqsave(&bc->lock, flags);
+	count = CIRC_SPACE(bc->head, bc->tail, BUF_SIZE);
+	spin_unlock_irqrestore(&bc->lock, flags);
+
+	return count;
+}
+
+/*
+ * Stop sending data to the tty layer
+ *
+ * This function is called when the tty layer's input buffers are getting full,
+ * so the driver should stop sending it data.  The easiest way to do this is to
+ * disable the RX IRQ, which will prevent ehv_bc_tty_rx_isr() from being
+ * called.
+ *
+ * The hypervisor will continue to queue up any incoming data.  If there is any
+ * data in the queue when the RX interrupt is enabled, we'll immediately get an
+ * RX interrupt.
+ */
+static void ehv_bc_tty_throttle(struct tty_struct *ttys)
+{
+	struct ehv_bc_data *bc = ttys->driver_data;
+
+	disable_irq(bc->rx_irq);
+}
+
+/*
+ * Resume sending data to the tty layer
+ *
+ * This function is called after previously calling ehv_bc_tty_throttle().  The
+ * tty layer's input buffers now have more room, so the driver can resume
+ * sending it data.
+ */
+static void ehv_bc_tty_unthrottle(struct tty_struct *ttys)
+{
+	struct ehv_bc_data *bc = ttys->driver_data;
+
+	/* If there is any data in the queue when the RX interrupt is enabled,
+	 * we'll immediately get an RX interrupt.
+	 */
+	enable_irq(bc->rx_irq);
+}
+
+static void ehv_bc_tty_hangup(struct tty_struct *ttys)
+{
+	struct ehv_bc_data *bc = ttys->driver_data;
+
+	ehv_bc_tx_dequeue(bc);
+	tty_port_hangup(&bc->port);
+}
+
+/*
+ * TTY driver operations
+ *
+ * If we could ask the hypervisor how much data is still in the TX buffer, or
+ * at least how big the TX buffers are, then we could implement the
+ * .wait_until_sent and .chars_in_buffer functions.
+ */
+static const struct tty_operations ehv_bc_ops = {
+	.open		= ehv_bc_tty_open,
+	.close		= ehv_bc_tty_close,
+	.write		= ehv_bc_tty_write,
+	.write_room	= ehv_bc_tty_write_room,
+	.throttle	= ehv_bc_tty_throttle,
+	.unthrottle	= ehv_bc_tty_unthrottle,
+	.hangup		= ehv_bc_tty_hangup,
+};
+
+/*
+ * initialize the TTY port
+ *
+ * This function will only be called once, no matter how many times
+ * ehv_bc_tty_open() is called.  That's why we register the ISR here, and also
+ * why we initialize tty_struct-related variables here.
+ */
+static int ehv_bc_tty_port_activate(struct tty_port *port,
+				    struct tty_struct *ttys)
+{
+	struct ehv_bc_data *bc = container_of(port, struct ehv_bc_data, port);
+	int ret;
+
+	ttys->driver_data = bc;
+
+	ret = request_irq(bc->rx_irq, ehv_bc_tty_rx_isr, 0, "ehv-bc", bc);
+	if (ret < 0) {
+		dev_err(bc->dev, "could not request rx irq %u (ret=%i)\n",
+		       bc->rx_irq, ret);
+		return ret;
+	}
+
+	/* request_irq also enables the IRQ */
+	bc->tx_irq_enabled = 1;
+
+	ret = request_irq(bc->tx_irq, ehv_bc_tty_tx_isr, 0, "ehv-bc", bc);
+	if (ret < 0) {
+		dev_err(bc->dev, "could not request tx irq %u (ret=%i)\n",
+		       bc->tx_irq, ret);
+		free_irq(bc->rx_irq, bc);
+		return ret;
+	}
+
+	/* The TX IRQ is enabled only when we can't write all the data to the
+	 * byte channel at once, so by default it's disabled.
+	 */
+	disable_tx_interrupt(bc);
+
+	return 0;
+}
+
+static void ehv_bc_tty_port_shutdown(struct tty_port *port)
+{
+	struct ehv_bc_data *bc = container_of(port, struct ehv_bc_data, port);
+
+	free_irq(bc->tx_irq, bc);
+	free_irq(bc->rx_irq, bc);
+}
+
+static const struct tty_port_operations ehv_bc_tty_port_ops = {
+	.activate = ehv_bc_tty_port_activate,
+	.shutdown = ehv_bc_tty_port_shutdown,
+};
+
+static int __devinit ehv_bc_tty_probe(struct platform_device *pdev)
+{
+	struct device_node *np = pdev->dev.of_node;
+	struct ehv_bc_data *bc;
+	const uint32_t *iprop;
+	unsigned int handle;
+	int ret;
+	static unsigned int index = 1;
+	unsigned int i;
+
+	iprop = of_get_property(np, "hv-handle", NULL);
+	if (!iprop) {
+		dev_err(&pdev->dev, "no 'hv-handle' property in %s node\n",
+			np->name);
+		return -ENODEV;
+	}
+
+	/* We already told the console layer that the index for the console
+	 * device is zero, so we need to make sure that we use that index when
+	 * we probe the console byte channel node.
+	 */
+	handle = be32_to_cpu(*iprop);
+	i = (handle == stdout_bc) ? 0 : index++;
+	bc = &bcs[i];
+
+	bc->handle = handle;
+	bc->head = 0;
+	bc->tail = 0;
+	spin_lock_init(&bc->lock);
+
+	bc->rx_irq = irq_of_parse_and_map(np, 0);
+	bc->tx_irq = irq_of_parse_and_map(np, 1);
+	if ((bc->rx_irq == NO_IRQ) || (bc->tx_irq == NO_IRQ)) {
+		dev_err(&pdev->dev, "no 'interrupts' property in %s node\n",
+			np->name);
+		ret = -ENODEV;
+		goto error;
+	}
+
+	bc->dev = tty_register_device(ehv_bc_driver, i, &pdev->dev);
+	if (IS_ERR(bc->dev)) {
+		ret = PTR_ERR(bc->dev);
+		dev_err(&pdev->dev, "could not register tty (ret=%i)\n", ret);
+		goto error;
+	}
+
+	tty_port_init(&bc->port);
+	bc->port.ops = &ehv_bc_tty_port_ops;
+
+	dev_set_drvdata(&pdev->dev, bc);
+
+	dev_info(&pdev->dev, "registered /dev/%s%u for byte channel %u\n",
+		ehv_bc_driver->name, i, bc->handle);
+
+	return 0;
+
+error:
+	irq_dispose_mapping(bc->tx_irq);
+	irq_dispose_mapping(bc->rx_irq);
+
+	memset(bc, 0, sizeof(struct ehv_bc_data));
+	return ret;
+}
+
+static int ehv_bc_tty_remove(struct platform_device *pdev)
+{
+	struct ehv_bc_data *bc = dev_get_drvdata(&pdev->dev);
+
+	tty_unregister_device(ehv_bc_driver, bc - bcs);
+
+	irq_dispose_mapping(bc->tx_irq);
+	irq_dispose_mapping(bc->rx_irq);
+
+	return 0;
+}
+
+static const struct of_device_id ehv_bc_tty_of_ids[] = {
+	{ .compatible = "epapr,hv-byte-channel" },
+	{}
+};
+
+static struct platform_driver ehv_bc_tty_driver = {
+	.driver = {
+		.owner = THIS_MODULE,
+		.name = "ehv-bc",
+		.of_match_table = ehv_bc_tty_of_ids,
+	},
+	.probe		= ehv_bc_tty_probe,
+	.remove		= ehv_bc_tty_remove,
+};
+
+/**
+ * ehv_bc_init - ePAPR hypervisor byte channel driver initialization
+ *
+ * This function is called when this module is loaded.
+ */
+static int __init ehv_bc_init(void)
+{
+	struct device_node *np;
+	unsigned int count = 0; /* Number of elements in bcs[] */
+	int ret;
+
+	pr_info("ePAPR hypervisor byte channel driver\n");
+
+	/* Count the number of byte channels */
+	for_each_compatible_node(np, NULL, "epapr,hv-byte-channel")
+		count++;
+
+	if (!count)
+		return -ENODEV;
+
+	/* The array index of an element in bcs[] is the same as the tty index
+	 * for that element.  If you know the address of an element in the
+	 * array, then you can use pointer math (e.g. "bc - bcs") to get its
+	 * tty index.
+	 */
+	bcs = kzalloc(count * sizeof(struct ehv_bc_data), GFP_KERNEL);
+	if (!bcs)
+		return -ENOMEM;
+
+	ehv_bc_driver = alloc_tty_driver(count);
+	if (!ehv_bc_driver) {
+		ret = -ENOMEM;
+		goto error;
+	}
+
+	ehv_bc_driver->owner = THIS_MODULE;
+	ehv_bc_driver->driver_name = "ehv-bc";
+	ehv_bc_driver->name = ehv_bc_console.name;
+	ehv_bc_driver->type = TTY_DRIVER_TYPE_CONSOLE;
+	ehv_bc_driver->subtype = SYSTEM_TYPE_CONSOLE;
+	ehv_bc_driver->init_termios = tty_std_termios;
+	ehv_bc_driver->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
+	tty_set_operations(ehv_bc_driver, &ehv_bc_ops);
+
+	ret = tty_register_driver(ehv_bc_driver);
+	if (ret) {
+		pr_err("ehv-bc: could not register tty driver (ret=%i)\n", ret);
+		goto error;
+	}
+
+	ret = platform_driver_register(&ehv_bc_tty_driver);
+	if (ret) {
+		pr_err("ehv-bc: could not register platform driver (ret=%i)\n",
+		       ret);
+		goto error;
+	}
+
+	return 0;
+
+error:
+	if (ehv_bc_driver) {
+		tty_unregister_driver(ehv_bc_driver);
+		put_tty_driver(ehv_bc_driver);
+	}
+
+	kfree(bcs);
+
+	return ret;
+}
+
+
+/**
+ * ehv_bc_exit - ePAPR hypervisor byte channel driver termination
+ *
+ * This function is called when this driver is unloaded.
+ */
+static void __exit ehv_bc_exit(void)
+{
+	tty_unregister_driver(ehv_bc_driver);
+	put_tty_driver(ehv_bc_driver);
+	kfree(bcs);
+}
+
+module_init(ehv_bc_init);
+module_exit(ehv_bc_exit);
+
+MODULE_AUTHOR("Timur Tabi <[email-masked]>");
+MODULE_DESCRIPTION("ePAPR hypervisor byte channel driver");
+MODULE_LICENSE("GPL v2");
-- 
1.7.3.4