Subject: [RFC PATCH 1/3] tracing: Add support for logging data to uncached buffer

From: [email-masked] (Steven Rostedt)
Date: Wed, 15 Aug 2018 22:59:55 -0400

Sorry for the late reply, I actually wrote this email over a week ago,
but never hit send. And the email was pushed back behind other
windows. :-/

On Fri,  3 Aug 2018 19:58:42 +0530
Sai Prakash Ranjan <[email-masked]> wrote:

> diff --git a/kernel/trace/trace_rtb.c b/kernel/trace/trace_rtb.c
> new file mode 100644
> index 000000000000..e8c24db71a2d
> --- /dev/null
> +++ b/kernel/trace/trace_rtb.c
> @@ -0,0 +1,160 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2018 The Linux Foundation. All rights reserved.
> + */
> +#include <linux/atomic.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/export.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/of_device.h>
> +#include <linux/platform_device.h>
> +#include <linux/rtb.h>
> +#include <linux/sched/clock.h>
> +
> +static struct platform_device *rtb_dev;
> +static atomic_t rtb_idx;
> +
> +struct rtb_state {
> +	struct rtb_layout *rtb;
> +	phys_addr_t phys;
> +	unsigned int nentries;
> +	unsigned int size;
> +	int enabled;
> +};
> +
> +static struct rtb_state rtb = {
> +	.enabled = 0,
> +};

No need for the initialization, you could just have:

static struct rtb_state rtb;

And it will be initialized to all zeros. Or did you do that to document
that it is not enabled at boot?

> +
> +static int rtb_panic_notifier(struct notifier_block *this,
> +					unsigned long event, void *ptr)
> +{
> +	rtb.enabled = 0;
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block rtb_panic_blk = {
> +	.notifier_call  = rtb_panic_notifier,
> +	.priority = INT_MAX,
> +};
> +
> +static void rtb_write_type(const char *log_type,
> +			struct rtb_layout *start)
> +{
> +	start->log_type = log_type;
> +}
> +
> +static void rtb_write_caller(u64 caller, struct rtb_layout *start)
> +{
> +	start->caller = caller;
> +}
> +
> +static void rtb_write_data(u64 data, struct rtb_layout *start)
> +{
> +	start->data = data;
> +}
> +
> +static void rtb_write_timestamp(struct rtb_layout *start)
> +{
> +	start->timestamp = sched_clock();
> +}

Why have the above static functions? They are not very helpful, and
appear to be actually confusing. They are used once.

> +
> +static void uncached_logk_pc_idx(const char *log_type, u64 caller,
> +				u64 data, int idx)
> +{
> +	struct rtb_layout *start;
> +
> +	start = &rtb.rtb[idx & (rtb.nentries - 1)];
> +
> +	rtb_write_type(log_type, start);
> +	rtb_write_caller(caller, start);
> +	rtb_write_data(data, start);
> +	rtb_write_timestamp(start);

How is the above better than:

	start->log_type = log_type;
	start->caller = caller;
	start->data = data;
 	start->timestamp = sched_clock();

??

> +	/* Make sure data is written */
> +	mb();
> +}
> +
> +static int rtb_get_idx(void)
> +{
> +	int i, offset;
> +
> +	i = atomic_inc_return(&rtb_idx);
> +	i--;
> +
> +	/* Check if index has wrapped around */
> +	offset = (i & (rtb.nentries - 1)) -
> +		 ((i - 1) & (rtb.nentries - 1));
> +	if (offset < 0) {
> +		i = atomic_inc_return(&rtb_idx);
> +		i--;
> +	}
> +
> +	return i;
> +}
> +
> +noinline void notrace uncached_logk(const char *log_type, void *data)

BTW, all files in this directory have their functions notrace by
default.

-- Steve

> +{
> +	int i;
> +
> +	if (!rtb.enabled)
> +		return;
> +
> +	i = rtb_get_idx();
> +	uncached_logk_pc_idx(log_type, (u64)(__builtin_return_address(0)),
> +				(u64)(data), i);
> +}
> +EXPORT_SYMBOL(uncached_logk);
> +
> +int rtb_init(void)
> +{
> +	struct device_node *np;
> +	u32 size;
> +	int ret;
> +
> +	np = of_find_node_by_name(NULL, "ramoops");
> +	if (!np)
> +		return -ENODEV;
> +
> +	ret = of_property_read_u32(np, "rtb-size", &size);
> +	if (ret) {
> +		of_node_put(np);
> +		return ret;
> +	}
> +
> +	rtb.size = size;
> +
> +	/* Create a dummy platform device to use dma api */
> +	rtb_dev = platform_device_register_simple("rtb", -1, NULL, 0);
> +	if (IS_ERR(rtb_dev))
> +		return PTR_ERR(rtb_dev);
> +
> +	/*
> +	 * The device is a dummy, so arch_setup_dma_ops
> +	 * is not called, thus leaving the device with dummy DMA ops
> +	 * which returns null in case of arm64.
> +	 */
> +	of_dma_configure(&rtb_dev->dev, NULL, true);
> +	rtb.rtb = dma_alloc_coherent(&rtb_dev->dev, rtb.size,
> +					&rtb.phys, GFP_KERNEL);
> +	if (!rtb.rtb)
> +		return -ENOMEM;
> +
> +	rtb.nentries = rtb.size / sizeof(struct rtb_layout);
> +	/* Round this down to a power of 2 */
> +	rtb.nentries = __rounddown_pow_of_two(rtb.nentries);
> +
> +	memset(rtb.rtb, 0, rtb.size);
> +	atomic_set(&rtb_idx, 0);
> +
> +	atomic_notifier_chain_register(&panic_notifier_list,
> +						&rtb_panic_blk);
> +	rtb.enabled = 1;
> +	return 0;
> +}
> +
> +void rtb_exit(void)
> +{
> +	dma_free_coherent(&rtb_dev->dev, rtb.size, rtb.rtb, rtb.phys);
> +	platform_device_unregister(rtb_dev);
> +}