Subject: [PATCH v3] drivers/tty: amba: defer probing DMA availability until hw_init

From: [email-masked] (Jorge Ramirez-Ortiz)
Date: Thu, 26 Feb 2015 10:15:21 -0500

On 02/26/2015 09:12 AM, Rob Herring wrote:
> On Wed, Feb 25, 2015 at 6:31 PM, Jorge Ramirez-Ortiz
> <[email-masked]> wrote:
>> Fix a race condition that happens when device_initcall(pl011_dma_initicall)
>> is executed before all the devices have been probed - this issue was observed on
>> an hisi_6220 SoC (HiKey board from Linaro).
>>
>> The deferred driver probing framework relies on late_initcall to trigger
>> deferred probes so it is just possible that, even with a valid DMA driver ready
>> to be loaded, we fail to synchronize with it.
>>
>> The proposed implementation delays probing of the DMA until the uart device is
>> opened. As hw_init is invoked on port startup and port resume - but the DMA
>> probe is only required once - we avoid calling multiple times using a new field
>> in uart_amba_port to track this scenario.
>>
>> Signed-off-by: Jorge Ramirez-Ortiz <[email-masked]>
>> ---
>>  drivers/tty/serial/amba-pl011.c | 66 ++++++++++-------------------------------
>>  1 file changed, 16 insertions(+), 50 deletions(-)
>>
>> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
>> index 8d94c19..3501ccc 100644
>> --- a/drivers/tty/serial/amba-pl011.c
>> +++ b/drivers/tty/serial/amba-pl011.c
>> @@ -29,6 +29,7 @@
>>   * and hooked into this driver.
>>   */
>>
>> +#define pr_fmt(fmt) "amba-pl011: "fmt
> You can drop this I think.

why not keep it? it makes the trace more readable (if debug is enabled only)

>
>>  #if defined(CONFIG_SERIAL_AMBA_PL011_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
>>  #define SUPPORT_SYSRQ
>> @@ -164,6 +165,7 @@ struct uart_amba_port {
>>         bool                    using_rx_dma;
>>         struct pl011_dmarx_data dmarx;
>>         struct pl011_dmatx_data dmatx;
>> +       bool                    dma_probed;
>>  #endif
>>  };
>>
>> @@ -261,7 +263,7 @@ static void pl011_sgbuf_free(struct dma_chan *chan, struct pl011_sgbuf *sg,
>>         }
>>  }
>>
>> -static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *uap)
>> +static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
>>  {
>>         /* DMA is the sole user of the platform data right now */
>>         struct amba_pl011_data *plat = dev_get_platdata(uap->port.dev);
>> @@ -275,9 +277,16 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>>         struct dma_chan *chan;
>>         dma_cap_mask_t mask;
>>
>> -       chan = dma_request_slave_channel(dev, "tx");
>> +       uap->dma_probed = true;
> I would not set this to true in the case of EPROBE_DEFER. That way if
> a DMA driver is loaded as a module some time later, DMA can be used.
> We're not really using deferred probe here, but using it as an
> indication that the UART is DMA capable.

ok. yes there is no need for the consistency I wast talking about on my previous
reply to RMK.
that was rubbish.

>
>> +
>> +       chan = dma_request_slave_channel_reason(dev, "tx");
>> +       if (IS_ERR(chan)) {
>> +               if (PTR_ERR(chan) == -EPROBE_DEFER) {
>> +                       dev_warn(uap->port.dev, "DMA driver not ready\n");

I'll change the warn for an info since there will be future opportunities to
load it.
I see value on the user knowing he might have a chance to use the DMA capability.

>> +                       return;
>> +               }
>> +               dev_info(uap->port.dev, "no OF or ACPI DMA data\n");
>>
>> -       if (!chan) {
>>                 /* We need platform data */
>>                 if (!plat || !plat->dma_filter) {
>>                         dev_info(uap->port.dev, "no DMA platform data\n");
>> @@ -383,54 +392,9 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
>>                 dev_info(uap->port.dev, "DMA channel RX %s\n",
>>                          dma_chan_name(uap->dmarx.chan));
>>         }
>> -}
>> -
>> -#ifndef MODULE
>> -/*
>> - * Stack up the UARTs and let the above initcall be done at device
>> - * initcall time, because the serial driver is called as an arch
>> - * initcall, and at this time the DMA subsystem is not yet registered.
>> - * At this point the driver will switch over to using DMA where desired.
>> - */
>> -struct dma_uap {
>> -       struct list_head node;
>> -       struct uart_amba_port *uap;
>> -       struct device *dev;
>> -};
>> -
>> -static LIST_HEAD(pl011_dma_uarts);
>> -
>> -static int __init pl011_dma_initcall(void)
>> -{
>> -       struct list_head *node, *tmp;
>> -
>> -       list_for_each_safe(node, tmp, &pl011_dma_uarts) {
>> -               struct dma_uap *dmau = list_entry(node, struct dma_uap, node);
>> -               pl011_dma_probe_initcall(dmau->dev, dmau->uap);
>> -               list_del(node);
>> -               kfree(dmau);
>> -       }
>> -       return 0;
>> -}
>> -
>> -device_initcall(pl011_dma_initcall);
>>
>> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
>> -{
>> -       struct dma_uap *dmau = kzalloc(sizeof(struct dma_uap), GFP_KERNEL);
>> -       if (dmau) {
>> -               dmau->uap = uap;
>> -               dmau->dev = dev;
>> -               list_add_tail(&dmau->node, &pl011_dma_uarts);
>> -       }
>> +       return;
> You don't need an explicit return.

yes, not sure how that came in.

>
>>  }
>> -#else
>> -static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
>> -{
>> -       pl011_dma_probe_initcall(dev, uap);
>> -}
>> -#endif
>> -interviewing
>>  static void pl011_dma_remove(struct uart_amba_port *uap)
>>  {
>>         /* TODO: remove the initcall if it has not yet executed */
>> @@ -1548,6 +1512,9 @@ static int pl011_hwinit(struct uart_port *port)
>>         uap->im = readw(uap->port.membase + UART011_IMSC);
>>         writew(UART011_RTIM | UART011_RXIM, uap->port.membase + UART011_IMSC);
>>
>> +       if (!uap->dma_probed)
>> +               pl011_dma_probe(uap->port.dev, uap);
>> +
>>         if (dev_get_platdata(uap->port.dev)) {
>>                 struct amba_pl011_data *plat;
>>
>> @@ -2218,7 +2185,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
>>         uap->port.ops = &amba_pl011_pops;
>>         uap->port.flags = UPF_BOOT_AUTOCONF;
>>         uap->port.line = i;
>> -       pl011_dma_probe(&dev->dev, uap);
>>
>>         /* Ensure interrupts from this UART are masked and cleared */
>>         writew(0, uap->port.membase + UART011_IMSC);
>> --
>> 1.9.1
>>