linux-kernel - Re: [PATCH] tty: serial: atmel: Check return code of dmaengine

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Message-ID: <1910ee9a-19f8-e155-9cc9-4a67412f3402@microchip.com>
Date:   Tue, 16 Nov 2021 06:14:23 +0000
From:   <Tudor.Ambarus@...rochip.com>
To:     <gregkh@...uxfoundation.org>
CC:     <richard.genoud@...il.com>, <jirislaby@...nel.org>,
        <Nicolas.Ferre@...rochip.com>, <alexandre.belloni@...tlin.com>,
        <Ludovic.Desroches@...rochip.com>, <linux-serial@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: serial: atmel: Check return code of
 dmaengine_submit()

On 11/15/21 5:58 PM, Greg KH wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Mon, Nov 15, 2021 at 04:30:04PM +0200, Tudor Ambarus wrote:
>> dma_cookie_t < 0 indicates an error code, check for it.
> 
> Very odd changelog text, please be more descriptive about what is
> happening here.
> 

The tx_submit() method of struct dma_async_tx_descriptor is entitled to do
sanity checks and return errors if encountered. It's not the case for the
DMA controller drivers that this client is using (at_h/xdmac), because they
currently don't do sanity checks and always return a positive cookie at
tx_submit() method. In case the controller drivers will implement sanity
checks and return errors, print a message so that the client will be informed
that something went wrong at tx_submit() level.

>>
>> Signed-off-by: Tudor Ambarus <tudor.ambarus@...rochip.com>
>> ---
>>  drivers/tty/serial/atmel_serial.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
>> index 2c99a47a2535..376f7a9c2868 100644
>> --- a/drivers/tty/serial/atmel_serial.c
>> +++ b/drivers/tty/serial/atmel_serial.c
>> @@ -1004,6 +1004,11 @@ static void atmel_tx_dma(struct uart_port *port)
>>               desc->callback = atmel_complete_tx_dma;
>>               desc->callback_param = atmel_port;
>>               atmel_port->cookie_tx = dmaengine_submit(desc);
>> +             if (dma_submit_error(atmel_port->cookie_tx)) {
>> +                     dev_err(port->dev, "dma_submit_error %d\n",
>> +                             atmel_port->cookie_tx);
>> +                     return;
> 
> What can a user do with this error message?
> 
will be informed that something went wrong at tx_submit() level.

> Have you seen this happen in real life?

No. I debugged a locking problem and I observed that dma_submit_error() is not called,
so I thought to update this.

> 
> What commit does this "fix"?

This is rather an improvement, but if you're looking for a fixes tag, I think
we can use:
Fixes: 08f738be88bb ("serial: at91: add tx dma support")

I'll send a v2 as part of a bigger series. If you find this patch does not worth it,
I can as well drop it.

Cheers,
ta