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

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Date:   Tue, 16 Nov 2021 09:26:59 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tudor.Ambarus@...rochip.com
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 Tue, Nov 16, 2021 at 06:14:23AM +0000, Tudor.Ambarus@...rochip.com wrote:
> 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.

Please resend it because as-is, I can not take it.

thanks,

greg k-h