Re: [RFC] serial: 8250: fix regression in 8250 uart driver

[Peter Hurley <peter@hurleysoftware.com>]

On 06/13/2016 01:13 PM, Dinh Nguyen wrote:
> Hi Andy,
> 
> I saw that you have discovered that commit ec5a11a91eec ("serial: 8250:
> Validate dmaengine rx chan meets requirements") introduced a regression
> in the 8250 uart driver. For SoCFPGA platform, I am seeing this error:
> 
> [    5.541751] ttyS0 - failed to request DMA
> 
> Reverting the commit ec5a11a91eec removes the error.

The error is that your dmaengine driver doesn't support the functionality
required by the 8250 driver.

Regards,
Peter Hurley

> I saw that you started the discussion, but I didn't see that a patch was
> included[1].
> 
> The following patch seems to fix the error, but I'm not sure if it's the
> same fix that you had in mind.
> 
> Thanks,
> Dinh
> 
> [1] http://marc.info/?l=linux-serial&m=146254187602862&w=2
> 
> ---------------8<------------
> diff --git a/drivers/tty/serial/8250/8250_dma.c
> b/drivers/tty/serial/8250/8250_dma.c
> index 7f33d1c..847a203 100644
> --- a/drivers/tty/serial/8250/8250_dma.c
> +++ b/drivers/tty/serial/8250/8250_dma.c
> @@ -176,7 +176,7 @@ int serial8250_request_dma(struct uart_8250_port *p)
>         ret = dma_get_slave_caps(dma->rxchan, &caps);
>         if (ret)
>                 goto release_rx;
> -       if (!caps.cmd_pause || !caps.cmd_terminate ||
> +       if ((!caps.cmd_pause || !caps.cmd_terminate) &&
>             caps.residue_granularity ==
> DMA_RESIDUE_GRANULARITY_DESCRIPTOR) {
>                 ret = -EINVAL;
>                 goto release_rx;
>