Re: [PATCH 1/3] tty: serial: 8250_omap: do not use RX DMA if pause is not supported

From: Sebastian Andrzej Siewior
Date: Mon Aug 10 2015 - 08:19:50 EST

On 08/10/2015 01:54 PM, Peter Ujfalusi wrote:
>> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
>> index 0340ee6ba970..07a11e0935e4 100644
>> --- a/drivers/tty/serial/8250/8250_omap.c
>> @@ -769,7 +771,9 @@ static void omap_8250_rx_dma_flush(struct uart_8250_port *p)
>> return;
>> }
>> - dmaengine_pause(dma->rxchan);
>> + ret = dmaengine_pause(dma->rxchan);
>> + if (WARN_ON_ONCE(ret))
>> + priv->rx_dma_broken = true;
> I don't think this is good thing for the stable _and_ for the mainline at the
> same time:
> in stable the rx DMA should not be allowed since the stable kernels does not
> allow pause/resume with omap-dma, so there the rx DMA should be just disabled
> for UART. This change will cause regression since it introduce a WARN_ON_ONCE,
> which will be printed if the user tries to use non working feature.

Okay. We do have pause support in mainline for edma since v4.2-rc1.
This driver can use edma or sdma depending on the configuration. But it
is not yet released. So you suggest remove RX-DMA support completely
from the 8250-omap, mark it stable, and revert that patch once we have
it fixed in sdma?

> In mainline you will eventually going to have pause/resume support so this
> patch will make no sense there.

The way this works is that it has to be fixed upstream before it can be
backported stable. Also Russell made clear (for a good reason) that the
RX problem has to be fixed upstream before he thinks about acking the
SDMA patch.
So if you prefer to instead remove RX-DMA until it is fixed, I am all

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