linux-kernel - Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

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Date:	Fri, 07 Aug 2015 17:44:03 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Peter Hurley <peter@...leysoftware.com>
CC:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

On 08/07/2015 05:29 PM, Russell King - ARM Linux wrote:
> On Fri, Aug 07, 2015 at 11:08:48AM -0400, Peter Hurley wrote:
>> [ + Greg KH ]
>>
>> On 08/07/2015 09:57 AM, Russell King - ARM Linux wrote:
>>> As it is something that the driver has _not_ supported, you are clearly
>>> adding a feature to an existing driver.  It's not a bug fix.
>>>
>>>>> If something else has been converted to pause channels and that is causing
>>>>> a problem, then _that_ conversion is where the bug lies, not the lack of
>>>>> support in the omap-dma.
>>
>> FWIW, the actual bug is the api that silently does nothing.
> 
> Incorrect.
> 
> static int omap_dma_pause(struct dma_chan *chan)
> {
>         struct omap_chan *c = to_omap_dma_chan(chan);
> 
>         /* Pause/Resume only allowed with cyclic mode */
>         if (!c->cyclic)
>                 return -EINVAL;
> 
> Asking for the channel to be paused will return an error code indicating
> that the request failed.  That will be propagated back through to the
> return code of dmaengine_pause().
> 
> If we look at what 8250-dma.c is doing:
> 
>                 if (dma->rx_running) {
>                         dmaengine_pause(dma->rxchan);
> 
> It's 8250-dma.c which is silently _ignoring_ the return code, failing
> to check that the operation it requested worked.  Maybe this should be
> WARN_ON(dmaengine_pause(dma->rxchan)) or at least it should print a
> message?

I think this is what Peter meant by saying "silently does nothing".

> So, I guess that means that older kernels will just have to remain broken -
> all because the basic testing of the original code was never undertaken
> to ensure that basic stuff like reception of characters worked properly.

Hehe. I wouldn't describe testing at 3mbaud as basic. This reads as I
didn't do any kind of testing at all prior submitting the driver. This
was not the case.

Sebastian
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