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

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>,
	Vinod Koul <vinod.koul@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
	nsekhar@ti.com, linux-omap@vger.kernel.org,
	linux-serial@vger.kernel.org, john.ogness@linutronix.de
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers
Date: Fri, 07 Aug 2015 15:42:06 +0200	[thread overview]
Message-ID: <55C4B5AE.10309@linutronix.de> (raw)
In-Reply-To: <20150807132241.GN7576@n2100.arm.linux.org.uk>

On 08/07/2015 03:22 PM, Russell King - ARM Linux wrote:
> On Fri, Aug 07, 2015 at 12:36:14PM +0200, Sebastian Andrzej Siewior wrote:
>> On 08/07/2015 11:44 AM, Peter Ujfalusi wrote:
>>> with a short testing audio did not broke (the only user of pause/resume)
>>> Some comments embedded.
>>>
>>>> Cc: <stable@vger.kernel.org>
>>>
>>> Why stable? This is not fixing any bugs since the PAUSE was not allowed for
>>> non cyclic transfers.
>>
>> Hmmm. The DRA7x was using pause before for UART. I just did not see it
>> coming that it was not allowed here. John made a similar change to the
>> edma driver and I assumed it went stable but now I see that it was just
>> cherry-picked into the ti tree.
> 
> This is *NOT* stable material.
> 
> Pause of these channels is something that omap-dma has *never* supported.
> Therefore, it is *not* a regression.  What you are doing is *adding* a
> feature to the omap-dma driver.  That is not stable material in any sense.
> Stable is for bug fixes to existing code, not feature enhancements.

I didn't consider this as a feature.

> 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.

So we had the 8250-DMA doing pause and all its current users implement
it. We have a DMA driver tree which is not used and it not implementing
pause (not implementing pause at all). Later we get a combo of 8250-DMA
+ DMA driver that is broken because the lack of pause and this is
noticed a few kernel releases later.
The only way of fixing the bug is by implementing the pause feature.
Now you are saying that even if I implement this missing feature in a
newer kernel I am not allowed to mark it stable despite the fact that
it fixes an existing problem in older kernels because it is not a
regression.

> If it's a result of using some new driver with omap-dma, then the problem
> is with whatever introduced that new combination - it's not that omap-dma
> is buggy.
> 
> Don't fix bugs in -stable by adding features.  That's _no_ way to fix bugs.
> 
> NAK on this feature patch having any kind of stable tag.

I already accepted this.

Sebastian