Message-ID: <5245A373.1090407@ti.com>
Date:	Fri, 27 Sep 2013 10:25:39 -0500
From:	Joel Fernandes <joelf@...com>
To:	Sekhar Nori <nsekhar@...com>
CC:	Olof Johansson <olof@...om.net>, Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <djbw@...com>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>, Nishanth Menon <nm@...com>,
	Pantel Antoniou <panto@...oniou-consulting.com>,
	Jason Kridner <jkridner@...gleboard.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	"arm@...nel.org" <arm@...nel.org>
Subject: Re: [PATCH v3] ARM: EDMA: Fix clearing of unused list for DT DMA
 resources

On 09/27/2013 04:04 AM, Sekhar Nori wrote:
> On 9/27/2013 5:58 AM, Joel Fernandes wrote:
>> On 09/26/2013 06:13 PM, Olof Johansson wrote:
>>> On Thu, Sep 26, 2013 at 2:55 PM, Joel Fernandes <joelf@...com> wrote:
>>>> HWMOD removal for MMC is breaking edma_start as the events are being manually
>>>> triggered due to unused channel list not being clear.
>>>>
>>>> The above issue is fixed by reading the "dmas" property from the DT node if it
>>>> exists and clearing the bits in the unused channel list if the dma controller
>>>> used by any device is EDMA. For this purpose we use the of_* helpers to parse
>>>> the arguments in the dmas phandle list.
>>>>
>>>> Also introduced is a minor clean up of a checkpatch error in old code.
>>>>
>>>> Reviewed-by: Sekhar Nori <nsekhar@...com>
>>>> Reported-by: Balaji T K <balajitk@...com>
>>>> Cc: Sekhar Nori <nsekhar@...com>
>>>> Cc: Tony Lindgren <tony@...mide.com>
>>>> Cc: Olof Johansson <olof@...om.net>
>>>> Cc: Nishanth Menon <nm@...com>
>>>> Cc: Pantel Antoniou <panto@...oniou-consulting.com>
>>>> Cc: Jason Kridner <jkridner@...gleboard.org>
>>>> Cc: Koen Kooi <koen@...inion.thruhere.net>
>>>> Signed-off-by: Joel Fernandes <joelf@...com>
>>>> ---
>>>> Just resending this patch after discussing with Sekhar and Olof.
>>>
>>> Actually, the patch you talked to me about was v3 of this. It seems
>>> that you have reposted v6 but labelled it v3. This is very confusing.
>>
>> Sorry about this. :-( This is indeed v6.
>>
>>>> AM335x is being booted by many users such as the beaglebone community. DT is
>>>> the only boot method available for all these users.  EDMA is required for the
>>>> operation for many common peripherals in AM335x SoC such as McASP, MMC and
>>>> Crypto.
>>>>
>>>> Although EDMA DT nodes are going in only for 3.13, in reality the kernel has
>>>> been used for more than a year with EDMA code and out of tree EDMA DTS patches.
>>>> Hence though the DT nodes are still not in mainline, this patch can be still be
>>>> considered a critical fix as such and it would be great if it could be included
>>>> in 3.12-rc release. Thanks.
>>>
>>> This is really the root of this problem. If you sit on code out of
>>> tree for a year, and something breaks that we couldn't even have
>>> detected since we didn't have the out-of-tree pieces. We'll help you
>>> the first few times (such as now) but we will eventually stop caring.
>>
>> When I started looking at EDMA in June, I noticed that a lot had already been
>> merged. EDMA DMA Engine driver itself was merged last year, no worries there.
>> but the long pending list of fixes to be made to the driver had to written and
>> rewritten multiple times which took a long time.
>>
>> Due to this, the EDMA device tree entries could not be merged in fear that doing
>> so would cause problems such as MMC/SD corruption etc.
>>
>>> If I was in a worse mood, then I'd just say that since your users
>>> already has to have out-of-tree code to even use this functionality,
>>> they could just add this fix on top of that stack of patches. But I'm
>>> in a slightly better mood than that and I'll pick it up this time. :)
>>
>> Thank you! :)
>>
>>>> More details about why this broke an existing feature folks were using:
>>>> Previously the DMA resources for platform devices were being populated through
>>>> HWMOD, however with the recent clean ups with HWMOD, this data has been moved
>>>> to Device tree. The EDMA code though is not aware of this so it fails to fetch
>>>> the DMA resources correctly which it needs to prepare the unused channel list
>>>> (basically doesn't properly clear the channels that are in use, in the unused
>>>> list).
>>>
>>> So that we can learn for next time: What should we (as in us
>>> maintainers and you TI) have done differently to avoid this?
>>
>> I think a little on both sides can be improved.
> 
> Since we are in lessons learnt mode, I think as developers we need to
> learn to prioritize fixes over other features we are working on. I went
> back to the chronology of this patch series.

Sure, I agree with this. Will definitely work on it.

> 
> 22nd July 2013: v2 posted
> 29th July 2013: Discussion on whether the patch can wait till *v3.12*
> 		merge window.
> 29th July 2013: comments given on v2
> 22nd Aug 2013: 	Pull request's sent by Sekhar for v3.12
> 24th Aug 2013: 	another v2 posted (all comments given earlier not
> 		addressed, received some comments on build warnings)
> 27th Aug 2013:	another v3 posted (all comments given on 29th July not
> 		addressed)
> 10th Sept 2013: another v3 posted (all comments given on 29th July not
> 		addressed)
> [some discussion on comments and why this cannot wait until v3.13]
> 17th Sept 2013:	Final version ready for merge posted.
> 26th Sept 2013: Another v3 posted, this time for Olof to send into
> 		v3.12-rc
> 
> See, early on, the patch was actually in consideration for v3.12 merge.
> The barrier of entry into -rc cycle is pretty high. So if you have an
> opportunity to hit a merge window, utilize that by prioritizing this
> work over anything else you may be doing.
> 
> I know you got busy with adding support for SG lists and all, but
> clearly this patch is critical in your mind. Plus the comments were not
> tough to fix. There is a need to keep looking at what provides the best
> return on time invested.


Yes, but we are not referring to this particular patch at all. Olof was asking
about the code that was not been merged and/or the reasons things are not
working. I was just responding to that. But thanks for the advice, will def keep
it in mind.

Regards,

-Joel

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