Re: [PATCH v3] ARM: EDMA: Fix clearing of unused list for DT DMA resources

[Joel Fernandes <joelf-l0cyMroinI0@public.gmane.org>]

On 09/27/2013 02:49 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-l0cyMroinI0@public.gmane.org> 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-l0cyMroinI0@public.gmane.org>
>>>> Reported-by: Balaji T K <balajitk-l0cyMroinI0@public.gmane.org>
>>>> Cc: Sekhar Nori <nsekhar-l0cyMroinI0@public.gmane.org>
>>>> Cc: Tony Lindgren <tony-4v6yS6AI5VpBDgjK7y7TUQ@public.gmane.org>
>>>> Cc: Olof Johansson <olof-nZhT3qVonbNeoWH0uzbU5w@public.gmane.org>
>>>> Cc: Nishanth Menon <nm-l0cyMroinI0@public.gmane.org>
>>>> Cc: Pantel Antoniou <panto-wVdstyuyKrO8r51toPun2/C9HSW9iNxf@public.gmane.org>
>>>> Cc: Jason Kridner <jkridner-hcmAuCOw+vXj4SYmN/TMmA@public.gmane.org>
>>>> Cc: Koen Kooi <koen-QLwJDigV5abLmq1fohREcCpxlwaOVQ5f@public.gmane.org>
>>>> Signed-off-by: Joel Fernandes <joelf-l0cyMroinI0@public.gmane.org>
>>>> ---
>>>> 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.
>>
>> For TI, a bit more work toward explaining patches better in change logs so that
>> maintainers understand and are willing to help to get the code upstream would
>> help. A lot of improvement to internal policies have been made for developing on
>> upstream, and that's certainly a good thing but there's still more room forard
>> improvement.
>>
>> For maintainers,  EDMA code would have been kept breathing all these months
>> (atleast 8 months) if those temporary fixes mentioned above would have been
>> merged into the kernel instead of not applied. With those fixes in place, dts
>> could have been enabled and EDMA would be fully working all these months. This
>> would have certainly made things a lot easier. Rewriting stuff the right way is
>> OK but if it is a _lot_ of effort, then to keep things alive, merging stuff "for
>> now" specially if they are one-liners could be made acceptable.
> 
> Joel, can you give a link to the "one-liner" "temporary fix" that was
> was not merged? I am unable to put it in context.

Sure, not exactly a one-line but maybe couple of lines (see below). Also wasn't
referring to anything you're maintaining. I was just continuing the original
discussion of where we can improve as a community.

My point was very trivial changes that keep things working such as this one
should be merged in time to keep things working:
http://lkml.indiana.edu/hypermail/linux/kernel/1210.2/01476.html

That one patch was reworked several times, and then was decided to do the right
way and finally merged with the limit-less series. All I am saying is all this
while the above patch(es) could be merged to keep things alive, and that way we
could avoid a lot of the pain and frustration that folks felt.

Its just my opinion though, I don't feel strongly about it but I think this may
be a helpful approach that's all.

regards,

-Joel