linux-kernel - Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep

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Date:   Fri, 2 Dec 2022 11:34:30 +0100
From:   Thorsten Leemhuis <regressions@...mhuis.info>
To:     Will Deacon <will@...nel.org>
Cc:     Amit Pundir <amit.pundir@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sibi Sankar <quic_sibis@...cinc.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Robin Murphy <robin.murphy@....com>, andersson@...nel.org,
        sumit.semwal@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, hch@....de,
        gregkh@...uxfoundation.org
Subject: Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from
 arch_dma_prep_coherent()"

On 02.12.22 11:03, Will Deacon wrote:
> On Fri, Dec 02, 2022 at 09:54:05AM +0100, Thorsten Leemhuis wrote:
>> On 02.12.22 09:26, Amit Pundir wrote:
>>> On Thu, 1 Dec 2022 at 23:15, Catalin Marinas <catalin.marinas@....com> wrote:
>>>>
>>>> On Thu, Dec 01, 2022 at 10:29:39AM +0100, Thorsten Leemhuis wrote:
>>>>> Has any progress been made to fix this regression? It afaics is not a
>>>>> release critical issue, but well, it still would be nice to get this
>>>>> fixed before 6.1 is released.
>>>>
>>>> The only (nearly) risk-free "fix" for 6.1 would be to revert the commit
>>>> that exposed the driver bug. It doesn't fix the actual bug, it only
>>>> makes it less likely to happen.
>>>>
>>>> I like the original commit removing the cache invalidation as it shows
>>>> drivers not behaving properly
>>
>> Yeah, I understand that, but I guess it's my job to ask at this point:
>> "is continuing to live with the old behavior for one or two more cycles"
>> that much of a problem"?
> 
> That wouldn't be a problem. The problem is that I haven't see any efforts
> from the Qualcomm side to actually fix the drivers [...]

Thx for sharing the details. I can fully understand your pain. But well,
in the end it looks to me like this commit it intentionally breaking
something that used to work -- which to my understanding of the "no
regression rule" is not okay, even if things only worked by chance and
not flawless.

But well, as with every rule there are misunderstandings, grey areas,
and situations where judgement calls have to be made. Then it's up to
Linus to decide how to handle things. Hence I'll just point him to this
thread and then he can decide. No biggie. And sorry if I'm being a PITA
here, I just thing doing that is my duty as regression tracker in
situations like this. Hope your won't mind.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.