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

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Date:   Fri, 2 Dec 2022 17:27:24 +0100
From:   Thorsten Leemhuis <regressions@...mhuis.info>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Will Deacon <will@...nel.org>,
        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
Subject: Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from
 arch_dma_prep_coherent()"



On 02.12.22 17:10, Greg KH wrote:
> On Fri, Dec 02, 2022 at 11:34:30AM +0100, Thorsten Leemhuis wrote:
>> 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.
> 
> "no regressions" for userspace code, this is broken, out-of-tree driver
> code, right?

If so: apologies. But that's not the impression I got, as Amit wrote "I
can reproduce this crash on vanilla v6.1-rc1 as well with no out-of-tree
drivers." here:
https://lore.kernel.org/linux-arm-kernel/CAMi1Hd3H2k1J8hJ6e-Miy5+nVDNzv6qQ3nN-9929B0GbHJkXEg@mail.gmail.com/

>  I do not think any in-kernel drivers have this issue today
> from what I can tell, but if I am wrong here, please let me know.
> 
> We don't keep stable apis, or even functionality, for out-of-tree kernel
> code as that would be impossible for us to do for obvious reasons.

Ciao, Thorsten