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

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Date:   Fri, 2 Dec 2022 22:44:37 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Thorsten Leemhuis <regressions@...mhuis.info>,
        Will Deacon <will@...nel.org>,
        Amit Pundir <amit.pundir@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sibi Sankar <quic_sibis@...cinc.com>,
        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 Fri, Dec 02, 2022 at 05:32:51PM +0100, Greg KH wrote:
> On Fri, Dec 02, 2022 at 05:27:24PM +0100, Thorsten Leemhuis wrote:
> > 
> > 
> > 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/
> 
> Ah, I missed that.
> 
> Ok, what in-tree drivers are having problems being buggy?  I can't seem
> to figure that out from that report at all.  Does anyone know?
> 

It is the Qualcomm Q6V5_MSS remoteproc driver:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/remoteproc/qcom_q6v5_mss.c

Qualcomm is working on the fix but the patches are not ready yet. So if we can
get this patch reverted in the meantime, that would be helpful.

Thanks,
Mani

> thanks,
> 
> greg k-h

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