Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()"

[Will Deacon]

On Tue, Dec 06, 2022 at 02:51:52PM +0530, Manivannan Sadhasivam wrote:
> On Mon, Dec 05, 2022 at 02:24:03PM +0000, Will Deacon wrote:
> > On Fri, Dec 02, 2022 at 10:44:37PM +0530, Manivannan Sadhasivam wrote:
> > > 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@xxxxxxx> 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@xxxxxxxxxxxxxx/
> > > > 
> > > > 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.
> > 
> > It's good to hear that you're working to fix this, even if it's happening
> > behind closed doors. Do you have a rough idea how soon you'll be able to
> > post the remoteproc driver fixes? That would help us to figure out when
> > to bring back the change if we were to revert it.
> > 
> 
> Sibi is the one working on the fix. I believe he should be able to post the
> patches within this week.

Oh nice, that's a lot sooner than I expected! I'll send a revert out now,
with a comment about where we're at.

Will