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

From: Greg KH <gregkh@linuxfoundation.org>
To: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: Will Deacon <will@kernel.org>,
	Amit Pundir <amit.pundir@linaro.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Sibi Sankar <quic_sibis@quicinc.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
	Robin Murphy <robin.murphy@arm.com>,
	andersson@kernel.org, sumit.semwal@linaro.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, hch@lst.de
Subject: Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()"
Date: Fri, 2 Dec 2022 17:10:07 +0100	[thread overview]
Message-ID: <Y4ojXyXMX2p+RVBR@kroah.com> (raw)
In-Reply-To: <92a148a3-a8ac-4065-123c-99b72ac3ebeb@leemhuis.info>

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@arm.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?  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.

thanks,

greg kh