Re: [PATCH 0/6] drm+dma: cache support for arm, etc

From: Christoph Hellwig <hch@lst.de>
To: Rob Clark <robdclark@chromium.org>
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Subject: Re: [PATCH 0/6] drm+dma: cache support for arm, etc
Date: Mon, 19 Aug 2019 07:23:37 +0200	[thread overview]
Message-ID: <20190819052337.GA16622@lst.de> (raw)
In-Reply-To: <CAJs_Fx6am7TeDFSG=CcTT=4KwhqrZX_jnn56NaWcDkGVizuakg@mail.gmail.com>

On Fri, Aug 16, 2019 at 02:04:35PM -0700, Rob Clark wrote:
> I don't disagree about needing an API to get uncached memory (or
> ideally just something outside of the linear map).  But I think this
> is a separate problem.
> 
> What I was hoping for, for v5.4, is a way to stop abusing dma_map/sync
> for cache ops to get rid of the hack I had to make for v5.3.  And also
> to fix vgem on non-x86.  (Unfortunately changing vgem to used cached
> mappings breaks x86 CI, but fixes CI on arm/arm64..)  We can do that
> without any changes in allocation.  There is still the possibility for
> problems due to cached alias, but that has been a problem this whole
> time, it isn't something new.

But that just means we start exposing random low-level APIs that
people will quickly abuse..  In fact even your simple plan to some
extent already is an abuse of the intent of these functions, and
it also requires a lot of knowledge in the driver that in the normal
cases drivers can't know (e.g. is the device dma coherent or not).