linux-kernel - Re: [PATCH] drm/vgem: fix cache synchronization on arm/arm64

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Date:   Thu, 01 Aug 2019 16:34:09 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Rob Clark <robdclark@...il.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Deepak Sharma <deepak.sharma@....com>,
        Emil Velikov <emil.velikov@...labora.com>,
        Eric Anholt <eric@...olt.net>,
        Eric Biggers <ebiggers@...gle.com>,
        Imre Deak <imre.deak@...el.com>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/vgem: fix cache synchronization on arm/arm64

Quoting Rob Clark (2019-08-01 16:18:45)
> On Thu, Aug 1, 2019 at 5:40 AM Chris Wilson <chris@...is-wilson.co.uk> wrote:
> >
> > Quoting Sean Paul (2019-07-31 20:23:31)
> > > On Fri, Jul 19, 2019 at 11:21:53AM +0200, Daniel Vetter wrote:
> > > > On Wed, Jul 17, 2019 at 02:15:37PM -0700, Rob Clark wrote:
> > > > > From: Rob Clark <robdclark@...omium.org>
> > > > >
> > > > > drm_cflush_pages() is no-op on arm/arm64.  But instead we can use
> > > > > dma_sync API.
> > > > >
> > > > > Fixes failures w/ vgem_test.
> > > > >
> > > > > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > > > > ---
> > > > > An alternative approach to the series[1] I sent yesterday
> > > > >
> > > > > On the plus side, it keeps the WC buffers and avoids any drm core
> > > > > changes.  On the minus side, I don't think it will work (at least
> > > > > on arm64) prior to v5.0[2], so the fix can't be backported very
> > > > > far.
> > > >
> > > > Yeah seems a lot more reasonable.
> > > >
> > > > Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>
> > >
> > > Applied to drm-misc-fixes, thanks!
> >
> > But it didn't actually fix the failures in CI.
> 
> Hmm, that is unfortunate, I'd assumed that silence meant latest
> version was working in CI..

Ah, takes a intel-gfx@ for CI to pick up patches atm.
 
> dma_sync_sg_* doesn't work on x86?  It would be kinda unfortunate to
> have vgem only work on x86 *or* arm..  maybe bringing back
> drm_cflush_pages() could make it work in both cases

I think it stems from the expectation that vgem provides "device
coherency" for CPU access. From the testing perspective, it's nice to
emulate HW interactions; but maybe that is just beyond the general
capabilities and we cannot simply use vgem as we do currently. That
would leave a hole for mocking prime in CI that needs filling :(
-Chris