From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Anholt <eric@anholt.net>
Subject: Re: [PATCH v3 3/3] drm/vgem: use normal cached mmap'ings
Date: Tue, 16 Jul 2019 16:39:11 -0700
Message-ID: <87lfwxh7mo.fsf@anholt.net>
References: <20190716213746.4670-1-robdclark@gmail.com> <20190716213746.4670-3-robdclark@gmail.com>
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To: Rob Clark <robdclark@gmail.com>, dri-devel@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>, Intel Graphics Development <intel-gfx@lists.freedesktop.org>, Rob Clark <robdclark@chromium.org>, David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>, Emil Velikov <emil.velikov@collabora.com>, Eric Biggers <ebiggers@google.com>, Deepak Sharma <deepak.sharma@amd.com>, linux-kernel@vger.kernel.org
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Rob Clark <robdclark@gmail.com> writes:

> From: Rob Clark <robdclark@chromium.org>
>
> Since there is no real device associated with VGEM, it is impossible to
> end up with appropriate dev->dma_ops, meaning that we have no way to
> invalidate the shmem pages allocated by VGEM.  So, at least on platforms
> without drm_cflush_pages(), we end up with corruption when cache lines
> from previous usage of VGEM bo pages get evicted to memory.
>
> The only sane option is to use cached mappings.

This may be an improvement, but...

pin/unpin is only on attaching/closing the dma-buf, right?  So, great,
you flushed the cached map once after exporting the vgem dma-buf to the
actual GPU device, but from then on you still have no interface for
getting coherent access through VGEM's mapping again, which still
exists.

I feel like this is papering over something that's really just broken,
and we should stop providing VGEM just because someone wants to write
dma-buf test code without driver-specific BO alloc ioctl code.

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