linux-kernel - Re: [PATCH v3 3/3] drm/vgem: use normal cached mmap'ings

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Date:   Tue, 16 Jul 2019 16:39:11 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Emil Velikov <emil.velikov@...labora.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Deepak Sharma <deepak.sharma@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] drm/vgem: use normal cached mmap'ings

Rob Clark <robdclark@...il.com> writes:

> From: Rob Clark <robdclark@...omium.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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