drm: Why shmem?

[Daniel Vetter]

On Mon, Aug 28, 2017 at 8:44 PM, Noralf Trønnes <noralf at tronnes.org> wrote:
> Hi,
>
> Currently I'm using the cma library with tinydrm because it was so
> simple to use even though I have to work around the fact that reads are
> uncached. A bigger problem that I have become aware of, is that it
> restricts the dma buffers it can import since they have to be continous.
>
> So I looked to udl and it uses shmem. Fine, let's make a shmem gem
> library similar to the cma library.
>
> Now I have done so and have started to think about the DOC: section,
> explaining what the library does. And I'm stuck, what's the benefit of
> using shmem compared to just using alloc_page()?

Gives you swapping (and eventually maybe even migration) since there's
a real filesystem behind it. Atm this only works if you register a
shrinker callback, which for display drivers is a bit overkill. See
i915 or msm for examples (or ttm, if you want an entire fancy
framework), and git grep shrinker -- drivers/gpu.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch