drm: Why shmem?

[Eric Anholt]

Noralf Trønnes <noralf at tronnes.org> writes:

> 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()?

Using shmem means that, when the buffer isn't pinned for DMA usage by
the device, the pages can be swapped out.
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