drm: Why shmem?

[Noralf Trønnes]

Den 30.08.2017 09.40, skrev Daniel Vetter:
> On Tue, Aug 29, 2017 at 10:40:04AM -0700, Eric Anholt wrote:
>> Daniel Vetter <daniel at ffwll.ch> writes:
>>
>>> 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.
>> The shrinker is only needed if you need some impetus to unbind objects
>> from your page tables, right?  If you're just binding the pages for the
>> moment that you're doing SPI transfers to the display, then in the
>> remaining time it could be swapped out, right?
> Yup, and for SPI the setup overhead shouldn't matter. But everyone else
> probably wants to cache mappings and page lists, and that means some kind
> of shrinker to drop them when needed.

Let me see if I've understood this correctly:

The first time I call drm_gem_get_pages() the buffer pages are
allocated and pinned.
When I then call drm_gem_put_pages() the pages are unpinned, but not freed.
The kernel is now free to swap out the pages if necessary.
Calling drm_gem_get_pages() a second time will swapin the pages if
necessary and pin them.

If this is correct, where are pages freed?

Noralf.