drm: Why shmem?

[Eric Anholt]

Noralf Trønnes <noralf at tronnes.org> writes:

> 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?

drm_gem_object_release() during freeing of the object.

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