Noralf Trønnes <noralf@xxxxxxxxxxx> 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@xxxxxxxx> writes: >>> >>>> On Mon, Aug 28, 2017 at 8:44 PM, Noralf Trønnes <noralf@xxxxxxxxxxx> 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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