Re: [PATCH v20 09/10] drm/shmem-helper: Switch drm_gem_shmem_vmap/vunmap to use pin/unpin

[Thomas Zimmermann]

Hi

Am 04.04.25 um 16:52 schrieb Boris Brezillon:
> On Fri, 4 Apr 2025 10:01:50 +0200
> Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
>
> > > > In your case, vmap an pin both intent to hold the shmem pages in memory.
> > > > They might be build on top of the same implementation, but one should
> > > > not be implemented with the other because of their different meanings.
> > > But that's not what we do, is it? Sure, in drm_gem_shmem_vmap_locked(),
> > > we call drm_gem_shmem_pin_locked(), but that's an internal function to
> > > make sure the pages are allocated and stay around until
> > > drm_gem_shmem_vunmap_locked() is called.
> > >
> > > I guess we could rename pin_count into hard_refcount or
> > > page_residency_count or xxx_count, and change the pin/unpin_locked()
> > > function names accordingly, but that's just a naming detail, it doesn't
> > > force you to call drm_gem_pin() to vmap() your GEM, it's something we
> > > do internally.
> > Such a rename would be much appreciated. page_residency_count seems
> > appropriate.
> On a second thought, I think I prefer
> 'unevictable_count/inc_unevictable()/dec_unevictable()'. But looking at

Ok

> the gem-vram changes you just posted, it looks like gem-shmem is not the
> only one to use the term 'pin' for this page pinning thing, so if we go
> and plan for a rename, I'd rather make it DRM-wide than gem-shmem being
> the outlier yet again :-).

Which one do you mean?

- The code in gem-vram does pinning in the TTM sense of the term.

- From the client code, the pinning got removed.

- The GEM pin/unpin callbacks are still there, but the long-term plan is 
to go to dma-buf pin callbacks AFAICT.

- Renaming the dma-buf pin/unpin would be a kernel-wide change. Not 
likely to happen.

Best regards
Thomas


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Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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