Re: [PATCH v14 02/12] drm/shmem-helper: Add pages_pin_count field

[Dmitry Osipenko]

On 7/31/23 15:27, Dmitry Osipenko wrote:
> On 7/25/23 11:32, Boris Brezillon wrote:
>>> Can we make it an atomic_t, so we can avoid taking the lock when the
>>> GEM has already been pinned. That's something I need to be able to grab
>>> a pin-ref in a path where the GEM resv lock is already held[1]. We could
>>> of course expose the locked version,
>> My bad, that's actually not true. The problem is not that I call
>> drm_gem_shmem_pin() with the resv lock already held, but that I call
>> drm_gem_shmem_pin() in a dma-signaling path where I'm not allowed to
>> take a resv lock. I know for sure pin_count > 0, because all GEM objects
>> mapped to a VM have their memory pinned right now, and this should
>> stand until we decide to add support for live-GEM eviction, at which
>> point we'll probably have a way to detect when a GEM is evicted, and
>> avoid calling drm_gem_shmem_pin() on it.
>>
>> TLDR; I can't trade the atomic_t for a drm_gem_shmem_pin_locked(),
>> because that wouldn't solve my problem. The other solution would be to
>> add an atomic_t at the driver-GEM level, and only call
>> drm_gem_shmem_[un]pin() on 0 <-> 1 transitions, but I thought using an
>> atomic at the GEM-shmem level, to avoid locking when we can, would be
>> beneficial to the rest of the eco-system. Let me know if that's not an
>> option, and I'll go back to the driver-specific atomic_t.
> 
> Could you please explain why do you need to pin GEM in a signal handler?
> This is not something drivers usually do or need to do. You likely also
> shouldn't need to detect that GEM is evicted in yours driver. I'd expect
> that Panthor shouldn't differ from Panfrost in regards to how GEM memory
> management is done and Panfrost doesn't need to do anything special.
> 
> Note that patch #14 makes locked pin/unpin functions public and turns
> the unlocked variants into helpers, you'll be able to experiment with
> these funcs in the Panthor driver.

correction: that's patch #10

> In general, using atomic_t or kref should be a good thing to do, but
> AFAICS it shouldn't bring benefits to the today's drm-shmem users. I'd
> want to understand what you're trying to achieve in the Panthor driver.
> 

-- 
Best regards,
Dmitry