[PATCH 4/7] drm/radeon: Pin buffers while they are vmap'ed

Thu Nov 26 11:59:32 UTC 2020

Hi

Am 26.11.20 um 12:04 schrieb Daniel Vetter:
> On Thu, Nov 26, 2020 at 11:15 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>>
>> Hi
>>
>> Am 25.11.20 um 17:32 schrieb Daniel Vetter:
>>> [...]
>>> I guess full locking is required :-/ I'm not exactly sure how to make this
>>> happen with the current plethora of helpers ... I think we need an
>>> _locked version of vmap/vunmap callbacks in drm_gem_object_funcs.
>>
>> I think we might be able to get away without new callbacks.
>>
>> I looked through the sources that implement and use vmap. All the
>> implementations are without taking resv locks. For locking, we can wrap
>> them in lock/unlock pairs. Having something like drm_gem_vmap_unlocked()
>> that locks and vmaps should make this easy.
>>
>> In terms of implementation, only vram helpers do a pin+map in their vmap
>> code. And as I mentioned before, this is actually wrong. The pattern
>> dates back to when the code was still in individual drivers. It's time
>> to clean this up. Vram helpers can use drm_gem_ttm_vmap() instead.
>>
>> Finally, there aren't that many users of vmap. A few drivers use it
>> while blitting framebuffers into HW buffers and ast does some permanent
>> mapping of the cursor BO. All this is trivial to turn into small pairs
>> of lock+vmap and vunmap+unlock.
>>
>> That leaves generic fbdev. The shadow buffer is also trivial to fix, as
>> outlined during this discussion.
>>
>> The code for fbdev in hardware buffers is a special case. It vmaps the
>> buffer during initialization and only vunmaps it during shutdown. As
>> this has worked so far without locking, I'd leave it as it is and put a
>> big comment next to is.
>>
>> Hardware fbdev buffers is only required by few drivers; namely those
>> that require the CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM config option to work.
>> We should consider to make the fbdev shadow buffer the default and have
>> drivers opt-in for the hardware buffer, if they need it.
>>
>>>
>>> And then document that if the callers of the _locked version wants a
>>> permanent mapping, it also needs to pin it. Plus I guess ideally implement
>>> the unlocked/permanent versions in terms of that, so that drivers only
>>> have to implement one or the other.
>>
>> For my understanding, pinning is only done in prepare_fb code. And ast
>> pins its cursor BOs into vram. We should document to hols vmap/vunmap
>> only for time and cover them with resv locks. Pinning is for cases where
>> the hardware requires buffers in a special location, but does not
>> protect against concurrent threat. I think those are the implicit rules
>> already.
>>
>> I updated the radeon patchset, where all this appears to be working well.
> 
> Hm yeah if you want to do the full change, then that works out too.
> It's just a pile of work.
> 
> But if we can finish off with an dma_resv_assert_locked in
> dma_buf_vmap/vunmap, then I think that's ok. It does mean that
> exporters must implement vmap caching, or performance will be
> terrible. So quite some update for the dma-buf docs.

Yeah, I remember a bug report about frequent page-table modifications 
wrt to vram helpers. So we implemented the lazy unmapping / vmap 
caching, as suggested by Christian back them. My guess is that anything 
TTM-based can use a similar pattern. Christian probably knows the corner 
cases.

CMA seems obviously working correctly already.

For SHMEM, I'd have to figure out the reference counting of the pages 
involved. My guess is that the vunmap in drm_gem_shmem_vunmap() could be 
moved into the free callback, plus a few other modifications.

Best regards
Thomas

> 
> But if you're willing to do all that conversion of callers, then of
> course I'm not stopping you. Not at all, it's great to see that kind
> of maze untangled.
> -Daniel
> 
>>
>> Best regards
>> Thomas
>>
>>>
>>> That should give us at least some way forward to gradually conver all the
>>> drivers and helpers over to dma_resv locking.
>>> -Daniel
>>>
>>>> The pin count is currently maintained by the vmap implementation in vram
>>>> helpers. Calling vmap is an implicit pin; calling vunmap is an implicit
>>>> unpin. This prevents eviction in the damage worker. But now I was told than
>>>> pinning is only for BOs that are controlled by userspace and internal users
>>>> should acquire the resv lock. So vram helpers have to be fixed, actually.
>>>>
>>>> In vram helpers, unmapping does not mean eviction. The unmap operation only
>>>> marks the BO as unmappable. The real unmap happens when the eviction takes
>>>> place. This avoids many modifications to the page tables. IOW an unpinned,
>>>> unmapped BO will remain in VRAM until the memory is actually needed.
>>>>
>>>> Best regards
>>>> Thomas
>>>>
>>>>>
>>>>> So I'm still not seeing how this can go boom.
>>>>>
>>>>> Now long term it'd be nice to cut everything over to dma_resv locking, but
>>>>> the issue there is that beyond ttm, none of the helpers (and few of the
>>>>> drivers) use dma_resv. So this is a fairly big uphill battle. Quick
>>>>> interim fix seems like the right solution to me.
>>>>> -Daniel
>>>>>
>>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>>>
>>>>>>>
>>>>>>> Best regards
>>>>>>> Thomas
>>>>>>>
>>>>>>>>
>>>>>>>>> There's no recursion taking place, so I guess the reservation
>>>>>>>>> lock could be
>>>>>>>>> acquired/release in drm_client_buffer_vmap/vunmap(), or a
>>>>>>>>> separate pair of
>>>>>>>>> DRM client functions could do the locking.
>>>>>>>>
>>>>>>>> Given how this "do the right locking" is a can of worms (and I think
>>>>>>>> it's
>>>>>>>> worse than what you dug out already) I think the fb_helper.lock hack is
>>>>>>>> perfectly good enough.
>>>>>>>>
>>>>>>>> I'm also somewhat worried that starting to use dma_resv lock in generic
>>>>>>>> code, while many helpers/drivers still have their hand-rolled locking,
>>>>>>>> will make conversion over to dma_resv needlessly more complicated.
>>>>>>>> -Daniel
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Best regards
>>>>>>>>> Thomas
>>>>>>>>>
>>>>>>>>> [1] https://cgit.freedesktop.org/drm/drm-tip/tree/drivers/gpu/drm/drm_fb_helper.c?id=ac60f3f3090115d21f028bffa2dcfb67f695c4f2#n394
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Please note that the reservation lock you need to take here is part of
>>>>>>>>>> the GEM object.
>>>>>>>>>>
>>>>>>>>>> Usually we design things in the way that the code needs to take a lock
>>>>>>>>>> which protects an object, then do some operations with the object and
>>>>>>>>>> then release the lock again.
>>>>>>>>>>
>>>>>>>>>> Having in the lock inside the operation can be done as well, but
>>>>>>>>>> returning with it is kind of unusual design.
>>>>>>>>>>
>>>>>>>>>>> Sorry for the noob questions. I'm still trying to understand the
>>>>>>>>>>> implications of acquiring these locks.
>>>>>>>>>>
>>>>>>>>>> Well this is the reservation lock of the GEM object we are
>>>>>>>>>> talking about
>>>>>>>>>> here. We need to take that for a couple of different operations,
>>>>>>>>>> vmap/vunmap doesn't sound like a special case to me.
>>>>>>>>>>
>>>>>>>>>> Regards,
>>>>>>>>>> Christian.
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Best regards
>>>>>>>>>>> Thomas
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> dri-devel mailing list
>>>>>>>>>> dri-devel at lists.freedesktop.org
>>>>>>>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Thomas Zimmermann
>>>>>>>>> Graphics Driver Developer
>>>>>>>>> SUSE Software Solutions Germany GmbH
>>>>>>>>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>>>>>>> (HRB 36809, AG Nürnberg)
>>>>>>>>> Geschäftsführer: Felix Imendörffer
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>> --
>>>> Thomas Zimmermann
>>>> Graphics Driver Developer
>>>> SUSE Software Solutions Germany GmbH
>>>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>> (HRB 36809, AG Nürnberg)
>>>> Geschäftsführer: Felix Imendörffer
>>>
>>>
>>>
>>>
>>>
>>>
>>
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Felix Imendörffer
> 
> 
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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