[PATCH v2 09/15] drm/ttm, drm/amdgpu: Allow the driver some control over swapping

[Christian König]

Am 18.05.21 um 17:20 schrieb Thomas Hellström:
>
> On 5/18/21 5:18 PM, Christian König wrote:
>>
>>
>> Am 18.05.21 um 17:15 schrieb Thomas Hellström:
>>>
>>> On 5/18/21 10:26 AM, Thomas Hellström wrote:
>>>> We are calling the eviction_valuable driver callback at eviction 
>>>> time to
>>>> determine whether we actually can evict a buffer object.
>>>> The upcoming i915 TTM backend needs the same functionality for 
>>>> swapout,
>>>> and that might actually be beneficial to other drivers as well.
>>>>
>>>> Add an eviction_valuable call also in the swapout path. Try to keep 
>>>> the
>>>> current behaviour for all drivers by returning true if the buffer 
>>>> object
>>>> is already in the TTM_PL_SYSTEM placement. We change behaviour for the
>>>> case where a buffer object is in a TT backed placement when swapped 
>>>> out,
>>>> in which case the drivers normal eviction_valuable path is run.
>>>>
>>>> Finally export ttm_tt_unpopulate() and don't swap out bos
>>>> that are not populated. This allows a driver to purge a bo at
>>>> swapout time if its content is no longer valuable rather than to
>>>> have TTM swap the contents out.
>>>>
>>>> Cc: Christian König <christian.koenig at amd.com>
>>>> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>>
>>> Christian,
>>>
>>> Here we have a ttm_tt_unpopulate() export as well at the end. I 
>>> figure you will push back on that one. What we really need is a 
>>> functionality to just drop the bo contents and end up in system 
>>> memory unpopulated. Should I perhaps add a utility function to do 
>>> that instead? like ttm_bo_purge()?
>>
>> We already have that. Just call ttm_bo_validate() without any place 
>> to put the buffer.
>>
>> See how ttm_bo_pipeline_gutting() is used.
>>
>> Christian.
>
> OK, so is that reentrant from the move() or swap_notify() callback.

That sounds like a design bug to me since you should never need to do this.

When you want to destroy the backing store of a buffer during eviction 
you should just do this by returning an empty placement from the 
evict_flags callback.

It is TTMs job to deal with the buffer placement and drivers are no 
longer allowed to mess with that.

Regards,
Christian.

>
> /Thomas
>
>
>
>>
>>>
>>> Thanks,
>>>
>>> Thomas
>>>
>>>
>>