[PATCH v2 6/6] drm/ttm: Clarify that the TTM_PL_SYSTEM buffers need to stay idle

[Christian König]

Am 05.11.21 um 20:38 schrieb Zack Rusin:
> TTM was designed to allow TTM_PL_SYSTEM buffer to be fenced but over
> the years the code that was meant to handle it was broken and new
> changes can not deal with buffers which have been placed in TTM_PL_SYSTEM
> buf do not remain idle.
> CPU buffers which need to be fenced and shared with accelerators should
> be placed in driver specific placements that can explicitly handle
> CPU/accelerator buffer fencing.
> Currently, apart, from things silently failing nothing is enforcing
> that requirement which means that it's easy for drivers and new
> developers to get this wrong. To avoid the confusion we can document
> this requirement and clarify the solution.
>
> This came up during a discussion on dri-devel:
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F232f45e9-8748-1243-09bf-56763e6668b3%40amd.com&data=04%7C01%7Cchristian.koenig%40amd.com%7C4f4ed0ab73894a482abd08d9a0942b98%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637717380521122147%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=1lGhEIESDwizXnC7APtlhGws8miB4xeh%2FsTewsEPfBY%3D&reserved=0
>
> Signed-off-by: Zack Rusin <zackr at vmware.com>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> ---
>   include/drm/ttm/ttm_placement.h | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/include/drm/ttm/ttm_placement.h b/include/drm/ttm/ttm_placement.h
> index 76d1b9119a2b..89dfb58ff199 100644
> --- a/include/drm/ttm/ttm_placement.h
> +++ b/include/drm/ttm/ttm_placement.h
> @@ -35,6 +35,16 @@
>   
>   /*
>    * Memory regions for data placement.
> + *
> + * Due to the fact that TTM_PL_SYSTEM BO's can be accessed by the hardware
> + * and are not directly evictable they're handled slightly differently
> + * from other placements. The most important and driver visible side-effect
> + * of that is that TTM_PL_SYSTEM BO's are not allowed to be fenced and have
> + * to remain idle. For BO's which reside in system memory but for which
> + * the accelerator requires direct access (i.e. their usage needs to be
> + * synchronized between the CPU and accelerator via fences) a new, driver
> + * private placement should be introduced that can handle such scenarios.
> + *

That we don't have fences on them is just the tip on the iceberg and 
mostly nice to have.

The major problem is that TTM_PL_SYSTEM are considered under TTMs 
control and swapped out whenever TTMs thinks it is a good idea.

That needs to be handled manually in the driver if a TTM_PL_SYSTEM is 
ever considered a valid placement.

Christian.

>    */
>   
>   #define TTM_PL_SYSTEM           0