[PATCH 3/3] drm/panthor: Relax the check on the tiler chunk size

[Steven Price]

On 25/04/2024 08:18, Boris Brezillon wrote:
> The field used to store the chunk size if 12 bits wide, and the encoding
NIT:                                     ^^ is

> is chunk_size = chunk_header.chunk_size << 12, which gives us a
> theoretical [4k:8M] range. This range is further limited by
> implementation constraints, but those shouldn't be enforced kernel side.
> 
> Fixes: 9cca48fa4f89 ("drm/panthor: Add the heap logical block")
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>

There's also a kerneldoc comment above panthor_heap_create that needs
updating too:

> /**
>  * panthor_heap_create() - Create a heap context
>  * @pool: Pool to instantiate the heap context from.
>  * @initial_chunk_count: Number of chunk allocated at initialization time.
>  * Must be at least 1.
>  * @chunk_size: The size of each chunk. Must be a power of two between 256k
>  * and 2M.

I'm also a little unsure on whether this is the right change. The
"implementation defined" min/max in the hardware docs say 128KiB to
8MiB. I'm not convinced it makes sense to increase the range beyond that.

As far as I'm aware the "must be a power of 2" isn't actually a
requirement (it's certainly not a requirement of the storage format) so
the kernel is already being more restrictive than necessary.

It seems like a good idea to keep the uAPI requirements stricter than
necessary and relax them in the future if we have a good reason (e.g.
new hardware supports a wider range). But matching the existing hardware
range of 128KB-8MB would obviously make sense now.

Steve

> ---
>  drivers/gpu/drm/panthor/panthor_heap.c | 2 +-
>  include/uapi/drm/panthor_drm.h         | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_heap.c b/drivers/gpu/drm/panthor/panthor_heap.c
> index 8728c9bb76e4..146ea2f57764 100644
> --- a/drivers/gpu/drm/panthor/panthor_heap.c
> +++ b/drivers/gpu/drm/panthor/panthor_heap.c
> @@ -285,7 +285,7 @@ int panthor_heap_create(struct panthor_heap_pool *pool,
>  		return -EINVAL;
>  
>  	if (hweight32(chunk_size) != 1 ||
> -	    chunk_size < SZ_256K || chunk_size > SZ_2M)
> +	    chunk_size < SZ_4K || chunk_size > SZ_8M)
>  		return -EINVAL;
>  
>  	down_read(&pool->lock);
> diff --git a/include/uapi/drm/panthor_drm.h b/include/uapi/drm/panthor_drm.h
> index 5db80a0682d5..80c0716361b9 100644
> --- a/include/uapi/drm/panthor_drm.h
> +++ b/include/uapi/drm/panthor_drm.h
> @@ -898,7 +898,7 @@ struct drm_panthor_tiler_heap_create {
>  	/** @initial_chunk_count: Initial number of chunks to allocate. Must be at least one. */
>  	__u32 initial_chunk_count;
>  
> -	/** @chunk_size: Chunk size. Must be a power of two at least 256KB large. */
> +	/** @chunk_size: Chunk size. Must be a power of two and lie in the [4k:8M] range. */
>  	__u32 chunk_size;
>  
>  	/**