Re: [PATCH v3 2/2] drm/i915/uapi: expose GTT alignment

["Das, Nirmoy" <nirmoy.das@linux.intel.com>]

On 10/4/2022 1:49 PM, Matthew Auld wrote:
> On some platforms we potentially have different alignment restrictions
> depending on the memory type. We also now have different alignment
> restrictions for the same region across different kernel versions.
> Extend the region query to return the minimum required GTT alignment.
>
> Testcase: igt@gem_create@create-ext-placement-alignment
> Testcase: igt@i915_query@query-regions-sanity-check
> Suggested-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Michal Mrozek <michal.mrozek@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Stuart Summers <stuart.summers@intel.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Yang A Shi <yang.a.shi@intel.com>
> Cc: Nirmoy Das <nirmoy.das@intel.com>

Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>


> Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_query.c |  1 +
>   include/uapi/drm/i915_drm.h       | 29 +++++++++++++++++++++++++++--
>   2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_query.c b/drivers/gpu/drm/i915/i915_query.c
> index 6ec9c9fb7b0d..111377f210ed 100644
> --- a/drivers/gpu/drm/i915/i915_query.c
> +++ b/drivers/gpu/drm/i915/i915_query.c
> @@ -498,6 +498,7 @@ static int query_memregion_info(struct drm_i915_private *i915,
>   		info.region.memory_class = mr->type;
>   		info.region.memory_instance = mr->instance;
>   		info.probed_size = mr->total;
> +		info.gtt_alignment = mr->min_page_size;
>   
>   		if (mr->type == INTEL_MEMORY_LOCAL)
>   			info.probed_cpu_visible_size = mr->io_size;
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 08d69e36fb66..2e613109356b 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -3346,8 +3346,33 @@ struct drm_i915_memory_region_info {
>   	/** @region: The class:instance pair encoding */
>   	struct drm_i915_gem_memory_class_instance region;
>   
> -	/** @rsvd0: MBZ */
> -	__u32 rsvd0;
> +	union {
> +		/** @rsvd0: MBZ */
> +		__u32 rsvd0;
> +		/**
> +		 * @gtt_alignment:
> +		 *
> +		 * The minimum required GTT alignment for this type of memory.
> +		 * When allocating a GTT address it must be aligned to this
> +		 * value or larger. On some platforms the kernel might opt to
> +		 * using 64K pages for I915_MEMORY_CLASS_DEVICE, where 64K GTT
> +		 * pages can then be used if we also use 64K GTT alignment.
> +		 *
> +		 * NOTE: If this is zero then this must be an older
> +		 * kernel which lacks support for this field.
> +		 *
> +		 * Side note: For larger objects (especially for
> +		 * I915_MEMORY_CLASS_DEVICE), like 2M+ in size, userspace should
> +		 * consider potentially bumping the GTT alignment to say 2M,
> +		 * which could potentially increase the likelihood of the kernel
> +		 * being able to utilise 2M GTT pages underneath, if the layout
> +		 * of the physical pages allows it.  On some configurations we
> +		 * can then also use a more efficient page-table layout, if we
> +		 * can't use the more desirable 2M GTT page, so long as we know
> +		 * that the entire page-table will be used by this object.
> +		 */
> +		__u32 gtt_alignment;
> +	};
>   
>   	/**
>   	 * @probed_size: Memory probed by the driver