Re: [PATCH 6/9] drm/i915/uapi: implement object placement extension

[Kenneth Graunke <kenneth@whitecape.org>]

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On Monday, April 26, 2021 2:38:58 AM PDT Matthew Auld wrote:
> Add new extension to support setting an immutable-priority-list of
> potential placements, at creation time.
> 
> If we use the normal gem_create or gem_create_ext without the
> extensions/placements then we still get the old behaviour with only
> placing the object in system memory.
> 
> v2(Daniel & Jason):
>     - Add a bunch of kernel-doc
>     - Simplify design for placements extension
> 
> Testcase: igt/gem_create/create-ext-placement-sanity-check
> Testcase: igt/gem_create/create-ext-placement-each
> Testcase: igt/gem_create/create-ext-placement-all
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Signed-off-by: CQ Tang <cq.tang@intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Kenneth Graunke <kenneth@whitecape.org>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> Cc: Dave Airlie <airlied@gmail.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: mesa-dev@lists.freedesktop.org
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_create.c    | 215 ++++++++++++++++--
>  drivers/gpu/drm/i915/gem/i915_gem_object.c    |   3 +
>  .../gpu/drm/i915/gem/i915_gem_object_types.h  |   6 +
>  .../drm/i915/gem/selftests/i915_gem_mman.c    |  26 +++
>  drivers/gpu/drm/i915/intel_memory_region.c    |  16 ++
>  drivers/gpu/drm/i915/intel_memory_region.h    |   4 +
>  include/uapi/drm/i915_drm.h                   |  62 +++++
>  7 files changed, 315 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_create.c b/drivers/gpu/drm/i915/gem/i915_gem_create.c
> index 90e9eb6601b5..895f1666a8d3 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_create.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_create.c
> @@ -4,12 +4,47 @@
>   */
>  
>  #include "gem/i915_gem_ioctls.h"
> +#include "gem/i915_gem_lmem.h"
>  #include "gem/i915_gem_region.h"
>  
>  #include "i915_drv.h"
>  #include "i915_trace.h"
>  #include "i915_user_extensions.h"
>  
> +static u32 object_max_page_size(struct drm_i915_gem_object *obj)
> +{
> +	u32 max_page_size = 0;
> +	int i;
> +
> +	for (i = 0; i < obj->mm.n_placements; i++) {
> +		struct intel_memory_region *mr = obj->mm.placements[i];
> +
> +		GEM_BUG_ON(!is_power_of_2(mr->min_page_size));
> +		max_page_size = max_t(u32, max_page_size, mr->min_page_size);
> +	}
> +
> +	GEM_BUG_ON(!max_page_size);
> +	return max_page_size;
> +}
> +
> +static void object_set_placements(struct drm_i915_gem_object *obj,
> +				  struct intel_memory_region **placements,
> +				  unsigned int n_placements)
> +{
> +	GEM_BUG_ON(!n_placements);
> +
> +	if (n_placements == 1) {
> +		struct intel_memory_region *mr = placements[0];
> +		struct drm_i915_private *i915 = mr->i915;
> +
> +		obj->mm.placements = &i915->mm.regions[mr->id];
> +		obj->mm.n_placements = 1;
> +	} else {
> +		obj->mm.placements = placements;
> +		obj->mm.n_placements = n_placements;
> +	}
> +}
> +

I found this helper function rather odd looking at first.  In the
general case, it simply sets fields based on the parameters...but in
the n == 1 case, it goes and uses something else as the array.

On further inspection, this makes sense: normally, we have an array
of multiple placements in priority order.  That array is (essentially)
malloc'd.  But if there's only 1 item, having a malloc'd array of 1
thing is pretty silly.  We can just point at it directly.  Which means
the callers can kfree the array, and the object destructor should not.

Maybe a comment saying

   /* 
    * For the common case of one memory region, skip storing an
    * allocated array and just point at the region directly.
    */

would be helpful?

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