Re: [PATCH v2 06/15] drm/i915/ttm: Embed a ttm buffer object in the i915 gem object

From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v2 06/15] drm/i915/ttm: Embed a ttm buffer object in the i915 gem object
Date: Tue, 18 May 2021 13:44:41 +0200	[thread overview]
Message-ID: <ce76a4ba-36a2-fb8c-bf47-366e8b030b29@linux.intel.com> (raw)
In-Reply-To: <20210518082701.997251-7-thomas.hellstrom@linux.intel.com>

Op 18-05-2021 om 10:26 schreef Thomas Hellström:
> Embed a struct ttm_buffer_object into the i915 gem object, making sure
> we alias the gem object part. It's a bit unfortunate that the
> struct ttm_buffer_ojbect embeds a gem object since we otherwise could
> make the TTM part private to the TTM backend, and use the usual
> i915 gem object for the other backends.
> To make this a bit more storage efficient for the other backends,
> we'd have to use a pointer for the gem object which would require
> a lot of changes in the driver. We postpone that for later.
>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_object.c       |  7 +++++++
>  drivers/gpu/drm/i915/gem/i915_gem_object_types.h | 12 +++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index abadf0994ad0..c8953e3f5c70 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> @@ -62,6 +62,13 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
>  			  const struct drm_i915_gem_object_ops *ops,
>  			  struct lock_class_key *key, unsigned flags)
>  {
> +	/*
> +	 * A gem object is embedded both in a struct ttm_buffer_object :/ and
> +	 * in a drm_i915_gem_object. Make sure they are aliased.
> +	 */
> +	BUILD_BUG_ON(offsetof(typeof(*obj), base) !=
> +		     offsetof(typeof(*obj), __do_not_access.base));
> +
>  	spin_lock_init(&obj->vma.lock);
>  	INIT_LIST_HEAD(&obj->vma.list);
>  
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> index dbd7fffe956e..98f69d8fd37d 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> @@ -10,6 +10,7 @@
>  #include <linux/mmu_notifier.h>
>  
>  #include <drm/drm_gem.h>
> +#include <drm/ttm/ttm_bo_api.h>
>  #include <uapi/drm/i915_drm.h>
>  
>  #include "i915_active.h"
> @@ -99,7 +100,16 @@ struct i915_gem_object_page_iter {
>  };
>  
>  struct drm_i915_gem_object {
> -	struct drm_gem_object base;
> +	/*
> +	 * We might have reason to revisit the below since it wastes
> +	 * a lot of space for non-ttm gem objects.
> +	 * In any case, always use the accessors for the ttm_buffer_object
> +	 * when accessing it.
> +	 */
> +	union {
> +		struct drm_gem_object base;
> +		struct ttm_buffer_object __do_not_access;
> +	};
>  
>  	const struct drm_i915_gem_object_ops *ops;
>  

Considering Dave did roughly the same in his patches, I don't think there's a better way to do this.

Although he just wrapped base.base using sizes. This works too. It probably needs someone else's r-b too, to ensure this is allowed.

Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>