Re: [Intel-gfx] [PATCH] drm/i915: Pin the ifbdev for the info->system_base GGTT mmapping

[Lukas Wunner <lukas@wunner.de>]

Another observation that occurred to me only after sending away my
previous message (sorry):

On Thu, Oct 08, 2015 at 01:50:21PM -0700, Wayne Boyer wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
> 
> A long time ago (before 3.14) we relied on a permanent pinning of the
> ifbdev to lock the fb in place inside the GGTT. However, the
> introduction of stealing the BIOS framebuffer and reusing its address in
> the GGTT for the fbdev has muddied waters and we use an inherited fb.
> However, the inherited fb is only pinned whilst it is active and we no
> longer have an explicit pin for the info->system_base mmapping used by
> the fbdev. The result is that after some aperture pressure the fbdev may
> be evicted, but we continue to write the fbcon into the same GGTT
> address - overwriting anything else that may be put into that offset.
> The effect is most pronounced across suspend/resume as
> intel_fbdev_set_suspend() does a full clear over the whole scanout.

Since this only concerns the case when the fb was inherited from BIOS,
why don't you invoke i915_gem_obj_ggtt_pin() only in this particular case?
It's discernible from the prealloc variable.

I think then you also don't need to call i915_gem_object_ggtt_unpin() twice.


> 
> v2: rebased on latest nightly (Wayne)
> v3: changed i915_gem_object_ggtt_pin() to i915_gem_obj_ggtt_pin() based
> on Chris' review. (Wayne)
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: "Goel, Akash" <akash.goel@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
> Cc: stable@vger.kernel.org
> Reviewed-by: Deepak S <deepak.s@linux.intel.com>
> Signed-off-by: Wayne Boyer <wayne.boyer@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_fbdev.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
> index 6532912..0ad46521 100644
> --- a/drivers/gpu/drm/i915/intel_fbdev.c
> +++ b/drivers/gpu/drm/i915/intel_fbdev.c
> @@ -215,6 +215,16 @@ static int intelfb_create(struct drm_fb_helper *helper,
>  	obj = intel_fb->obj;
>  	size = obj->base.size;
>  
> +	/* The fb constructor will have already pinned us (or inherited a
> +	 * GGTT region from the BIOS) suitable for a scanout, so
> +	 * this should just be a no-op and increment the pin count for the
> +	 * fbdev mmapping. It does have a useful side-effect of validating
> +	 * the pin for fbdev's use via a GGTT mmapping.
> +	 */
> +	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE);
> +	if (ret)
> +		goto out_unlock;
> +
>  	info = drm_fb_helper_alloc_fbi(helper);
>  	if (IS_ERR(info)) {
>  		ret = PTR_ERR(info);
> @@ -274,6 +284,9 @@ static int intelfb_create(struct drm_fb_helper *helper,
>  out_destroy_fbi:
>  	drm_fb_helper_release_fbi(helper);
>  out_unpin:
> +	/* Once for info->screen_base mmaping... */
> +	i915_gem_object_ggtt_unpin(obj);
> +	/* ...and once for the intel_fb */
>  	i915_gem_object_ggtt_unpin(obj);
>  	drm_gem_object_unreference(&obj->base);
>  out_unlock:
> @@ -514,6 +527,8 @@ static const struct drm_fb_helper_funcs intel_fb_helper_funcs = {
>  static void intel_fbdev_destroy(struct drm_device *dev,
>  				struct intel_fbdev *ifbdev)
>  {
> +	/* Release the pinning for the info->screen_base mmaping. */
> +	i915_gem_object_ggtt_unpin(ifbdev->fb->obj);
>  
>  	drm_fb_helper_unregister_fbi(&ifbdev->helper);
>  	drm_fb_helper_release_fbi(&ifbdev->helper);
> -- 
> 1.9.1
> 
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