[Intel-gfx] [PATCH] drm/i915: Do NOT skip the first 4k of stolen memory for pre-allocated buffers

[Chris Wilson]

Quoting Hans de Goede (2018-03-30 13:27:15)
> Before this commit the WaSkipStolenMemoryFirstPage workaround code was
> skipping the first 4k by passing 4096 as start of the address range passed
> to drm_mm_init(). This means that calling drm_mm_reserve_node() to try and
> reserve the firmware framebuffer so that we can inherit it would always
> fail, as the firmware framebuffer starts at address 0.
> 
> Commit d43537610470 ("drm/i915: skip the first 4k of stolen memory on
> everything >= gen8") says in its commit message: "This is confirmed to fix
> Skylake screen flickering issues (probably caused by the fact that we
> initialized a ring in the first page of stolen, but I didn't 100% confirm
> this theory)."
> 
> Which suggests that it is safe to use the first page for a linear
> framebuffer as the firmware is doing.
> 
> This commit always passes 0 as start to drm_mm_init() and works around
> WaSkipStolenMemoryFirstPage in i915_gem_stolen_insert_node_in_range()
> by insuring the start address passed by to drm_mm_insert_node_in_range()
> is always 4k or more. All entry points to i915_gem_stolen.c go through
> i915_gem_stolen_insert_node_in_range(), so that any newly allocated
> objects such as ring-buffers will not be allocated in the first 4k.
> 
> The one exception is i915_gem_object_create_stolen_for_preallocated()
> which directly calls drm_mm_reserve_node() which now will be able to
> use the first 4k.
> 
> This fixes the i915 driver no longer being able to inherit the firmware
> framebuffer on gen8+, which fixes the video output changing from the
> vendor logo to a black screen as soon as the i915 driver is loaded
> (on systems without fbcon).

We've been told by the HW guys not to use the first page. (That's my
understanding from every time this gets questioned.)
-Chris