[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 13:37:40)
> Hi,
> 
> On 30-03-18 14:30, Chris Wilson wrote:
> > 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.)
> 
> Yet the GOP is happily using the first page. I think we may need to make
> a difference here between the GPU not using the first page and the
> display engine/pipeline not using the first page. Note that my patch
> only influences the inheriting of the initial framebuffer as allocated
> by the GOP. It does not influence any other allocations from the
> reserved range, those will still all avoid the first page.
> 
> Without this patch fastboot / flickerfree support is essentially broken
> on any gen8+ hardware given that one of the goals of atomic is to be
> able to do flickerfree transitions I think that this warrants a closer
> look then just simply saying never use the first page.

The concern is what else (i.e. nothing that we allocated ourselves) that
may be in the first page...
-Chris