drm/i915: Do NOT skip the first 4k of stolen memory for pre-allocated buffers v2

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 d4353761 ("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 (see note below).

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).

Some notes about the mapping of the BIOS framebuffer:

v1 led to some discussion if the assumption of the intel_display.c code
that the firmware framebuffer is a linear mapping of the stolen memory
starting at offset 0 is still correct, because that would mean that the
GOP does not implement the WaSkipStolenMemoryFirstPage workaround.

To verify this the following code was added at the end of
i915_gem_object_create_stolen_for_preallocated() :

pr_err("first ggtt entry before bind: 0x%016llx\n",
       readq(dev_priv->ggtt.gsm));
ret = i915_vma_bind(vma,
            HAS_LLC(dev_priv) ? I915_CACHE_LLC : I915_CACHE_NONE,
            PIN_UPDATE);
pr_err("i915_vma_bind ret %d\n", ret);
pr_err("first ggtt entry after bind: 0x%016llx\n",
       readq(dev_priv->ggtt.gsm));

Which prints the mapping of the first page, then does a vma_bind() to
force update the mapping with our linear view of the framebuffer and
then prints the mapping of the first page again.

On an Asrock B150M Pro4S/D3 mainboard with i5-6500 CPU this prints:

[    1.651141] first ggtt entry before bind: 0x0000000078c00001
[    1.651151] i915_vma_bind ret 0
[    1.651152] first ggtt entry after bind: 0x0000000078c00083

And "sudo cat /proc/iomem | grep Stolen" gives:
  78c00000-88bfffff : Graphics Stolen Memory

There are no visual changes with this patch (BIOS vendor logo still
stays in place when we inherit the BIOS framebuffer), so the vma_bind()
does not impact which memory is being scanned out.

The address of the first ggtt entry matches with the start of stolen
and the i915_vma_bind call only changes the first gtt entry's flags,
or-ing in _PAGE_RW (BIT(1)) and PPAT_CACHED (BIT(7)), which perfectly
matches what we would expect based on gen8_pte_encode()'s behavior.

So it seems that the GOP indeed does NOT implement the wa and the i915's
code assuming a linear mapping at the start of stolen for the BIOS fb
still holds true for gen8+.

I've also tested this on a Cherry Trail based device (a GPD Win)
with identical results (the flags are 0x1b after the vma_bind
on CHT, which matches with I915_CACHE_NONE).

Changed in v2: No code changes, extended the commit message with the
verification that the intel_display.c BIOS framebuffer mapping is still
correct.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180420095933.16442-1-hdegoede@redhat.com