video: hyperv_fb: Fix the mmap() regression for v5.4.y and older

db49200b is backported from the mainline commit
5f1251a4 ("video: hyperv_fb: Fix the cache type when mapping the VRAM"),
to v5.4.y and older stable branches, but unluckily db49200b causes
mmap() to fail for /dev/fb0 due to EINVAL:

[ 5797.049560] x86/PAT: a.out:1910 map pfn expected mapping type
  uncached-minus for [mem 0xf8200000-0xf85cbfff], got write-back

This means the v5.4.y kernel detects an incompatibility issue about the
mapping type of the VRAM: db49200b changes to use Write-Back when
mapping the VRAM, while the mmap() syscall tries to use Uncached-minus.
That’s to say, the kernel thinks Uncached-minus is incompatible with
Write-Back: see drivers/video/fbdev/core/fbmem.c: fb_mmap() ->
vm_iomap_memory() -> io_remap_pfn_range() -> ... -> track_pfn_remap() ->
reserve_pfn_range().

Note: any v5.5 and newer kernel doesn't have the issue, because they
have commit
d21987d7 ("video: hyperv: hyperv_fb: Support deferred IO for Hyper-V frame buffer driver")
, and when the hyperv_fb driver has the deferred_io support,
fb_deferred_io_init() overrides info->fbops->fb_mmap with
fb_deferred_io_mmap(), which doesn’t check the mapping type
incompatibility. Note: since it's VRAM here, the checking is not really
necessary.

Fix the regression by ioremap_wc(), which uses Write-combining. The kernel
thinks it's compatible with Uncached-minus. The VRAM mappped by
ioremap_wc() is slightly slower than mapped by ioremap_cache(), but is
still significantly faster than by ioremap().

Change the comment accordingly. Linux VM on ARM64 Hyper-V is still not
working in the latest mainline yet, and when it works in future, the ARM64
support is unlikely to be backported to v5.4 and older, so using
ioremap_wc() in v5.4 and older should be ok.

Note: this fix is only targeted at the stable branches:
v5.4.y, v4.19.y, v4.14.y, v4.9.y and v4.4.y.

Fixes: db49200b ("video: hyperv_fb: Fix the cache type when mapping the VRAM")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>