From: Dexuan Cui decui@microsoft.com
db49200b1dad is backported from the mainline commit 5f1251a48c17 ("video: hyperv_fb: Fix the cache type when mapping the VRAM"), to v5.4.y and older stable branches, but unluckily db49200b1dad 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: db49200b1dad 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 d21987d709e8 ("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: db49200b1dad ("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 --- drivers/video/fbdev/hyperv_fb.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c index 81671272aa58f..1c6ae98710a01 100644 --- a/drivers/video/fbdev/hyperv_fb.c +++ b/drivers/video/fbdev/hyperv_fb.c @@ -704,11 +704,9 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info) }
/* - * Map the VRAM cacheable for performance. This is also required for - * VM Connect to display properly for ARM64 Linux VM, as the host also - * maps the VRAM cacheable. + * Map the VRAM cacheable for performance. */ - fb_virt = ioremap_cache(par->mem->start, screen_fb_size); + fb_virt = ioremap_wc(par->mem->start, screen_fb_size); if (!fb_virt) goto err2;