Re: [Xen-devel][PATCH] drm/xen-front: Fix mmap attributes for display buffers

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

On 1/29/19 9:07 PM, Julien Grall wrote:
> Hi Oleksandr,
>
> On 1/29/19 3:04 PM, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>
>> When GEM backing storage is allocated those are normal pages,
>> so there is no point using pgprot_writecombine while mmaping.
>> This fixes mismatch of buffer pages' memory attributes between
>> the frontend and backend which may cause screen artifacts.
>>
>> Fixes: c575b7eeb89f ("drm/xen-front: Add support for Xen PV display 
>> frontend")
>>
>> Signed-off-by: Oleksandr Andrushchenko 
>> <oleksandr_andrushchenko@epam.com>
>> Suggested-by: Julien Grall <julien.grall@arm.com>
>> ---
>>   drivers/gpu/drm/xen/xen_drm_front_gem.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c 
>> b/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> index d303a2e17f5e..9d5c03d7668d 100644
>> --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
>> @@ -235,8 +235,7 @@ static int gem_mmap_obj(struct xen_gem_object 
>> *xen_obj,
>>       vma->vm_flags &= ~VM_PFNMAP;
>>       vma->vm_flags |= VM_MIXEDMAP;
>>       vma->vm_pgoff = 0;
>> -    vma->vm_page_prot =
>> - pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
>> +    vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
>
> The patch looks good to me. It would be worth expanding the comment a 
> bit before to explain that we overwrite vm_page_prot to use cacheable 
> attribute as required by the Xen ABI.
>
> With the comment updated:
>
> Acked-by: Julien Grall <julien.grall@arm.com>
>
> Cheers,
>
Applied to drm-misc-next