Re: [PATCH] drm/xe/display: skip FORCE_WC and vm_bound check for external dma-bufs

[Matthew Auld <matthew.auld@intel.com>]

On 25/06/2026 16:35, Maarten Lankhorst wrote:
> Hello,
> 
> On 6/12/26 19:05, Matthew Auld wrote:
>> Currently, xe_display_bo_framebuffer_init() unconditionally attempts to
>> apply XE_BO_FLAG_FORCE_WC to the buffer and rejects the FB creation with
>> -EINVAL if the BO is already VM_BINDed.
>>
>> However, for imported dma-bufs (ttm_bo_type_sg), this check doesn't seem
>> to make much sense since CPU caching policy is entirely controlled by
>> the exporter. Plus there is no place to set this flag, in the first
>> place. Also this is not rejected if not yet vm_binded, but that seems
>> arbitrary since setting or not setting FORCE_WC should a noop either
>> way, at this stage, and whether it is currently VM_BINDed makes no
>> difference.
>>
>> Currently if we run an app and offload rendering to an external dGPU,
>> like NV or another xe device, the dma-buf passed back to the compositor
>> (igpu) will be an actual external import from xe pov, and it will be
>> missing FORCE_WC, and if the compositor side did a VM_BIND before
>> turning into it into an fb the whole thing gets rejected.
>>
>> So it looks like we either need to reject outright, no matter what, or
>> this usecase is valid and we need to loosen the restriction for sg
>> buffers.  Proposing here to loosen the restriction.
>>
>> Assisted-by: Gemini:gemini-3.1-pro-preview
>> Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/7919
>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Cc: Maarten Lankhorst <dev@lankhorst.se>
>> ---
>>   drivers/gpu/drm/xe/display/xe_display_bo.c | 3 ++-
>>   drivers/gpu/drm/xe/display/xe_fb_pin.c     | 3 ++-
>>   2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/xe/display/xe_display_bo.c b/drivers/gpu/drm/xe/display/xe_display_bo.c
>> index 7fbac223b097..8953da0136dc 100644
>> --- a/drivers/gpu/drm/xe/display/xe_display_bo.c
>> +++ b/drivers/gpu/drm/xe/display/xe_display_bo.c
>> @@ -48,7 +48,8 @@ static int xe_display_bo_framebuffer_init(struct drm_gem_object *obj,
>>   	if (ret)
>>   		goto err;
>>   
>> -	if (!(bo->flags & XE_BO_FLAG_FORCE_WC)) {
>> +	if (!(bo->flags & XE_BO_FLAG_FORCE_WC) &&
>> +	    bo->ttm.type != ttm_bo_type_sg) {
>>   		/*
>>   		 * XE_BO_FLAG_FORCE_WC should ideally be set at creation, or is
>>   		 * automatically set when creating FB. We cannot change caching
>> diff --git a/drivers/gpu/drm/xe/display/xe_fb_pin.c b/drivers/gpu/drm/xe/display/xe_fb_pin.c
>> index f93c98bec5b5..5f4a0cd8deca 100644
>> --- a/drivers/gpu/drm/xe/display/xe_fb_pin.c
>> +++ b/drivers/gpu/drm/xe/display/xe_fb_pin.c
>> @@ -331,7 +331,8 @@ static struct i915_vma *__xe_pin_fb_vma(struct drm_gem_object *obj, bool is_dpt,
>>   	int ret = 0;
>>   
>>   	/* We reject creating !SCANOUT fb's, so this is weird.. */
>> -	drm_WARN_ON(bo->ttm.base.dev, !(bo->flags & XE_BO_FLAG_FORCE_WC));
>> +	drm_WARN_ON(bo->ttm.base.dev, !(bo->flags & XE_BO_FLAG_FORCE_WC) &&
>> +		    bo->ttm.type != ttm_bo_type_sg);
>>   
>>   	if (!vma)
>>   		return ERR_PTR(-ENODEV);
> 
> Yeah we cannot actually set the force flag on imported bo's for mmap coherency, but mmap on an imported dma-buf should be forbidden anyway, so that is fine.
> 
> Best we can hope for is that dirtyfb is called correctly.
> 
> Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>

Thanks!

>