Re: [PATCH] drm/i915/dmabuf: Flush the cache in vmap

[Jocelyn Falempe <jfalempe@redhat.com>]

On 24/10/2025 17:55, Tvrtko Ursulin wrote:
> 
> On 24/10/2025 16:18, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 24.10.25 um 15:33 schrieb Jocelyn Falempe:
>>> On 24/10/2025 14:40, Thomas Zimmermann wrote:
>>>> Hi
>>>>
>>>> Am 24.10.25 um 13:53 schrieb Tvrtko Ursulin:
>>>>>
>>>>> On 24/10/2025 12:04, Jocelyn Falempe wrote:
>>>>>> On a lenovo se100 server, when using i915 GPU for rendering, and the
>>>>>> ast driver for display, the graphic output is corrupted, and almost
>>>>>> unusable.
>>>>>>
>>>>>> Adding a clflush call in the vmap function fixes this issue
>>>>>> completely.
>>>>>
>>>>> AST is importing i915 allocated buffer in this use case, or how 
>>>>> exactly is the relationship?
>>>>>
>>>>> Wondering if some path is not calling dma_buf_begin/end_cpu_access().
>>>>
>>>> Yes, ast doesn't call begin/end_cpu_access in [1].
>>>>
>>>> Jocelyn, if that fixes the issue, feel free to send me a patch for 
>>>> review.
>>>>
>>>> [1] https://elixir.bootlin.com/linux/v6.17.4/source/drivers/gpu/drm/ 
>>>> ast/ ast_mode.c
>>>
>>> I tried the following patch, but that doesn't fix the graphical issue:
>>>
>>> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ 
>>> ast_mode.c
>>> index b4e8edc7c767..e50f95a4c8a9 100644
>>> --- a/drivers/gpu/drm/ast/ast_mode.c
>>> +++ b/drivers/gpu/drm/ast/ast_mode.c
>>> @@ -564,6 +564,7 @@ static void 
>>> ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
>>>         struct drm_crtc_state *crtc_state = 
>>> drm_atomic_get_new_crtc_state(state, crtc);
>>>         struct drm_rect damage;
>>>         struct drm_atomic_helper_damage_iter iter;
>>> +       int ret;
>>>
>>>         if (!old_fb || (fb->format != old_fb->format) || crtc_state- 
>>> >mode_changed) {
>>>                 struct ast_crtc_state *ast_crtc_state = 
>>> to_ast_crtc_state(crtc_state);
>>> @@ -572,11 +573,16 @@ static void 
>>> ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
>>>                 ast_set_vbios_color_reg(ast, fb->format, 
>>> ast_crtc_state->vmode);
>>>         }
>>>
>>> +       ret = drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE);
>>> +       pr_info("AST begin_cpu_access %d\n", ret);
>>
>> Presumably, you end up in [1]. I cannot find the cflush there or in 
>> [2]. Maybe you need to add this call somewhere in there, similar to 
>> [3]. Just guessing.
> 
> Near [2] clflush can happen at [4] *if* the driver thinks it is needed. 
> Most GPUs are cache coherent so mostly it isn't. But if this is a 
> Meteorlake machine (when I google Lenovo se100 it makes me think so?) 
> then the userspace has some responsibility to manage things since there 
> it is only 1-way coherency. Or userspace could have even told the driver 
> to stay off in which case it then needs to manage everything. From the 
> top of my head I am not sure how exactly this used to work, or how it is 
> supposed to interact with exported buffers.
> 
> If this is indeed on Meteorlake, maybe Joonas or Rodrigo remember better 
> how the special 1-way coherency is supposed to be managed there?

I've made an experiment, and if I add:

* a calls to drm_gem_fb_begin_cpu_access() in the ast driver.
* and in i915_gem_domain.c flush_write_domain():
         case I915_GEM_DOMAIN_RENDER:
+               i915_gem_clflush_object(obj, I915_CLFLUSH_SYNC | 
I915_CLFLUSH_FORCE);

Then that fixes the issue too.

So I think there are two things to fix:
  * The missing call to drm_gem_fb_begin_cpu_access() in ast.
  * The missing cache flush in i915 for the Arrowlake iGPU (but probably 
not the way I've done it).

Regards,

-- 

Jocelyn

> 
> Regards,
> 
> Tvrtko
> 
> [4] https://elixir.bootlin.com/linux/v6.17.4/source/drivers/gpu/drm/ 
> i915/gem/i915_gem_domain.c#L510
> 
>> [1] https://elixir.bootlin.com/linux/v6.17.4/source/drivers/gpu/drm/ 
>> i915/gem/i915_gem_dmabuf.c#L117
>> [2] https://elixir.bootlin.com/linux/v6.17.4/source/drivers/gpu/drm/ 
>> i915/gem/i915_gem_domain.c#L493
>> [3] https://elixir.bootlin.com/linux/v6.17.4/source/drivers/gpu/drm/ 
>> i915/gem/i915_gem_object.c#L509
>>
>>> +
>>>         drm_atomic_helper_damage_iter_init(&iter, old_plane_state, 
>>> plane_state);
>>>         drm_atomic_for_each_plane_damage(&iter, &damage) {
>>>                 ast_handle_damage(ast_plane, shadow_plane_state- 
>>> >data, fb, &damage);
>>>         }
>>>
>>> +       drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
>>> +
>>>         /*
>>>          * Some BMCs stop scanning out the video signal after the driver
>>>          * reprogrammed the offset. This stalls display output for 
>>> several
>>>
>>>
>>>
>>> Best regards,
>>>
>>
>