Re: [PATCH v6 4/8] drm/i915/gem: Add i915_gem_object_panic_map()

[Jocelyn Falempe <jfalempe@redhat.com>]

On 01/04/2025 19:47, Ville Syrjälä wrote:
> On Tue, Apr 01, 2025 at 02:51:10PM +0200, Jocelyn Falempe wrote:
>> Prepare the work for drm_panic support. This is used to map the
>> current framebuffer, so the CPU can overwrite it with the panic
>> message.
>>
>> Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
>> ---
>>
>> v5:
>>   * Use iosys_map for intel_bo_panic_map().
>>
>>   drivers/gpu/drm/i915/display/intel_bo.c    |  5 ++++
>>   drivers/gpu/drm/i915/display/intel_bo.h    |  1 +
>>   drivers/gpu/drm/i915/gem/i915_gem_object.h |  2 ++
>>   drivers/gpu/drm/i915/gem/i915_gem_pages.c  | 29 ++++++++++++++++++++++
>>   drivers/gpu/drm/xe/display/intel_bo.c      | 10 ++++++++
>>   5 files changed, 47 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_bo.c b/drivers/gpu/drm/i915/display/intel_bo.c
>> index fbd16d7b58d9..ac904e9ec7d5 100644
>> --- a/drivers/gpu/drm/i915/display/intel_bo.c
>> +++ b/drivers/gpu/drm/i915/display/intel_bo.c
>> @@ -57,3 +57,8 @@ void intel_bo_describe(struct seq_file *m, struct drm_gem_object *obj)
>>   {
>>   	i915_debugfs_describe_obj(m, to_intel_bo(obj));
>>   }
>> +
>> +void intel_bo_panic_map(struct drm_gem_object *obj, struct iosys_map *map)
>> +{
>> +	i915_gem_object_panic_map(to_intel_bo(obj), map);
>> +}
>> diff --git a/drivers/gpu/drm/i915/display/intel_bo.h b/drivers/gpu/drm/i915/display/intel_bo.h
>> index ea7a2253aaa5..5b6c63d99786 100644
>> --- a/drivers/gpu/drm/i915/display/intel_bo.h
>> +++ b/drivers/gpu/drm/i915/display/intel_bo.h
>> @@ -23,5 +23,6 @@ struct intel_frontbuffer *intel_bo_set_frontbuffer(struct drm_gem_object *obj,
>>   						   struct intel_frontbuffer *front);
>>   
>>   void intel_bo_describe(struct seq_file *m, struct drm_gem_object *obj);
>> +void intel_bo_panic_map(struct drm_gem_object *obj, struct iosys_map *map);
>>   
>>   #endif /* __INTEL_BO__ */
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>> index a5f34542135c..b16092707ea5 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>> @@ -692,6 +692,8 @@ i915_gem_object_unpin_pages(struct drm_i915_gem_object *obj)
>>   int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj);
>>   int i915_gem_object_truncate(struct drm_i915_gem_object *obj);
>>   
>> +void i915_gem_object_panic_map(struct drm_i915_gem_object *obj, struct iosys_map *map);
>> +
>>   /**
>>    * i915_gem_object_pin_map - return a contiguous mapping of the entire object
>>    * @obj: the object to map into kernel address space
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_pages.c b/drivers/gpu/drm/i915/gem/i915_gem_pages.c
>> index 8780aa243105..718bea6474d7 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_pages.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_pages.c
>> @@ -355,6 +355,35 @@ static void *i915_gem_object_map_pfn(struct drm_i915_gem_object *obj,
>>   	return vaddr ?: ERR_PTR(-ENOMEM);
>>   }
>>   
>> +/* Map the current framebuffer for CPU access. Called from panic handler, so no
>> + * need to pin or cleanup.
>> + */
>> +void i915_gem_object_panic_map(struct drm_i915_gem_object *obj, struct iosys_map *map)
>> +{
>> +	enum i915_map_type has_type;
>> +	void *ptr;
>> +
>> +	ptr = page_unpack_bits(obj->mm.mapping, &has_type);
>> +
>> +
>> +	if (!ptr) {
>> +		if (i915_gem_object_has_struct_page(obj))
>> +			ptr = i915_gem_object_map_page(obj, I915_MAP_WB);
>> +		else
>> +			ptr = i915_gem_object_map_pfn(obj, I915_MAP_WB);
> 
> WB mapping would require clflushing to make it to the display.
> Is that being done somewhere?

Yes, it's done in intel_panic_flush() in patch 5, otherwise the panic 
screen is not displayed.

> 
>> +
>> +		if (IS_ERR(ptr))
>> +			return;
> 
> What happens when the mapping fails?

In intel_get_scanout_buffer(), the iosys_map is cleared before calling 
this function. Then it checks iosys_map_is_null(), and returns an error 
if it is.
I can add a comment, or I can change the function type to return an int, 
  that would probably be cleaner.

> 
>> +
>> +		obj->mm.mapping = page_pack_bits(ptr, I915_MAP_WB);
>> +	}
>> +
>> +	if (i915_gem_object_has_iomem(obj))
>> +		iosys_map_set_vaddr_iomem(map, (void __iomem *) ptr);
>> +	else
>> +		iosys_map_set_vaddr(map, ptr);
>> +}
>> +
>>   /* get, pin, and map the pages of the object into kernel space */
>>   void *i915_gem_object_pin_map(struct drm_i915_gem_object *obj,
>>   			      enum i915_map_type type)
>> diff --git a/drivers/gpu/drm/xe/display/intel_bo.c b/drivers/gpu/drm/xe/display/intel_bo.c
>> index 27437c22bd70..c68166a64336 100644
>> --- a/drivers/gpu/drm/xe/display/intel_bo.c
>> +++ b/drivers/gpu/drm/xe/display/intel_bo.c
>> @@ -59,3 +59,13 @@ void intel_bo_describe(struct seq_file *m, struct drm_gem_object *obj)
>>   {
>>   	/* FIXME */
>>   }
>> +
>> +void intel_bo_panic_map(struct drm_gem_object *obj, struct iosys_map *map)
>> +{
>> +	struct xe_bo *bo = gem_to_xe_bo(obj);
>> +	int ret;
>> +
>> +	ret = ttm_bo_vmap(&bo->ttm, map);
>> +	if (ret)
>> +		iosys_map_clear(map);
>> +}
>> -- 
>> 2.49.0
>