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

[Jocelyn Falempe <jfalempe@redhat.com>]

On 01/04/2025 19:57, Ville Syrjälä wrote:
> On Tue, Apr 01, 2025 at 08:47:50PM +0300, 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?
> 
> This also seems to have a bunch of race conditions:
> - what happens if the oops happens before the pages have
>    even been swapped in?
> - what happens if the oops happens before we've committed
>    the fb to the hardware?
> 

The panic handler tries to take the panic_lock from the 
device->mode_config, which should ensure we're not in the middle of a 
page swap.

https://elixir.bootlin.com/linux/v6.14-rc6/source/include/drm/drm_panic.h#L70

https://elixir.bootlin.com/linux/v6.14-rc6/source/include/drm/drm_mode_config.h#L500

If the lock is already taken when the panic handler run, it will skip 
this device, and won't draw the panic screen on it.

Best regards,

-- 

Jocelyn

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