[PATCH v8 4/9] drm/ttm: Add ttm_bo_kmap_try_from_panic()

[Jocelyn Falempe]

On 06/06/2025 14:28, Christian König wrote:
> On 6/6/25 13:48, Jocelyn Falempe wrote:
>> If the ttm bo is backed by pages, then it's possible to safely kmap
>> one page at a time, using kmap_try_from_panic().
> 
> I strongly assume that we don't care about locking anything in this case, don't we?

Yes, normally it's called for the current framebuffer, so I assume it's 
properly allocated, and isn't growing/shrinking while being displayed.

> 
>> Unfortunately there is no way to do the same with ioremap, so it
>> only supports the kmap case.
> 
> Oh, there actually is on most modern systems.
> 
> At least on 64bit systems amdgpu maps the whole VRAM BAR into kernel address space on driver load.
> 
> So as long as you have a large BAR system you can trivially have access to the MMIO memory.

For amdgpu, I used the indirect MMIO access, so I didn't need to ioremap
https://elixir.bootlin.com/linux/v6.15/source/drivers/gpu/drm/amd/amdgpu/amdgpu_display.c#L1800

For the xe driver, I only tested on integrated GPU, using system RAM, so 
this first approach is good enough.
But I'm still interested to find a solution, is there a way to get the 
current io-mapping if it exists?


> 
>> This is needed for proper drm_panic support with xe driver.
>>
>> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
>> ---
>>
>> v8:
>>   * Added in v8
>>
>>   drivers/gpu/drm/ttm/ttm_bo_util.c | 27 +++++++++++++++++++++++++++
>>   include/drm/ttm/ttm_bo.h          |  1 +
>>   2 files changed, 28 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> index 15cab9bda17f..9c3f3b379c2a 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> @@ -377,6 +377,33 @@ static int ttm_bo_kmap_ttm(struct ttm_buffer_object *bo,
>>   	return (!map->virtual) ? -ENOMEM : 0;
>>   }
>>   
>> +/**
>> + *
>> + * ttm_bo_kmap_try_from_panic
>> + *
>> + * @bo: The buffer object
>> + * @page: The page to map
>> + *
>> + * Sets up a kernel virtual mapping using kmap_local_page_try_from_panic().
>> + * This can safely be called from the panic handler, if you make sure the bo
> 
> "This can *only* be called from the panic handler..."

Yes, I will fix that, it shouldn't be called for normal operations.

> 
> Apart from those open questions, looks sane to me.
> 
> Regards,
> Christian.
> 
>> + * is the one being displayed, so is properly allocated, and won't be modified.
>> + *
>> + * Returns the vaddr, that you can use to write to the bo, and that you should
>> + * pass to kunmap_local() when you're done with this page, or NULL if the bo
>> + * is in iomem.
>> + */
>> +void *ttm_bo_kmap_try_from_panic(struct ttm_buffer_object *bo, unsigned long page)
>> +{
>> +	if (page + 1 > PFN_UP(bo->resource->size))
>> +		return NULL;
>> +
>> +	if (!bo->resource->bus.is_iomem && bo->ttm->pages && bo->ttm->pages[page])
>> +		return kmap_local_page_try_from_panic(bo->ttm->pages[page]);
>> +
>> +	return NULL;
>> +}
>> +EXPORT_SYMBOL(ttm_bo_kmap_try_from_panic);
>> +
>>   /**
>>    * ttm_bo_kmap
>>    *
>> diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
>> index cf027558b6db..8c0ce3fa077f 100644
>> --- a/include/drm/ttm/ttm_bo.h
>> +++ b/include/drm/ttm/ttm_bo.h
>> @@ -429,6 +429,7 @@ int ttm_bo_init_validate(struct ttm_device *bdev, struct ttm_buffer_object *bo,
>>   int ttm_bo_kmap(struct ttm_buffer_object *bo, unsigned long start_page,
>>   		unsigned long num_pages, struct ttm_bo_kmap_obj *map);
>>   void ttm_bo_kunmap(struct ttm_bo_kmap_obj *map);
>> +void *ttm_bo_kmap_try_from_panic(struct ttm_buffer_object *bo, unsigned long page);
>>   int ttm_bo_vmap(struct ttm_buffer_object *bo, struct iosys_map *map);
>>   void ttm_bo_vunmap(struct ttm_buffer_object *bo, struct iosys_map *map);
>>   int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo);
>