Re: [PATCH v2 4/5] drm/i915: ttm backend dont provide mmap_offset for kernel buffers

[Robert Beckett <bob.beckett@collabora.com>]

On 14/04/2022 15:05, Thomas Hellström wrote:
> On Tue, 2022-04-12 at 15:18 +0000, Robert Beckett wrote:
>> stolen/kernel buffers should not be mmapable by userland.
>> do not provide callbacks to facilitate this for these buffers.
>>
>> Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 32 +++++++++++++++++++++--
>> --
>>   1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>> b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>> index a878910a563c..b20f81836c54 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>> @@ -1092,8 +1092,8 @@ static void i915_ttm_unmap_virtual(struct
>> drm_i915_gem_object *obj)
>>          ttm_bo_unmap_virtual(i915_gem_to_ttm(obj));
>>   }
>>   
>> -static const struct drm_i915_gem_object_ops i915_gem_ttm_obj_ops = {
>> -       .name = "i915_gem_object_ttm",
>> +static const struct drm_i915_gem_object_ops
>> i915_gem_ttm_user_obj_ops = {
>> +       .name = "i915_gem_object_ttm_user",
>>          .flags = I915_GEM_OBJECT_IS_SHRINKABLE |
>>                   I915_GEM_OBJECT_SELF_MANAGED_SHRINK_LIST,
>>   
>> @@ -1111,6 +1111,21 @@ static const struct drm_i915_gem_object_ops
>> i915_gem_ttm_obj_ops = {
>>          .mmap_ops = &vm_ops_ttm,
>>   };
>>   
>> +static const struct drm_i915_gem_object_ops
>> i915_gem_ttm_kern_obj_ops = {
>> +       .name = "i915_gem_object_ttm_kern",
>> +       .flags = I915_GEM_OBJECT_IS_SHRINKABLE |
>> +                I915_GEM_OBJECT_SELF_MANAGED_SHRINK_LIST,
>> +
>> +       .get_pages = i915_ttm_get_pages,
>> +       .put_pages = i915_ttm_put_pages,
>> +       .truncate = i915_ttm_truncate,
>> +       .shrink = i915_ttm_shrink,
>> +
>> +       .adjust_lru = i915_ttm_adjust_lru,
>> +       .delayed_free = i915_ttm_delayed_free,
>> +       .migrate = i915_ttm_migrate,
>> +};
> 
> Do we really need two different ops here?
> 
> Since if we don't have mmap ops, basically that tells GEM it should do
> the mmapping rather than TTM.
> 
> That might of course come in handy for the shmem backend, but I don't
> fully follow why we need this for stolen.

the main rationale for doing this was to avoid 
drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c:can_mmap() presuming 
that is can use I915_MMAP_TYPE_FIXED

As the original backend also did not have mmap_offset handlers for 
stolen, this seemed like a reasonable design.

If desired, we could add a special case for the testing logic, but those 
special cases have tendency to multiply.

> 
> Also for the framebuffer handed over from BIOS to fbdev, Does that need
> mmapping and if so, how do we handle that?
> 

I'm not sure of the usecase there. Do you know of any igt test that 
tests this? I can investigate further if you do not.

> 
> /Thomas
> 
> 
> 
> 
>> +
>>   void i915_ttm_bo_destroy(struct ttm_buffer_object *bo)
>>   {
>>          struct drm_i915_gem_object *obj = i915_ttm_to_gem(bo);
>> @@ -1165,10 +1180,19 @@ int __i915_gem_ttm_object_init(struct
>> intel_memory_region *mem,
>>                  .no_wait_gpu = false,
>>          };
>>          enum ttm_bo_type bo_type;
>> +       const struct drm_i915_gem_object_ops *ops;
>>          int ret;
>>   
>>          drm_gem_private_object_init(&i915->drm, &obj->base, size);
>> -       i915_gem_object_init(obj, &i915_gem_ttm_obj_ops, &lock_class,
>> flags);
>> +
>> +       if (flags & I915_BO_ALLOC_USER &&
>> intel_region_to_ttm_type(mem) != I915_PL_STOLEN) {
>> +               bo_type = ttm_bo_type_device;
>> +               ops = &i915_gem_ttm_user_obj_ops;
>> +       } else {
>> +               bo_type = ttm_bo_type_kernel;
>> +               ops = &i915_gem_ttm_kern_obj_ops;
>> +       }
>> +       i915_gem_object_init(obj, ops, &lock_class, flags);
>>   
>>          obj->bo_offset = offset;
>>   
>> @@ -1178,8 +1202,6 @@ int __i915_gem_ttm_object_init(struct
>> intel_memory_region *mem,
>>   
>>          INIT_RADIX_TREE(&obj->ttm.get_io_page.radix, GFP_KERNEL |
>> __GFP_NOWARN);
>>          mutex_init(&obj->ttm.get_io_page.lock);
>> -       bo_type = (obj->flags & I915_BO_ALLOC_USER) ?
>> ttm_bo_type_device :
>> -               ttm_bo_type_kernel;
>>   
>>          obj->base.vma_node.driver_private = i915_gem_to_ttm(obj);
>>   
> 
>