[PATCH 2/2] drm/lima: driver for ARM Mali4xx GPUs

[Christian König]

Am 14.02.19 um 03:52 schrieb Alex Deucher via dri-devel:
> [SNIP]
>>>>> +static int lima_ioctl_gem_va(struct drm_device *dev, void *data, struct drm_file *file)
>>>>> +{
>>>>> +       struct drm_lima_gem_va *args = data;
>>>>> +
>>>>> +       switch (args->op) {
>>>>> +       case LIMA_VA_OP_MAP:
>>>>> +               return lima_gem_va_map(file, args->handle, args->flags, args->va);
>>>>> +       case LIMA_VA_OP_UNMAP:
>>>>> +               return lima_gem_va_unmap(file, args->handle, args->va);
>>>> These are mapping to GPU VA. Why not do that on GEM object creation or
>>>> import or when the objects are submitted with cmd queue as other
>>>> drivers do?
>>>>
>>>> To put it another way, These ioctls look different than what other
>>>> drivers do. Why do you need to do things differently? My understanding
>>>> is best practice is to map and return the GPU offset when the GEM
>>>> object is created. This is what v3d does. I think Intel is moving to
>>>> that. And panfrost will do that.
>>> I think it would be a good idea to look at the amdgpu driver.  This
>>> driver is heavily modeled after it.  Basically the GEM VA ioctl allows
>>> userspace to manage per process (per fd really) virtual addresses.
>> Why do you want userspace to manage assigning VAs versus the kernel to
>> do so? Exposing that detail to userspace means the driver must support
>> a per process address space. Letting the kernel assign addresses means
>> it can either be a single address space or be a per process address
>> space. It seems to me more flexible to allow the kernel driver to
>> evolve without that ABI.
> Having it in userspace provides a lot more flexibility and makes it
> easier to support things like unified address space between CPU and
> GPU. I guess it depends on the hw as to what is the right choice.

To summarize we actually have tried this approach with the radeon and it 
turned out to be a really bad mistake.

To implement features like partial residential textures and shared 
virtual address space you absolutely need userspace to be in charge of 
allocating virtual addresses.

Regards,
Christian.