[PATCH RFC 00/24] Lima DRM driver

[Christian König]

Am 24.05.2018 um 02:31 schrieb Qiang Yu:
> On Wed, May 23, 2018 at 11:44 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Wed, May 23, 2018 at 3:52 PM, Qiang Yu <yuq825 at gmail.com> wrote:
>>> On Wed, May 23, 2018 at 5:29 PM, Christian König
>>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>>> Am 18.05.2018 um 11:27 schrieb Qiang Yu:
>>>>> Kernel DRM driver for ARM Mali 400/450 GPUs.
>>>>>
>>>>> This implementation mainly take amdgpu DRM driver as reference.
>>>>>
>>>>> - Mali 4xx GPUs have two kinds of processors GP and PP. GP is for
>>>>>     OpenGL vertex shader processing and PP is for fragment shader
>>>>>     processing. Each processor has its own MMU so prcessors work in
>>>>>     virtual address space.
>>>>> - There's only one GP but multiple PP (max 4 for mali 400 and 8
>>>>>     for mali 450) in the same mali 4xx GPU. All PPs are grouped
>>>>>     togather to handle a single fragment shader task divided by
>>>>>     FB output tiled pixels. Mali 400 user space driver is
>>>>>     responsible for assign target tiled pixels to each PP, but mali
>>>>>     450 has a HW module called DLBU to dynamically balance each
>>>>>     PP's load.
>>>>> - User space driver allocate buffer object and map into GPU
>>>>>     virtual address space, upload command stream and draw data with
>>>>>     CPU mmap of the buffer object, then submit task to GP/PP with
>>>>>     a register frame indicating where is the command stream and misc
>>>>>     settings.
>>>>> - There's no command stream validation/relocation due to each user
>>>>>     process has its own GPU virtual address space. GP/PP's MMU switch
>>>>>     virtual address space before running two tasks from different
>>>>>     user process. Error or evil user space code just get MMU fault
>>>>>     or GP/PP error IRQ, then the HW/SW will be recovered.
>>>>> - Use TTM as MM. TTM_PL_TT type memory is used as the content of
>>>>>     lima buffer object which is allocated from TTM page pool. all
>>>>>     lima buffer object gets pinned with TTM_PL_FLAG_NO_EVICT when
>>>>>     allocation, so there's no buffer eviction and swap for now. We
>>>>>     need reverse engineering to see if and how GP/PP support MMU
>>>>>     fault recovery (continue execution). Otherwise we have to
>>>>>     pin/unpin each envolved buffer when task creation/deletion.
>>>>
>>>> Well pinning all memory is usually a no-go for upstreaming. But since you
>>>> are already using the drm_sched for GPU task scheduling why are you actually
>>>> needing this?
>>>>
>>>> The scheduler should take care of signaling all fences when the hardware is
>>>> done with it's magic and that is enough for TTM to note that a buffer object
>>>> is movable again (e.g. unpin them).
>>> Please correct me if I'm wrong.
>>>
>>> One way to implement eviction/swap is like this:
>>> call validation on each buffers involved in a task, but this won't
>>> prevent it from
>>> eviction/swap when executing, so a GPU MMU fault may happen and in the
>>> handler we need to recover the buffer evicted/swapped.
>>>
>>> Another way is pin/unpin buffers evolved when task create/free.
>>>
>>> First way is better when memory load is low and second way is better when
>>> memory load is high. First way also need less memory.
>>>
>>> So I'd prefer first way but due to the GPU MMU fault
>>> HW op need reverse engineering, I have to pin all buffers now. After
>>> the HW op is clear, I can choose one way to implement.
>> All the drivers using ttm have something that looks like vram, or a
>> requirement to move buffers around. Afaiui that includes virtio drm
>> driver.
> Does virtio drm driver need to move buffers around? amdgpu also
> has no vram when APU.
>
>>  From your description you don't have such a requirement, and
>> then doing what etnaviv has done would be a lot simpler. Everything
>> that's not related to buffer movement handling is also available
>> outside of ttm already.
> Yeah, I could do like etnaviv, but it's not simpler than using ttm
> directly especially want some optimization (like ttm page pool,
> ttm_eu_reserve_buffers, ttm_bo_mmap). If I have/want to implement
> them, why not just use TTM directly with all those helper functions.

Well TTM has some design flaws (e.g. heavily layered design etc...), but 
it also offers some rather nice functionality.

Regards,
Christian.

>
> Regards,
> Qiang