Re: [Intel-gfx] [RFC v3 3/3] drm/doc/rfc: VM_BIND uapi definition

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/06/2022 07:53, Niranjana Vishwanathapura wrote:
> On Wed, Jun 01, 2022 at 10:08:35PM -0700, Niranjana Vishwanathapura wrote:
>> On Wed, Jun 01, 2022 at 11:27:17AM +0200, Daniel Vetter wrote:
>>> On Wed, 1 Jun 2022 at 11:03, Dave Airlie <airlied@gmail.com> wrote:
>>>>
>>>> On Tue, 24 May 2022 at 05:20, Niranjana Vishwanathapura
>>>> <niranjana.vishwanathapura@intel.com> wrote:
>>>>>
>>>>> On Thu, May 19, 2022 at 04:07:30PM -0700, Zanoni, Paulo R wrote:
>>>>> >On Tue, 2022-05-17 at 11:32 -0700, Niranjana Vishwanathapura wrote:
>>>>> >> VM_BIND and related uapi definitions
>>>>> >>
>>>>> >> v2: Ensure proper kernel-doc formatting with cross references.
>>>>> >>     Also add new uapi and documentation as per review comments
>>>>> >>     from Daniel.
>>>>> >>
>>>>> >> Signed-off-by: Niranjana Vishwanathapura 
>>>>> <niranjana.vishwanathapura@intel.com>
>>>>> >> ---
>>>>> >>  Documentation/gpu/rfc/i915_vm_bind.h | 399 
>>>>> +++++++++++++++++++++++++++
>>>>> >>  1 file changed, 399 insertions(+)
>>>>> >>  create mode 100644 Documentation/gpu/rfc/i915_vm_bind.h
>>>>> >>
>>>>> >> diff --git a/Documentation/gpu/rfc/i915_vm_bind.h 
>>>>> b/Documentation/gpu/rfc/i915_vm_bind.h
>>>>> >> new file mode 100644
>>>>> >> index 000000000000..589c0a009107
>>>>> >> --- /dev/null
>>>>> >> +++ b/Documentation/gpu/rfc/i915_vm_bind.h
>>>>> >> @@ -0,0 +1,399 @@
>>>>> >> +/* SPDX-License-Identifier: MIT */
>>>>> >> +/*
>>>>> >> + * Copyright © 2022 Intel Corporation
>>>>> >> + */
>>>>> >> +
>>>>> >> +/**
>>>>> >> + * DOC: I915_PARAM_HAS_VM_BIND
>>>>> >> + *
>>>>> >> + * VM_BIND feature availability.
>>>>> >> + * See typedef drm_i915_getparam_t param.
>>>>> >> + */
>>>>> >> +#define I915_PARAM_HAS_VM_BIND               57
>>>>> >> +
>>>>> >> +/**
>>>>> >> + * DOC: I915_VM_CREATE_FLAGS_USE_VM_BIND
>>>>> >> + *
>>>>> >> + * Flag to opt-in for VM_BIND mode of binding during VM creation.
>>>>> >> + * See struct drm_i915_gem_vm_control flags.
>>>>> >> + *
>>>>> >> + * A VM in VM_BIND mode will not support the older execbuff 
>>>>> mode of binding.
>>>>> >> + * In VM_BIND mode, execbuff ioctl will not accept any execlist 
>>>>> (ie., the
>>>>> >> + * &drm_i915_gem_execbuffer2.buffer_count must be 0).
>>>>> >> + * Also, &drm_i915_gem_execbuffer2.batch_start_offset and
>>>>> >> + * &drm_i915_gem_execbuffer2.batch_len must be 0.
>>>>> >> + * DRM_I915_GEM_EXECBUFFER_EXT_BATCH_ADDRESSES extension must 
>>>>> be provided
>>>>> >> + * to pass in the batch buffer addresses.
>>>>> >> + *
>>>>> >> + * Additionally, I915_EXEC_NO_RELOC, I915_EXEC_HANDLE_LUT and
>>>>> >> + * I915_EXEC_BATCH_FIRST of &drm_i915_gem_execbuffer2.flags 
>>>>> must be 0
>>>>> >> + * (not used) in VM_BIND mode. I915_EXEC_USE_EXTENSIONS flag 
>>>>> must always be
>>>>> >> + * set (See struct drm_i915_gem_execbuffer_ext_batch_addresses).
>>>>> >> + * The buffers_ptr, buffer_count, batch_start_offset and 
>>>>> batch_len fields
>>>>> >> + * of struct drm_i915_gem_execbuffer2 are also not used and 
>>>>> must be 0.
>>>>> >> + */
>>>>> >
>>>>> >From that description, it seems we have:
>>>>> >
>>>>> >struct drm_i915_gem_execbuffer2 {
>>>>> >        __u64 buffers_ptr;              -> must be 0 (new)
>>>>> >        __u32 buffer_count;             -> must be 0 (new)
>>>>> >        __u32 batch_start_offset;       -> must be 0 (new)
>>>>> >        __u32 batch_len;                -> must be 0 (new)
>>>>> >        __u32 DR1;                      -> must be 0 (old)
>>>>> >        __u32 DR4;                      -> must be 0 (old)
>>>>> >        __u32 num_cliprects; (fences)   -> must be 0 since using 
>>>>> extensions
>>>>> >        __u64 cliprects_ptr; (fences, extensions) -> contains an 
>>>>> actual pointer!
>>>>> >        __u64 flags;                    -> some flags must be 0 (new)
>>>>> >        __u64 rsvd1; (context info)     -> repurposed field (old)
>>>>> >        __u64 rsvd2;                    -> unused
>>>>> >};
>>>>> >
>>>>> >Based on that, why can't we just get drm_i915_gem_execbuffer3 instead
>>>>> >of adding even more complexity to an already abused interface? While
>>>>> >the Vulkan-like extension thing is really nice, I don't think what
>>>>> >we're doing here is extending the ioctl usage, we're completely
>>>>> >changing how the base struct should be interpreted based on how 
>>>>> the VM
>>>>> >was created (which is an entirely different ioctl).
>>>>> >
>>>>> >From Rusty Russel's API Design grading, drm_i915_gem_execbuffer2 is
>>>>> >already at -6 without these changes. I think after vm_bind we'll need
>>>>> >to create a -11 entry just to deal with this ioctl.
>>>>> >
>>>>>
>>>>> The only change here is removing the execlist support for VM_BIND
>>>>> mode (other than natual extensions).
>>>>> Adding a new execbuffer3 was considered, but I think we need to be 
>>>>> careful
>>>>> with that as that goes beyond the VM_BIND support, including any 
>>>>> future
>>>>> requirements (as we don't want an execbuffer4 after VM_BIND).
>>>>
>>>> Why not? it's not like adding extensions here is really that different
>>>> than adding new ioctls.
>>>>
>>>> I definitely think this deserves an execbuffer3 without even
>>>> considering future requirements. Just  to burn down the old
>>>> requirements and pointless fields.
>>>>
>>>> Make execbuffer3 be vm bind only, no relocs, no legacy bits, leave the
>>>> older sw on execbuf2 for ever.
>>>
>>> I guess another point in favour of execbuf3 would be that it's less
>>> midlayer. If we share the entry point then there's quite a few vfuncs
>>> needed to cleanly split out the vm_bind paths from the legacy
>>> reloc/softping paths.
>>>
>>> If we invert this and do execbuf3, then there's the existing ioctl
>>> vfunc, and then we share code (where it even makes sense, probably
>>> request setup/submit need to be shared, anything else is probably
>>> cleaner to just copypaste) with the usual helper approach.
>>>
>>> Also that would guarantee that really none of the old concepts like
>>> i915_active on the vma or vma open counts and all that stuff leaks
>>> into the new vm_bind execbuf.
>>>
>>> Finally I also think that copypasting would make backporting easier,
>>> or at least more flexible, since it should make it easier to have the
>>> upstream vm_bind co-exist with all the other things we have. Without
>>> huge amounts of conflicts (or at least much less) that pushing a pile
>>> of vfuncs into the existing code would cause.
>>>
>>> So maybe we should do this?
>>
>> Thanks Dave, Daniel.
>> There are a few things that will be common between execbuf2 and
>> execbuf3, like request setup/submit (as you said), fence handling 
>> (timeline fences, fence array, composite fences), engine selection,
>> etc. Also, many of the 'flags' will be there in execbuf3 also (but
>> bit position will differ).
>> But I guess these should be fine as the suggestion here is to
>> copy-paste the execbuff code and having a shared code where possible.
>> Besides, we can stop supporting some older feature in execbuff3
>> (like fence array in favor of newer timeline fences), which will
>> further reduce common code.
>>
>> Ok, I will update this series by adding execbuf3 and send out soon.
>>
> 
> Does this sound reasonable?
> 
> struct drm_i915_gem_execbuffer3 {
>         __u32 ctx_id;        /* previously execbuffer2.rsvd1 */
> 
>         __u32 batch_count;
>         __u64 batch_addr_ptr;    /* Pointer to an array of batch gpu 
> virtual addresses */

Casual stumble upon..

Alternatively you could embed N pointers to make life a bit easier for 
both userspace and kernel side. Yes, but then "N batch buffers should be 
enough for everyone" problem.. :)

> 
>         __u64 flags;
> #define I915_EXEC3_RING_MASK              (0x3f)
> #define I915_EXEC3_DEFAULT                (0<<0)
> #define I915_EXEC3_RENDER                 (1<<0)
> #define I915_EXEC3_BSD                    (2<<0)
> #define I915_EXEC3_BLT                    (3<<0)
> #define I915_EXEC3_VEBOX                  (4<<0)
> 
> #define I915_EXEC3_SECURE               (1<<6)
> #define I915_EXEC3_IS_PINNED            (1<<7)
> 
> #define I915_EXEC3_BSD_SHIFT     (8)
> #define I915_EXEC3_BSD_MASK      (3 << I915_EXEC3_BSD_SHIFT)
> #define I915_EXEC3_BSD_DEFAULT   (0 << I915_EXEC3_BSD_SHIFT)
> #define I915_EXEC3_BSD_RING1     (1 << I915_EXEC3_BSD_SHIFT)
> #define I915_EXEC3_BSD_RING2     (2 << I915_EXEC3_BSD_SHIFT)

I'd suggest legacy engine selection is unwanted, especially not with the 
convoluted BSD1/2 flags. Can we just require context with engine map and 
index? Or if default context has to be supported then I'd suggest 
...class_instance for that mode.

> #define I915_EXEC3_FENCE_IN             (1<<10)
> #define I915_EXEC3_FENCE_OUT            (1<<11)
> #define I915_EXEC3_FENCE_SUBMIT         (1<<12)

People are likely to object to submit fence since generic mechanism to 
align submissions was rejected.

> 
>         __u64 in_out_fence;        /* previously execbuffer2.rsvd2 */

New ioctl you can afford dedicated fields.

In any case I suggest you involve UMD folks in designing it.

Regards,

Tvrtko

> 
>         __u64 extensions;        /* currently only for 
> DRM_I915_GEM_EXECBUFFER_EXT_TIMELINE_FENCES */
> };
> 
> With this, user can pass in batch addresses and count directly,
> instead of as an extension (as this rfc series was proposing).
> 
> I have removed many of the flags which were either legacy or not
> applicable to BM_BIND mode.
> I have also removed fence array support (execbuffer2.cliprects_ptr)
> as we have timeline fence array support. Is that fine?
> Do we still need FENCE_IN/FENCE_OUT/FENCE_SUBMIT support?
> 
> Any thing else needs to be added or removed?
> 
> Niranjana
> 
>> Niranjana
>>
>>> -Daniel
>>> -- 
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch