[PATCH v3 4/4] drm/doc/rfc: i915 DG1 uAPI
Jason Ekstrand
jason at jlekstrand.net
Wed Apr 21 13:54:29 UTC 2021
On Wed, Apr 21, 2021 at 3:22 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
> On 20/04/2021 18:00, Jason Ekstrand wrote:
> > On Tue, Apr 20, 2021 at 11:34 AM Tvrtko Ursulin
> > <tvrtko.ursulin at linux.intel.com> wrote:
> >>
> >>
> >> On 19/04/2021 16:19, Jason Ekstrand wrote:
> >>> On Mon, Apr 19, 2021 at 7:02 AM Matthew Auld <matthew.auld at intel.com> wrote:
> >>>>
> >>>> On 16/04/2021 17:38, Jason Ekstrand wrote:
> >>>>> On Thu, Apr 15, 2021 at 11:04 AM Matthew Auld <matthew.auld at intel.com> wrote:
> >>>>>>
> >>>>>> Add an entry for the new uAPI needed for DG1.
> >>>>>>
> >>>>>> v2(Daniel):
> >>>>>> - include the overall upstreaming plan
> >>>>>> - add a note for mmap, there are differences here for TTM vs i915
> >>>>>> - bunch of other suggestions from Daniel
> >>>>>> v3:
> >>>>>> (Daniel)
> >>>>>> - add a note for set/get caching stuff
> >>>>>> - add some more docs for existing query and extensions stuff
> >>>>>> - add an actual code example for regions query
> >>>>>> - bunch of other stuff
> >>>>>> (Jason)
> >>>>>> - uAPI change(!):
> >>>>>> - try a simpler design with the placements extension
> >>>>>> - rather than have a generic setparam which can cover multiple
> >>>>>> use cases, have each extension be responsible for one thing
> >>>>>> only
> >>>>>>
> >>>>>> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> >>>>>> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> >>>>>> Cc: Jordan Justen <jordan.l.justen at intel.com>
> >>>>>> Cc: Daniel Vetter <daniel.vetter at intel.com>
> >>>>>> Cc: Kenneth Graunke <kenneth at whitecape.org>
> >>>>>> Cc: Jason Ekstrand <jason at jlekstrand.net>
> >>>>>> Cc: Dave Airlie <airlied at gmail.com>
> >>>>>> Cc: dri-devel at lists.freedesktop.org
> >>>>>> Cc: mesa-dev at lists.freedesktop.org
> >>>>>> ---
> >>>>>> Documentation/gpu/rfc/i915_gem_lmem.h | 255 ++++++++++++++++++++++++
> >>>>>> Documentation/gpu/rfc/i915_gem_lmem.rst | 139 +++++++++++++
> >>>>>> Documentation/gpu/rfc/index.rst | 4 +
> >>>>>> 3 files changed, 398 insertions(+)
> >>>>>> create mode 100644 Documentation/gpu/rfc/i915_gem_lmem.h
> >>>>>> create mode 100644 Documentation/gpu/rfc/i915_gem_lmem.rst
> >>>>>>
> >>>>>> diff --git a/Documentation/gpu/rfc/i915_gem_lmem.h b/Documentation/gpu/rfc/i915_gem_lmem.h
> >>>>>> new file mode 100644
> >>>>>> index 000000000000..2a82a452e9f2
> >>>>>> --- /dev/null
> >>>>>> +++ b/Documentation/gpu/rfc/i915_gem_lmem.h
> >>>>>> @@ -0,0 +1,255 @@
> >>>>>> +/*
> >>>>>> + * Note that drm_i915_query_item and drm_i915_query are existing bits of uAPI.
> >>>>>> + * For the regions query we are just adding a new query id, so no actual new
> >>>>>> + * ioctl or anything, but including it here for reference.
> >>>>>> + */
> >>>>>> +struct drm_i915_query_item {
> >>>>>> +#define DRM_I915_QUERY_MEMORY_REGIONS 0xdeadbeaf
> >>>>>> + ....
> >>>>>> + __u64 query_id;
> >>>>>> +
> >>>>>> + /*
> >>>>>> + * When set to zero by userspace, this is filled with the size of the
> >>>>>> + * data to be written at the data_ptr pointer. The kernel sets this
> >>>>>> + * value to a negative value to signal an error on a particular query
> >>>>>> + * item.
> >>>>>> + */
> >>>>>> + __s32 length;
> >>>>>> +
> >>>>>> + __u32 flags;
> >>>>>> + /*
> >>>>>> + * Data will be written at the location pointed by data_ptr when the
> >>>>>> + * value of length matches the length of the data to be written by the
> >>>>>> + * kernel.
> >>>>>> + */
> >>>>>> + __u64 data_ptr;
> >>>>>> +};
> >>>>>> +
> >>>>>> +struct drm_i915_query {
> >>>>>> + __u32 num_items;
> >>>>>> + /*
> >>>>>> + * Unused for now. Must be cleared to zero.
> >>>>>> + */
> >>>>>> + __u32 flags;
> >>>>>> + /*
> >>>>>> + * This points to an array of num_items drm_i915_query_item structures.
> >>>>>> + */
> >>>>>> + __u64 items_ptr;
> >>>>>> +};
> >>>>>> +
> >>>>>> +#define DRM_IOCTL_I915_QUERY DRM_IOWR(DRM_COMMAND_BASE + DRM_I915_QUERY, struct drm_i915_query)
> >>>>>> +
> >>>>>> +/**
> >>>>>> + * enum drm_i915_gem_memory_class
> >>>>>> + */
> >>>>>> +enum drm_i915_gem_memory_class {
> >>>>>> + /** @I915_MEMORY_CLASS_SYSTEM: system memory */
> >>>>>> + I915_MEMORY_CLASS_SYSTEM = 0,
> >>>>>> + /** @I915_MEMORY_CLASS_DEVICE: device local-memory */
> >>>>>> + I915_MEMORY_CLASS_DEVICE,
> >>>>>> +};
> >>>>>> +
> >>>>>> +/**
> >>>>>> + * struct drm_i915_gem_memory_class_instance
> >>>>>> + */
> >>>>>> +struct drm_i915_gem_memory_class_instance {
> >>>>>> + /** @memory_class: see enum drm_i915_gem_memory_class */
> >>>>>> + __u16 memory_class;
> >>>>>> +
> >>>>>> + /** @memory_instance: which instance */
> >>>>>> + __u16 memory_instance;
> >>>>>> +};
> >>>>>> +
> >>>>>> +/**
> >>>>>> + * struct drm_i915_memory_region_info
> >>>>>> + *
> >>>>>> + * Describes one region as known to the driver.
> >>>>>> + *
> >>>>>> + * Note that we reserve quite a lot of stuff here for potential future work. As
> >>>>>> + * an example we might want expose the capabilities(see caps) for a given
> >>>>>> + * region, which could include things like if the region is CPU
> >>>>>> + * mappable/accessible etc.
> >>>>>
> >>>>> I get caps but I'm seriously at a loss as to what the rest of this
> >>>>> would be used for. Why are caps and flags both there and separate?
> >>>>> Flags are typically something you set, not query. Also, what's with
> >>>>> rsvd1 at the end? This smells of substantial over-building to me.
> >>>>>
> >>>>> I thought to myself, "maybe I'm missing a future use-case" so I looked
> >>>>> at the internal tree and none of this is being used there either.
> >>>>> This indicates to me that either I'm missing something and there's
> >>>>> code somewhere I don't know about or, with three years of building on
> >>>>> internal branches, we still haven't proven that any of this is needed.
> >>>>> If it's the latter, which I strongly suspect, maybe we should drop the
> >>>>> unnecessary bits and only add them back in if and when we have proof
> >>>>> that they're useful.
> >>>>
> >>>> Do you mean just drop caps/flags here, but keep/inflate rsvd0/rsvd1,
> >>>> which is less opinionated about future unknowns? If so, makes sense to me.
> >>>
> >>> I meant drop flags and rsvd1. We need rsvd0 for padding and I can
> >>> see some value to caps. We may want to advertise, for instance, what
> >>> mapping coherency types are available per-heap. But I don't see any
> >>> use for any of the other fields.
> >>
> >> I'd suggest making sure at least enough rsvd fields remain so that flags
> >> could be added later if needed. Experience from engine info shows that
> >> both were required in order to extend the query via re-purposing the
> >> rsvds and adding flag bits to indicate when a certain rsvd contains a
> >> new piece of information.
> >
> > Looking at DII, all I see is we started using caps. I already said
> > I'm fine with caps. I can already imagine some useful ones like
> > specifying what kinds of mappings we can do.
> >
> > If we're concerned about more complicated stuff, I argue that we have
> > no ability to predict what that will look like and so just throwing in
> > a bunch of __u32 rsvd[N] is blind guessing. I'm seeing a lot of that
> > in the recently added APIs such as the flags and rsvd[4] in
> > i915_user_extension. What's that there for? Why can't you put that
> > information in the extension struct which derives from it? Maybe it's
> > so that we can extend it. But we added that struct as part of an
> > extension mechanism!?!
> >
> > If we want to make things extensible, Vulkan actually provides some
> > prior art for this in the form of allowing queries to be extended just
> > like input structs. We could add a __u64 extensions field to
> > memory_region_info and, if we ever need to query more info, the client
> > can provide a chain of additional per-region queries. Yeah, there are
> > problems with it and it gets a bit clunky but it does work pretty
> > well.
> >
> >> I probably cannot go into too much detail
> >> here, but anyway the point is just to make sure too much is not stripped
> >> out so that instead of simply adding fields/flags we have to add a new
> >> query in the future. IMO some rsvd fields are not really harmful and if
> >> they can make things easier in the future why not.
> >
> > Maybe it's my tired and generally grumpy state of mind but I'm not
> > particularly favorable towards "why not?" as a justification for
> > immutable kernel APIs. We've already found a few places where
> > Zoidberg API design has caused us problems. We need an answer to
> > "why?" Future extensibility is a potentially valid answer but we need
> > to do a better job of thinking through it than we have in the past.
>
> I did not simply say why not, did I?
You literally did: "...and if they can make things easier in the
future why not."
> It is a balance thing between cost
> and benefit. I see the cost of rsvd fields as approaching zero really ,
> and cost of having to add query v2 if we end up having not enough rsvd
> as definitely way bigger.
>
> If you look at the mentioned engine info query you will see that as soon
> as you add some caps, flags become useful (so userspace can answer the
> question of does the object not support this cap or does the kernel does
> not even know about the cap).
>
> Furthermore, in that uapi, caps pertain to the property of the
> underlying object being queried, while the flags pertain to the query
> itself. I find that separation logical and useful.
Ok, that answers the question I asked above: "what are flags for and
why are they different?" At the very least, that should be
documented. Then again... We really want a GETPARAM query any time a
kernel interface changes, such as adding caps, and we can say that
userspace should ignore caps it doesn't understand. I think that
solves both directions of the negotiation without flags.
> I am not claiming to know memory region query will end up the same, and
> I definitely agree we cannot guess the future. I am just saying rsvd
> fields are inconsequential really in terms of maintenance burden and
> have been proven useful in the past. So I disagree with the drive to
> kick them all out.
Sure, it doesn't cost anything to have extra zeros in the struct.
However, if/when the API grows using rsvd fields, we end up with "if
CAP_FOO is set, rsvd[5] means blah" which makes for a horribly
confusing API. As a userspace person who has to remember how to use
this stuff, I'd rather make another call or chain in a struct than try
to remember and/or figure out what all 8 rsvd fields mean.
> Btw extension chains also work for me. I this a bit more complicated and
> we don't have prior art in i915 to use them on the read/get/query side
> but we could add if a couple of rsvd is so objectionable.
Another option, which I think I mentioned somewhere, is that we could
add a secondary query which takes a memory region class and instance
and lets you query additional properties one-at-a-time. That might be
easier to extend. Sure, it means more ioctls but they're not that
expensive and they should only happen at driver initialization so I'm
not that inclined to care about the cost there.
--Jason
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