[Mesa-dev] [PATCH v3 4/4] drm/doc/rfc: i915 DG1 uAPI
Jason Ekstrand
jason at jlekstrand.net
Mon Apr 19 15:19:06 UTC 2021
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.
> >
> > To be clear, I don't mind the query API as such and the class/instance
> > stuff seems fine and I really like being able to get the sizes
> > directly. What concerns me is all this extra future-proofing that we
> > have zero proof is actually useful. In my experience, when you build
> > out like this without so much as a use-case, you always end up
> > building the wrong thing.
> >
> >> + */
> >> +struct drm_i915_memory_region_info {
> >> + /** @region: class:instance pair encoding */
> >> + struct drm_i915_gem_memory_class_instance region;
> >> +
> >> + /** @rsvd0: MBZ */
> >> + __u32 rsvd0;
> >> +
> >> + /** @caps: MBZ */
> >> + __u64 caps;
> >> +
> >> + /** @flags: MBZ */
> >> + __u64 flags;
> >> +
> >> + /** @probed_size: Memory probed by the driver (-1 = unknown) */
> >> + __u64 probed_size;
> >> +
> >> + /** @unallocated_size: Estimate of memory remaining (-1 = unknown) */
> >> + __u64 unallocated_size;
> >> +
> >> + /** @rsvd1: MBZ */
> >> + __u64 rsvd1[8];
> >> +};
> >> +
> >> +/**
> >> + * struct drm_i915_query_memory_regions
> >> + *
> >> + * Region info query enumerates all regions known to the driver by filling in
> >> + * an array of struct drm_i915_memory_region_info structures.
> >> + *
> >> + * Example for getting the list of supported regions:
> >> + *
> >> + * .. code-block:: C
> >> + *
> >> + * struct drm_i915_query_memory_regions *info;
> >> + * struct drm_i915_query_item item = {
> >> + * .query_id = DRM_I915_QUERY_MEMORY_REGIONS;
> >> + * };
> >> + * struct drm_i915_query query = {
> >> + * .num_items = 1,
> >> + * .items_ptr = (uintptr_t)&item,
> >> + * };
> >> + * int err, i;
> >> + *
> >> + * // First query the size of the blob we need, this needs to be large
> >> + * // enough to hold our array of regions. The kernel will fill out the
> >> + * // item.length for us, which is the number of bytes we need.
> >> + * err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> >> + * if (err) ...
> >> + *
> >> + * info = calloc(1, item.length);
> >> + * // Now that we allocated the required number of bytes, we call the ioctl
> >> + * // again, this time with the data_ptr pointing to our newly allocated
> >> + * // blob, which the kernel can then populate with the all the region info.
> >> + * item.data_ptr = (uintptr_t)&info,
> >> + *
> >> + * err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> >> + * if (err) ...
> >> + *
> >> + * // We can now access each region in the array
> >> + * for (i = 0; i < info->num_regions; i++) {
> >> + * struct drm_i915_memory_region_info mr = info->regions[i];
> >> + * u16 class = mr.region.class;
> >> + * u16 instance = mr.region.instance;
> >> + *
> >> + * ....
> >> + * }
> >> + *
> >> + * free(info);
> >> + */
> >> +struct drm_i915_query_memory_regions {
> >> + /** @num_regions: Number of supported regions */
> >> + __u32 num_regions;
> >> +
> >> + /** @rsvd: MBZ */
> >> + __u32 rsvd[3];
> >
> > Why pad to 16B instead of 8B?
>
> It's copy-pasta from engine_info. I can shrink it if you want? I don't
> have a strong opinion.
Yeah, I'd shrink to ust a __u32. We could probably drop it entirely
but aligning to 8B seems like a good idea before an array.
--Jason
> >
> >> +
> >> + /** @regions: Info about each supported region */
> >> + struct drm_i915_memory_region_info regions[];
> >> +};
> >> +
> >> +#define DRM_I915_GEM_CREATE_EXT 0xdeadbeaf
> >> +#define DRM_IOCTL_I915_GEM_CREATE_EXT DRM_IOWR(DRM_COMMAND_BASE + DRM_I915_GEM_CREATE_EXT, struct drm_i915_gem_create_ext)
> >> +
> >> +/**
> >> + * struct drm_i915_gem_create_ext
> >> + *
> >> + * Existing gem_create behaviour, with added extension support.
> >> + *
> >> + * Note that in the future we want to have our buffer flags here, at least for
> >> + * the stuff that is immutable. Previously we would have two ioctls, one to
> >> + * create the object with gem_create, and another to apply various parameters,
> >> + * however this creates some ambiguity for the params which are considered
> >> + * immutable. Also in general we're phasing out the various SET/GET ioctls.
> >> + */
> >> +struct drm_i915_gem_create_ext {
> >> + /**
> >> + * @size: Requested size for the object.
> >> + *
> >> + * The (page-aligned) allocated size for the object will be returned.
> >> + *
> >> + * Note that for some devices we have might have further minimum
> >> + * page-size restrictions(larger than 4K), like for device local-memory.
> >> + * However in general the final size here should always reflect any
> >> + * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
> >> + * extension to place the object in device local-memory.
> >> + */
> >> + __u64 size;
> >> + /**
> >> + * @handle: Returned handle for the object.
> >> + *
> >> + * Object handles are nonzero.
> >> + */
> >> + __u32 handle;
> >> + /** @flags: MBZ */
> >> + __u32 flags;
> >> + /**
> >> + * @extensions: The chain of extensions to apply to this object.
> >> + *
> >> + * This will be useful in the future when we need to support several
> >> + * different extensions, and we need to apply more than one when
> >> + * creating the object. See struct i915_user_extension.
> >> + *
> >> + * If we don't supply any extensions then we get the same old gem_create
> >> + * behaviour.
> >> + *
> >> + * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
> >> + * drm_i915_gem_create_ext_memory_regions
> >> + */
> >> +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
> >> + __u64 extensions;
> >> +};
> >> +
> >> +/**
> >> + * struct drm_i915_gem_create_ext_memory_regions
> >> + *
> >> + * I915_GEM_CREATE_EXT_MEMORY_REGIONS extension:
> >> + *
> >> + * Set the object with the desired set of placements/regions in priority
> >> + * order(each entry must be unique and supported by the device), as an array of
> >> + * drm_i915_gem_memory_class_instance, or an equivalent layout of class:instance
> >> + * pair encodings. See DRM_I915_QUERY_MEMORY_REGIONS for how to query the
> >> + * supported regions for a device.
> >> + *
> >> + * As an example, on discrete devices, if we wish to set the placement as
> >> + * device local-memory we can do something like:
> >> + *
> >> + * .. code-block:: C
> >> + *
> >> + * struct drm_i915_gem_memory_class_instance region_lmem = {
> >> + * .memory_class = I915_MEMORY_CLASS_DEVICE,
> >> + * .memory_instance = 0,
> >> + * };
> >> + * struct drm_i915_gem_create_ext_memory_regions regions = {
> >> + * .base = { .name = I915_GEM_CREATE_EXT_MEMORY_REGIONS },
> >> + * .regions = (uintptr_t)®ion_lmem,
> >> + * .num_regions = 1,
> >> + * };
> >> + * struct drm_i915_gem_create_ext create_ext = {
> >> + * .size = 16 * PAGE_SIZE,
> >> + * .extensions = (uintptr_t)®ions,
> >> + * };
> >> + *
> >> + * int err = ioctl(fd, DRM_IOCTL_I915_GEM_CREATE_EXT, &create_ext);
> >> + * if (err) ...
> >> + *
> >> + * At which point we get the object handle in create_ext.handle, if all went
> >> + * well.
> >> + */
> >> +struct drm_i915_gem_create_ext_memory_regions {
> >> + /** @base: Extension link. See struct i915_user_extension. */
> >> + struct i915_user_extension base;
> >> +
> >> + /** @pad: MBZ */
> >> + __u32 pad;
> >> + /** @num_regions: Number of elements in the placements array. */
> >> + __u32 num_regions;
> >> + /**
> >> + * @regions: The placements array.
> >> + *
> >> + * Should be an array of drm_i915_gem_memory_class_instance.
> >> + */
> >> + __u64 regions;
> >> +};
> >> diff --git a/Documentation/gpu/rfc/i915_gem_lmem.rst b/Documentation/gpu/rfc/i915_gem_lmem.rst
> >> new file mode 100644
> >> index 000000000000..52f1db15ae94
> >> --- /dev/null
> >> +++ b/Documentation/gpu/rfc/i915_gem_lmem.rst
> >> @@ -0,0 +1,139 @@
> >> +=========================
> >> +I915 DG1/LMEM RFC Section
> >> +=========================
> >> +
> >> +Upstream plan
> >> +=============
> >> +For upstream the overall plan for landing all the DG1 stuff and turning it for
> >> +real, with all the uAPI bits is:
> >> +
> >> +* Merge basic HW enabling of DG1(still without pciid)
> >> +* Merge the uAPI bits behind special CONFIG_BROKEN(or so) flag
> >> + * At this point we can still make changes, but importantly this lets us
> >> + start running IGTs which can utilize local-memory in CI
> >> +* Convert over to TTM, make sure it all keeps working
> >> +* Add pciid for DG1 and turn on uAPI for real
> >> +
> >> +New object placement and region query uAPI
> >> +==========================================
> >> +Starting from DG1 we need to give userspace the ability to allocate buffers from
> >> +device local-memory. Currently the driver supports gem_create, which can place
> >> +buffers in system memory via shmem, and the usual assortment of other
> >> +interfaces, like dumb buffers and userptr.
> >> +
> >> +To support this new capability, while also providing a uAPI which will work
> >> +beyond just DG1, we propose to offer three new bits of uAPI:
> >> +
> >> +Query uAPI
> >> +----------
> >> +Existing query interface
> >> +^^^^^^^^^^^^^^^^^^^^^^^^
> >> +.. kernel-doc:: include/uapi/drm/i915_drm.h
> >> + :functions: drm_i915_query_item drm_i915_query
> >> +
> >> +DRM_I915_QUERY_MEMORY_REGIONS
> >> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >> +New query ID which allows userspace to discover the list of supported memory
> >> +regions(like system-memory and local-memory) for a given device. We identify
> >> +each region with a class and instance pair, which should be unique. The class
> >> +here would be DEVICE or SYSTEM, and the instance would be zero, on platforms
> >> +like DG1.
> >> +
> >> +Side note: The class/instance design is borrowed from our existing engine uAPI,
> >> +where we describe every physical engine in terms of its class, and the
> >> +particular instance, since we can have more than one per class.
> >> +
> >> +In the future we also want to expose more information which can further
> >> +describe the capabilities of a region.
> >> +
> >> +.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
> >> + :functions: drm_i915_gem_memory_class drm_i915_gem_memory_class_instance drm_i915_memory_region_info drm_i915_query_memory_regions
> >> +
> >> +GEM_CREATE_EXT
> >> +--------------
> >> +New ioctl which is basically just gem_create but now allows userspace to
> >> +provide a chain of possible extensions. Note that if we don't provide any
> >> +extensions then we get the exact same behaviour as gem_create.
> >> +
> >> +Side note: We also need to support PXP[1] in the near future, which is also
> >> +applicable to integrated platforms, and adds its own gem_create_ext extension,
> >> +which basically lets userspace mark a buffer as "protected".
> >
> > A bit off-topic, but do we really need a whole extension for that? Or
> > can we just throw a bit in flags? I'm a big fan of landing create_ext
> > anyway; I like extensibility. I'm just questioning whether or not
> > that one needs its own struct.
> >
> > --Jason
> >
> >
> >> +.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
> >> + :functions: drm_i915_gem_create_ext
> >> +
> >> +It's raining extensions
> >> +^^^^^^^^^^^^^^^^^^^^^^^
> >> +As noted above, extensions can be supplied as a chain in gem_create_ext using the
> >> +existing i915_user_extension. This will be useful in the future when we need to
> >> +support several different extensions, and we need to apply more than one when
> >> +creating the object.
> >> +
> >> +.. kernel-doc:: include/uapi/drm/i915_drm.h
> >> + :functions: i915_user_extension
> >> +
> >> +I915_GEM_CREATE_EXT_MEMORY_REGIONS
> >> +----------------------------------
> >> +Implemented as an extension for gem_create_ext, we would now allow userspace to
> >> +optionally provide an immutable list of preferred placements at creation time,
> >> +in priority order, for a given buffer object. For the placements we expect
> >> +them each to use the class/instance encoding, as per the output of the regions
> >> +query. Having the list in priority order will be useful in the future when
> >> +placing an object, say during eviction.
> >> +
> >> +.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
> >> + :functions: drm_i915_gem_create_ext_memory_regions
> >> +
> >> +One fair criticism here is that this seems a little over-engineered[2]. If we
> >> +just consider DG1 then yes, a simple gem_create.flags or something is totally
> >> +all that's needed to tell the kernel to allocate the buffer in local-memory or
> >> +whatever. However looking to the future we need uAPI which can also support
> >> +upcoming Xe HP multi-tile architecture in a sane way, where there can be
> >> +multiple local-memory instances for a given device, and so using both class and
> >> +instance in our uAPI to describe regions is desirable, although specifically
> >> +for DG1 it's uninteresting, since we only have a single local-memory instance.
> >> +
> >> +Existing uAPI issues
> >> +====================
> >> +Some potential issues we still need to resolve.
> >> +
> >> +I915 MMAP
> >> +---------
> >> +In i915 there are multiple ways to MMAP GEM object, including mapping the same
> >> +object using different mapping types(WC vs WB), i.e multiple active mmaps per
> >> +object. TTM expects one MMAP at most for the lifetime of the object. If it
> >> +turns out that we have to backpedal here, there might be some potential
> >> +userspace fallout.
> >> +
> >> +I915 SET/GET_CACHING
> >> +--------------------
> >> +In i915 we have set/get_caching ioctl. TTM doesn't let us to change this, but
> >> +DG1 doesn't support non-snooped pcie transactions, so we can just always
> >> +allocate as WB for smem-only buffers. If/when our hw gains support for
> >> +non-snooped pcie transactions then we must fix this mode at allocation time as
> >> +a new GEM extension.
> >> +
> >> +This is related to the mmap problem, because in general (meaning, when we're
> >> +not running on intel cpus) the cpu mmap must not, ever, be inconsistent with
> >> +allocation mode.
> >> +
> >> +Possible idea is to let the kernel picks the mmap mode for userspace from the
> >> +following table:
> >> +
> >> +smem-only: WB. Userspace does not need to call clflush.
> >> +
> >> +smem+lmem: We allocate uncached memory, and give userspace a WC mapping
> >> +for when the buffer is in smem, and WC when it's in lmem. GPU does snooped
> >> +access, which is a bit inefficient.
> >> +
> >> +lmem only: always WC
> >> +
> >> +This means on discrete you only get a single mmap mode, all others must be
> >> +rejected. That's probably going to be a new default mode or something like
> >> +that.
> >> +
> >> +Links
> >> +=====
> >> +[1] https://patchwork.freedesktop.org/series/86798/
> >> +
> >> +[2] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5599#note_553791
> >> diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
> >> index a8621f7dab8b..05670442ca1b 100644
> >> --- a/Documentation/gpu/rfc/index.rst
> >> +++ b/Documentation/gpu/rfc/index.rst
> >> @@ -15,3 +15,7 @@ host such documentation:
> >>
> >> * Once the code has landed move all the documentation to the right places in
> >> the main core, helper or driver sections.
> >> +
> >> +.. toctree::
> >> +
> >> + i915_gem_lmem.rst
> >> --
> >> 2.26.3
> >>
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