Re: [PATCH 1/9] drm/doc/rfc: i915 DG1 uAPI

[Matthew Auld <matthew.auld@intel.com>]

On 26/04/2021 16:11, Jason Ekstrand wrote:
> On Mon, Apr 26, 2021 at 4:42 AM Matthew Auld <matthew.auld@intel.com> wrote:
>>
>> Add an entry for the new uAPI needed for DG1. Also add the overall
>> upstream plan, including some notes for the TTM conversion.
>>
>> 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
>> v4:
>>   (Daniel)
>>    - add some more notes for ttm conversion
>>    - bunch of other stuff
>>   (Jason)
>>    - uAPI change(!):
>>          - drop all the extra rsvd members for the region_query and
>>            region_info, just keep the bare minimum needed for padding
>>
>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
>> Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
>> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Daniel Vetter <daniel.vetter@intel.com>
>> Cc: Kenneth Graunke <kenneth@whitecape.org>
>> Cc: Jason Ekstrand <jason@jlekstrand.net>
>> Cc: Dave Airlie <airlied@gmail.com>
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: mesa-dev@lists.freedesktop.org
>> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Acked-by: Dave Airlie <airlied@redhat.com>
>> ---
>>   Documentation/gpu/rfc/i915_gem_lmem.h   | 212 ++++++++++++++++++++++++
>>   Documentation/gpu/rfc/i915_gem_lmem.rst | 130 +++++++++++++++
>>   Documentation/gpu/rfc/index.rst         |   4 +
>>   3 files changed, 346 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..7ed59b6202d5
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_gem_lmem.h
>> @@ -0,0 +1,212 @@
>> +/**
>> + * enum drm_i915_gem_memory_class - Supported memory classes
>> + */
>> +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 - Identify particular memory region
>> + */
>> +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 some 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, what are
>> + * the supported mapping types etc.
>> + *
>> + * Note this is using both struct drm_i915_query_item and struct drm_i915_query.
>> + * For this new query we are adding the new query id DRM_I915_QUERY_MEMORY_REGIONS
>> + * at &drm_i915_query_item.query_id.
>> + */
>> +struct drm_i915_memory_region_info {
>> +       /** @region: The class:instance pair encoding */
>> +       struct drm_i915_gem_memory_class_instance region;
>> +
>> +       /** @pad: MBZ */
>> +       __u32 pad;
>> +
>> +       /** @caps: MBZ */
>> +       __u64 caps;
>> +
>> +       /** @probed_size: Memory probed by the driver (-1 = unknown) */
>> +       __u64 probed_size;
>> +
>> +       /** @unallocated_size: Estimate of memory remaining (-1 = unknown) */
>> +       __u64 unallocated_size;
>> +};
>> +
>> +/**
>> + * struct drm_i915_query_memory_regions
>> + *
>> + * The 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;
>> +
>> +       /** @pad: MBZ */
>> +       __u32 pad;
>> +
>> +       /** @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)
> 
> Here's another thought:  Instead of burning a new IOCTL number, should
> we just re-use DRM_I915_GEM_CREATE?  The different structure size
> should let us tell the two apart.

Yeah, it was exactly like that in the original version[1]. Scrolling 
through the review comments, I think the concern was with an older 
kernel just silently ignoring the extensions with new userspace.

[1] https://patchwork.freedesktop.org/patch/404455/?series=84344&rev=1

> 
> --Jason
> 
> 
>> +
>> +/**
>> + * struct drm_i915_gem_create_ext - Existing gem_create behaviour, with added
>> + * extension support using struct i915_user_extension.
>> + *
>> + * 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
>> +        * struct 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 - The
>> + * 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.
>> + *
>> + * This is provided as an array of struct drm_i915_gem_memory_class_instance, or
>> + * an equivalent layout of class:instance pair encodings. See struct
>> + * drm_i915_query_memory_regions and 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)&region_lmem,
>> + *              .num_regions = 1,
>> + *      };
>> + *      struct drm_i915_gem_create_ext create_ext = {
>> + *              .size = 16 * PAGE_SIZE,
>> + *              .extensions = (uintptr_t)&regions,
>> + *      };
>> + *
>> + *      int err = ioctl(fd, DRM_IOCTL_I915_GEM_CREATE_EXT, &create_ext);
>> + *      if (err) ...
>> + *
>> + * At which point we get the object handle in &drm_i915_gem_create_ext.handle,
>> + * along with the final object size in &drm_i915_gem_create_ext.size, which
>> + * should account for any rounding up, if required.
>> + */
>> +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 @regions array. */
>> +       __u32 num_regions;
>> +       /**
>> +        * @regions: The regions/placements array.
>> +        *
>> +        * An array of struct 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..462f1efd9003
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_gem_lmem.rst
>> @@ -0,0 +1,130 @@
>> +=========================
>> +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. Some of the work items:
>> +        * TTM shrinker for discrete
>> +        * dma_resv_lockitem for full dma_resv_lock, i.e not just trylock
>> +        * Use TTM CPU pagefault handler
>> +        * Route shmem backend over to TTM SYSTEM for discrete
>> +        * TTM purgeable object support
>> +        * Move i915 buddy allocator over to TTM
>> +        * MMAP ioctl mode(see `I915 MMAP`_)
>> +        * SET/GET ioctl caching(see `I915 SET/GET CACHING`_)
>> +* 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:
>> +
>> +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".
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_gem_lmem.h
>> +        :functions: drm_i915_gem_create_ext
>> +
>> +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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