drm/i915: Drop I915_CONTEXT_PARAM_RINGSIZE

This reverts commit 88be76cdafc7 ("drm/i915: Allow userspace to specify
ringsize on construction").  This API was originally added for OpenCL
but the compute-runtime PR has sat open for a year without action so we
can still pull it out if we want.  I argue we should drop it for three
reasons:

 1. If the compute-runtime PR has sat open for a year, this clearly
    isn't that important.

 2. It's a very leaky API.  Ring size is an implementation detail of the
    current execlist scheduler and really only makes sense there.  It
    can't apply to the older ring-buffer scheduler on pre-execlist
    hardware because that's shared across all contexts and it won't
    apply to the GuC scheduler that's in the pipeline.

 3. Having userspace set a ring size in bytes is a bad solution to the
    problem of having too small a ring.  There is no way that userspace
    has the information to know how to properly set the ring size so
    it's just going to detect the feature and always set it to the
    maximum of 512K.  This is what the compute-runtime PR does.  The
    scheduler in i915, on the other hand, does have the information to
    make an informed choice.  It could detect if the ring size is a
    problem and grow it itself.  Or, if that's too hard, we could just
    increase the default size from 16K to 32K or even 64K instead of
    relying on userspace to do it.

Let's drop this API for now and, if someone decides they really care
about solving this problem, they can do it properly.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-2-jason@jlekstrand.net

1 files changed, 2 insertions, 18 deletions

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 2f70c48567c0..f229c0abcbb5 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1722,24 +1722,8 @@ struct drm_i915_gem_context_param {
  */
 #define I915_CONTEXT_PARAM_PERSISTENCE	0xb
 
-/*
- * I915_CONTEXT_PARAM_RINGSIZE:
- *
- * Sets the size of the CS ringbuffer to use for logical ring contexts. This
- * applies a limit of how many batches can be queued to HW before the caller
- * is blocked due to lack of space for more commands.
- *
- * Only reliably possible to be set prior to first use, i.e. during
- * construction. At any later point, the current execution must be flushed as
- * the ring can only be changed while the context is idle. Note, the ringsize
- * can be specified as a constructor property, see
- * I915_CONTEXT_CREATE_EXT_SETPARAM, but can also be set later if required.
- *
- * Only applies to the current set of engine and lost when those engines
- * are replaced by a new mapping (see I915_CONTEXT_PARAM_ENGINES).
- *
- * Must be between 4 - 512 KiB, in intervals of page size [4 KiB].
- * Default is 16 KiB.
+/* This API has been removed.  On the off chance someone somewhere has
+ * attempted to use it, never re-use this context param number.
  */
 #define I915_CONTEXT_PARAM_RINGSIZE	0xc
 /* Must be kept compact -- no holes and well documented */