[Mesa-dev] [PATCH 2/4] i965: Document the sad story of the kernel command parser.

[Kenneth Graunke]

This should help us figure out the complexities of which kernel
versions we need to get various features on various platforms.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/intel_screen.c | 97 ++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/intel_screen.c b/src/mesa/drivers/dri/i965/intel_screen.c
index 225a387fa90..d952062e27b 100644
--- a/src/mesa/drivers/dri/i965/intel_screen.c
+++ b/src/mesa/drivers/dri/i965/intel_screen.c
@@ -1746,6 +1746,103 @@ __DRIconfig **intelInitScreen2(__DRIscreen *dri_screen)
       screen->subslice_total = 1 << (devinfo->gt - 1);
    }
 
+   /* Gen7-7.5 kernel requirements / command parser saga:
+    *
+    * - pre-v3.16:
+    *   Haswell and Baytrail cannot use any privileged batchbuffer features.
+    *
+    *   Ivybridge has aliasing PPGTT on by default, which accidentally marks
+    *   all batches secure, allowing them to use any feature with no checking.
+    *   This is effectively equivalent to a command parser version of
+    *   \infinity - everything is possible.
+    *
+    *   The command parser does not exist, and querying the version will
+    *   return -EINVAL.
+    *
+    * - v3.16:
+    *   The kernel enables the command parser by default, for systems with
+    *   aliasing PPGTT enabled (Ivybridge and Haswell).  However, the
+    *   hardware checker is still enabled, so Haswell and Baytrail cannot
+    *   do anything.
+    *
+    *   Ivybridge goes from "everything is possible" to "only what the
+    *   command parser allows" (if the user boots with i915.cmd_parser=0,
+    *   then everything is possible again).  We can only safely use features
+    *   allowed by the supported command parser version.
+    *
+    *   Annoyingly, I915_PARAM_CMD_PARSER_VERSION reports the static version
+    *   implemented by the kernel, even if it's turned off.  So, checking
+    *   for version > 0 does not mean that you can write registers.  We have
+    *   to try it and see.  The version does, however, indicate the age of
+    *   the kernel.
+    *
+    *   Instead of matching the hardware checker's behavior of converting
+    *   privileged commands to MI_NOOP, it makes execbuf2 start returning
+    *   -EINVAL, making it dangerous to try and use privileged features.
+    *
+    *   Effective command parser versions:
+    *   - Haswell:   0 (reporting 1, writes don't work)
+    *   - Baytrail:  0 (reporting 1, writes don't work)
+    *   - Ivybridge: 1 (enabled) or infinite (disabled)
+    *
+    * - v3.17:
+    *   Baytrail aliasing PPGTT is enabled, making it like Ivybridge:
+    *   effectively version 1 (enabled) or infinite (disabled).
+    *
+    * - v3.19: f1f55cc0556031c8ee3fe99dae7251e78b9b653b
+    *   Command parser v2 supports predicate writes.
+    *
+    *   - Haswell:   0 (reporting 1, writes don't work)
+    *   - Baytrail:  2 (enabled) or infinite (disabled)
+    *   - Ivybridge: 2 (enabled) or infinite (disabled)
+    *
+    *   So version >= 2 is enough to know that Ivybridge and Baytrail
+    *   will work.  Haswell still can't do anything.
+    *
+    * - v4.0: Version 3 happened.  Largely not relevant.
+    *
+    * - v4.1: 6702cf16e0ba8b0129f5aa1b6609d4e9c70bc13b
+    *   L3 config registers are properly saved and restored as part
+    *   of the hardware context.  We can approximately detect this point
+    *   in time by checking if I915_PARAM_REVISION is recognized - it
+    *   landed in a later commit, but in the same release cycle.
+    *
+    * - v4.2: 245054a1fe33c06ad233e0d58a27ec7b64db9284
+    *   Command parser finally gains secure batch promotion.  On Haswell,
+    *   the hardware checker gets disabled, which finally allows it to do
+    *   privileged commands.
+    *
+    *   I915_PARAM_CMD_PARSER_VERSION reports 3.  Effective versions:
+    *   - Haswell:   3 (enabled) or 0 (disabled)
+    *   - Baytrail:  3 (enabled) or infinite (disabled)
+    *   - Ivybridge: 3 (enabled) or infinite (disabled)
+    *
+    *   Unfortunately, detecting this point in time is tricky, because
+    *   no version bump happened when this important change occurred.
+    *   On Haswell, if we can write any register, then the kernel is at
+    *   least this new, and we can start trusting the version number.
+    *
+    * - v4.4: 2bbe6bbb0dc94fd4ce287bdac9e1bd184e23057b and
+    *   Command parser reaches version 4, allowing access to Haswell
+    *   atomic scratch and chicken3 registers.  If version >= 4, we know
+    *   the kernel is new enough to support privileged features on all
+    *   hardware.  However, the user might have disabled it...and the
+    *   kernel will still report version 4.  So we still have to guess
+    *   and check.
+    *
+    * - v4.4: 7b9748cb513a6bef4af87b79f0da3ff7e8b56cd8
+    *   Command parser v5 whitelists indirect compute shader dispatch
+    *   registers, needed for OpenGL 4.3 and later.
+    *
+    * - v4.8:
+    *   Command parser v7 lets us use MI_MATH on Haswell.
+    *
+    *   Additionally, the kernel begins reporting version 0 when
+    *   the command parser is disabled, allowing us to skip the
+    *   guess-and-check step on Haswell.  Unfortunately, this also
+    *   means that we can no longer use it as an indicator of the
+    *   age of the kernel.
+    */
    if (intel_detect_pipelined_so(screen))
       screen->kernel_features |= KERNEL_ALLOWS_SOL_OFFSET_WRITES;
 
-- 
2.12.0