[Intel-gfx] [PATCH 1/1] drm/i915: Version the MOCS settings

Ben Widawsky ben at bwidawsk.net
Thu Jul 6 23:27:01 UTC 2017


From: Ben Widawsky <benjamin.widawsky at intel.com>

Starting with GEN9, Memory Object Control State (MOCS) becomes an index
into a table as opposed to the direct programming within the command.
The table has 62 usable entries (ie 6 bits can represent all settings),
and each buffer type may use one of these 62 entries to describe
cacheability type, and age (and some other less useful fields).

Because we hadn't dealt with MOCS settings like this, we didn't think
ahead too well and have ended up with a mess for GEN9 (and soon GEN10)
platform. The plan for for future platforms is that the ideal MOCS
settings will be determined, defined, and written in the public PRMs.
After this point, the i915.ko will absorb these settings and sometime
afterwards flip the alpha switch. All driver releases without the final
MOCS table must be considered alpha. Here on, userspace can assume the
MOCS table is definitively done. There will be some reserved entries for
'oh shit' scenarios. This avoids versioning the MOCS table which leaves
somewhat of a mess in userspace trying to handle arbitrarily many MOCS
versions.

But we do have a mess on GEN9. In the beginning, the MOCS table entries
were pre-populated by the hardware based on estimations made prior to
tapeout and we could just use that. Subsequently much performance tuning
was done to determine optimal settings that the i915 driver should load
on top of the hardware defaults. That was posted last as v6 of the
original per-engine MOCS settings:
https://patchwork.freedesktop.org/patch/53237/. Since the MOCS table is
not context saved/restored, it isn't feasible to let userspace upload
its own MOCS table. After a good amount of debate, it was decided that
we'd utilize only the minimal set of entires in mesa anyway, and so we
took only those entries for our MOCS entries.

Now we've come to the realization that indeed there are other MOCS
entries which are more optimal for various buffer types and workloads.
The problem is that the meaning of the indices is ABI (we assume index 0
is the uncached entry, and that there are only 3 entries total).

What this patch [simply] aims to do is expose a parameter to inform
userspace which "version" of the table was loaded by i915. Upon
sufficient data, new entries can be added, and the version can be
bumped. For example, from my original mesa mocs branch:

commit c9b0481bce24af032386701de0266eb5bc24e988
Author: Ben Widawsky <benjamin.widawsky at intel.com>
Date:   Fri Apr 8 10:21:16 2016 -0700

    i965: Use PTE mocs

    Signed-off-by: Ben Widawsky <benjamin.widawsky at intel.com>

diff --git a/src/mesa/drivers/dri/i965/brw_mocs.c b/src/mesa/drivers/dri/i965/brw_mocs.c
index 5df154eb86..b7bfdab671 100644
--- a/src/mesa/drivers/dri/i965/brw_mocs.c
+++ b/src/mesa/drivers/dri/i965/brw_mocs.c
@@ -14,6 +14,9 @@
 /* Skylake: MOCS is now an index into an array of 62 different caching
  * configurations programmed by the kernel.
  */
+
+/* TC=PTE, LeCC=PTE, LRUM=3, L3CC=WB */
+#define SKL_MOCS_PTE_PTE (3 << 1)
 /* TC=LLC/eLLC, LeCC=WB, LRUM=3, L3CC=WB */
 #define SKL_MOCS_WB  (2 << 1)
 /* TC=LLC/eLLC, LeCC=PTE, LRUM=3, L3CC=WB */
@@ -26,6 +29,9 @@ brw_mocs_get_control_state(const struct brw_context *brw,
    switch (brw->gen) {
    default:
    case 9:
+      if (brw->intelScreen->mocs_version > 1)
+         return SKL_MOCS_PTE_PTE;
+
       return type == INTEL_MOCS_PTE ? SKL_MOCS_PTE : SKL_MOCS_WB;
    case 8:
       return type == INTEL_MOCS_PTE ? BDW_MOCS_PTE : BDW_MOCS_WB;

tl;dr: A versioned MOCS table will allow userspace to be aware of new
and potentially interesting cacheability settings. Next GEN platforms
will not be considered production worthy until the MOCS table is
finalized.

v2: Update 1.5 year old patch. Add comments. Update commit message.

Signed-off-by: Ben Widawsky <benjamin.widawsky at intel.com>
---
 drivers/gpu/drm/i915/i915_drv.c |  3 +++
 drivers/gpu/drm/i915/i915_drv.h |  2 ++
 drivers/gpu/drm/i915/i915_pci.c | 13 +++++++++----
 include/uapi/drm/i915_drm.h     |  8 ++++++++
 4 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 9167a73f3c69..26c27b6ae814 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -401,6 +401,9 @@ static int i915_getparam(struct drm_device *dev, void *data,
 		if (!value)
 			return -ENODEV;
 		break;
+	case I915_PARAM_MOCS_TABLE_VERSION:
+		value = INTEL_INFO(dev_priv)->mocs_version;
+		break;
 	default:
 		DRM_DEBUG("Unknown parameter %d\n", param->param);
 		return -EINVAL;
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index effbe4f72a64..9b30f6e6ef9b 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -859,6 +859,8 @@ struct intel_device_info {
 		u16 degamma_lut_size;
 		u16 gamma_lut_size;
 	} color;
+
+	u8 mocs_version;
 };
 
 struct intel_display_error_state;
diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
index 04aaf553e3fa..9697eb91d972 100644
--- a/drivers/gpu/drm/i915/i915_pci.c
+++ b/drivers/gpu/drm/i915/i915_pci.c
@@ -356,7 +356,8 @@ static const struct intel_device_info intel_cherryview_info = {
 	.platform = INTEL_SKYLAKE, \
 	.has_csr = 1, \
 	.has_guc = 1, \
-	.ddb_size = 896
+	.ddb_size = 896, \
+	.mocs_version = MOCS_TABLE_VERSION
 
 static const struct intel_device_info intel_skylake_info = {
 	SKL_PLATFORM,
@@ -390,6 +391,7 @@ static const struct intel_device_info intel_skylake_gt3_info = {
 	.has_full_ppgtt = 1, \
 	.has_full_48bit_ppgtt = 1, \
 	.has_reset_engine = 1, \
+	.mocs_version = MOCS_TABLE_VERSION, \
 	GEN_DEFAULT_PIPEOFFSETS, \
 	IVB_CURSOR_OFFSETS, \
 	BDW_COLORS
@@ -413,7 +415,8 @@ static const struct intel_device_info intel_geminilake_info = {
 	.platform = INTEL_KABYLAKE, \
 	.has_csr = 1, \
 	.has_guc = 1, \
-	.ddb_size = 896
+	.ddb_size = 896, \
+	.mocs_version = MOCS_TABLE_VERSION
 
 static const struct intel_device_info intel_kabylake_info = {
 	KBL_PLATFORM,
@@ -431,7 +434,8 @@ static const struct intel_device_info intel_kabylake_gt3_info = {
 	.platform = INTEL_COFFEELAKE, \
 	.has_csr = 1, \
 	.has_guc = 1, \
-	.ddb_size = 896
+	.ddb_size = 896, \
+	.mocs_version = MOCS_TABLE_VERSION
 
 static const struct intel_device_info intel_coffeelake_info = {
 	CFL_PLATFORM,
@@ -448,7 +452,8 @@ static const struct intel_device_info intel_cannonlake_info = {
 	.platform = INTEL_CANNONLAKE,
 	.gen = 10,
 	.ddb_size = 1024,
-	.has_csr = 1,
+	.has_csr = 1, \
+	.mocs_version = MOCS_TABLE_VERSION
 };
 
 /*
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 7ccbd6a2bbe0..2e370c40e48d 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -431,6 +431,14 @@ typedef struct drm_i915_irq_wait {
  */
 #define I915_PARAM_HAS_EXEC_BATCH_FIRST	 48
 
+/* What version of the MOCS table we have. For GEN9 GPUs, the PRM defined
+ * non-optimal settings for the MOCS table. As a result, we were required to use a
+ * small subset, and later add new settings. This param allows userspace to
+ * determine which settings are there.
+ */
+#define MOCS_TABLE_VERSION		  1 /* Build time MOCS table version */
+#define I915_PARAM_MOCS_TABLE_VERSION	 49
+
 typedef struct drm_i915_getparam {
 	__s32 param;
 	/*
-- 
2.13.2



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