[PATCH 38/39] drm/i915/cmdparser: Use binary search for faster register lookup

[Chris Wilson]

A significant proportion of the cmdparsing time for some batches is the
cost to find the register in the mmiotable. We ensure that those tables
are in ascending order such that we could do a binary search if it was
ever merited. It is.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld at intel.com>
---
 drivers/gpu/drm/i915/i915_cmd_parser.c | 42 ++++++++++++++++------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
index 690aaf51d715..e128e3ab8452 100644
--- a/drivers/gpu/drm/i915/i915_cmd_parser.c
+++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
@@ -921,36 +921,37 @@ find_cmd(struct intel_engine_cs *engine,
 }
 
 static const struct drm_i915_reg_descriptor *
-find_reg(const struct drm_i915_reg_descriptor *table,
-	 int count, u32 addr)
+__find_reg(const struct drm_i915_reg_descriptor *table, int count, u32 addr)
 {
-	int i;
-
-	for (i = 0; i < count; i++) {
-		if (i915_mmio_reg_offset(table[i].addr) == addr)
-			return &table[i];
+	int start = 0, end = count;
+	while (start < end) {
+		int mid = start + (end - start) / 2;
+		int ret = addr - i915_mmio_reg_offset(table[mid].addr);
+		if (ret < 0)
+			end = mid;
+		else if (ret > 0)
+			start = mid + 1;
+		else
+			return &table[mid];
 	}
-
 	return NULL;
 }
 
 static const struct drm_i915_reg_descriptor *
-find_reg_in_tables(const struct drm_i915_reg_table *tables,
-		   int count, bool is_master, u32 addr)
+find_reg(const struct intel_engine_cs *engine, bool is_master, u32 addr)
 {
-	int i;
-	const struct drm_i915_reg_table *table;
-	const struct drm_i915_reg_descriptor *reg;
+	const struct drm_i915_reg_table *table = engine->reg_tables;
+	int count = engine->reg_table_count;
 
-	for (i = 0; i < count; i++) {
-		table = &tables[i];
+	do {
 		if (!table->master || is_master) {
-			reg = find_reg(table->regs, table->num_regs,
-				       addr);
+			const struct drm_i915_reg_descriptor *reg;
+
+			reg = __find_reg(table->regs, table->num_regs, addr);
 			if (reg != NULL)
 				return reg;
 		}
-	}
+	} while (table++, --count);
 
 	return NULL;
 }
@@ -1073,10 +1074,7 @@ static bool check_cmd(const struct intel_engine_cs *engine,
 		     offset += step) {
 			const u32 reg_addr = cmd[offset] & desc->reg.mask;
 			const struct drm_i915_reg_descriptor *reg =
-				find_reg_in_tables(engine->reg_tables,
-						   engine->reg_table_count,
-						   is_master,
-						   reg_addr);
+				find_reg(engine, is_master, reg_addr);
 
 			if (!reg) {
 				DRM_DEBUG_DRIVER("CMD: Rejected register 0x%08X in command: 0x%08X (exec_id=%d)\n",
-- 
2.9.3