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Commit d6a4ead7 authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915/cmdparser: Improve hash function 



The existing code's hashfunction is very suboptimal (most 3D commands
use the same bucket degrading the hash to a long list). The code even
acknowledge that the issue was known and the fix simple:

/*
 * If we attempt to generate a perfect hash, we should be able to look at bits
 * 31:29 of a command from a batch buffer and use the full mask for that
 * client. The existing INSTR_CLIENT_MASK/SHIFT defines can be used for this.
 */

Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: default avatarMatthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-35-chris@chris-wilson.co.uk
parent ed13033f

drivers/gpu/drm/i915/i915_cmd_parser.c

+31 −20
Original line number Diff line number Diff line
@@ -86,24 +86,24 @@ 
 * general bitmasking mechanism.

 */



#define STD_MI_OPCODE_MASK  0xFF800000

#define STD_3D_OPCODE_MASK  0xFFFF0000

#define STD_2D_OPCODE_MASK  0xFFC00000

#define STD_MFX_OPCODE_MASK 0xFFFF0000

#define STD_MI_OPCODE_SHIFT  (32 - 9)

#define STD_3D_OPCODE_SHIFT  (32 - 16)

#define STD_2D_OPCODE_SHIFT  (32 - 10)

#define STD_MFX_OPCODE_SHIFT (32 - 16)



#define CMD(op, opm, f, lm, fl, ...)				\

	{							\

		.flags = (fl) | ((f) ? CMD_DESC_FIXED : 0),	\

		.cmd = { (op), (opm) },				\

		.cmd = { (op), ~0u << (opm) },			\

		.length = { (lm) },				\

		__VA_ARGS__					\

	}



/* Convenience macros to compress the tables */

#define SMI STD_MI_OPCODE_MASK

#define S3D STD_3D_OPCODE_MASK

#define S2D STD_2D_OPCODE_MASK

#define SMFX STD_MFX_OPCODE_MASK

#define SMI STD_MI_OPCODE_SHIFT

#define S3D STD_3D_OPCODE_SHIFT

#define S2D STD_2D_OPCODE_SHIFT

#define SMFX STD_MFX_OPCODE_SHIFT

#define F true

#define S CMD_DESC_SKIP

#define R CMD_DESC_REJECT
@@ -696,12 +696,26 @@ struct cmd_node { 
 * non-opcode bits being set. But if we don't include those bits, some 3D

 * commands may hash to the same bucket due to not including opcode bits that

 * make the command unique. For now, we will risk hashing to the same bucket.

 *

 * If we attempt to generate a perfect hash, we should be able to look at bits

 * 31:29 of a command from a batch buffer and use the full mask for that

 * client. The existing INSTR_CLIENT_MASK/SHIFT defines can be used for this.

 */

#define CMD_HASH_MASK STD_MI_OPCODE_MASK

static inline u32 cmd_header_key(u32 x)

{

	u32 shift;



	switch (x >> INSTR_CLIENT_SHIFT) {

	default:

	case INSTR_MI_CLIENT:

		shift = STD_MI_OPCODE_SHIFT;

		break;

	case INSTR_RC_CLIENT:

		shift = STD_3D_OPCODE_SHIFT;

		break;

	case INSTR_BC_CLIENT:

		shift = STD_2D_OPCODE_SHIFT;

		break;

	}



	return x >> shift;

}



static int init_hash_table(struct intel_engine_cs *engine,

			   const struct drm_i915_cmd_table *cmd_tables,
@@ -725,7 +739,7 @@ static int init_hash_table(struct intel_engine_cs *engine, 


			desc_node->desc = desc;

			hash_add(engine->cmd_hash, &desc_node->node,

				 desc->cmd.value & CMD_HASH_MASK);

				 cmd_header_key(desc->cmd.value));

		}

	}
@@ -859,12 +873,9 @@ find_cmd_in_table(struct intel_engine_cs *engine, 
	struct cmd_node *desc_node;



	hash_for_each_possible(engine->cmd_hash, desc_node, node,

			       cmd_header & CMD_HASH_MASK) {

			       cmd_header_key(cmd_header)) {

		const struct drm_i915_cmd_descriptor *desc = desc_node->desc;

		u32 masked_cmd = desc->cmd.mask & cmd_header;

		u32 masked_value = desc->cmd.value & desc->cmd.mask;



		if (masked_cmd == masked_value)

		if (((cmd_header ^ desc->cmd.value) & desc->cmd.mask) == 0)

			return desc;

	}