linux-kernel - [PATCH] regmap: flat: introduce register striding to save some memories

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Date:	Mon, 14 Dec 2015 15:14:34 +0800
From:	Xiubo Li <lixiubo@...s.chinamobile.com>
To:	broonie@...nel.org
Cc:	linux-kernel@...r.kernel.org,
	Xiubo Li <lixiubo@...s.chinamobile.com>
Subject: [PATCH] regmap: flat: introduce register striding to save some memories

Throughout the regcache-flat code, for the flat cache array, the
actual register address offsets are used to index the values, and
this will waste some memory spaces.

For example, on 64-BIT platform, device A has three registers:
    register offsets: {0x00, 0x08, 0x10}
    max_register: 0x10
    regsiter striding: 8

And the size of flat cache memory space will be:
    (max_register + 1 ) * sizeof(unsigned int) =
    (0x10 + 1) * sizeof(unsigned int) = 17 * 8 = 136 Bytes

Since map->reg_stride has been introduced and all extant register
addresses are a multiple of this value, it could use the address
offsets divide by the stride to determine the index.

Then the size of flat cache memory space will be:
    (max_register / reg_stride + 1 ) * sizeof(unsigned int) =
    (0x10 / 8 + 1) * sizeof(unsigned int) = 3 * 8 = 24 Bytes

And the bigger of the striding value, there will be more memory
space wasted. After introducing the register striding here can
save some memeories for the system.

Signed-off-by: Xiubo Li <lixiubo@...s.chinamobile.com>
---
 drivers/base/regmap/regcache-flat.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/base/regmap/regcache-flat.c b/drivers/base/regmap/regcache-flat.c
index 686c9e0..160d636 100644
--- a/drivers/base/regmap/regcache-flat.c
+++ b/drivers/base/regmap/regcache-flat.c
@@ -19,17 +19,19 @@
 static int regcache_flat_init(struct regmap *map)
 {
 	int i;
-	unsigned int *cache;
+	unsigned int index, *cache;
 
-	map->cache = kcalloc(map->max_register + 1, sizeof(unsigned int),
-			     GFP_KERNEL);
+	map->cache = kcalloc(map->max_register / map->reg_stride + 1,
+			     sizeof(unsigned int), GFP_KERNEL);
 	if (!map->cache)
 		return -ENOMEM;
 
 	cache = map->cache;
 
-	for (i = 0; i < map->num_reg_defaults; i++)
-		cache[map->reg_defaults[i].reg] = map->reg_defaults[i].def;
+	for (i = 0; i < map->num_reg_defaults; i++) {
+		index =  map->reg_defaults[i].reg / map->reg_stride;
+		cache[index] = map->reg_defaults[i].def;
+	}
 
 	return 0;
 }
@@ -45,9 +47,10 @@ static int regcache_flat_exit(struct regmap *map)
 static int regcache_flat_read(struct regmap *map,
 			      unsigned int reg, unsigned int *value)
 {
+	unsigned int index = reg / map->reg_stride;
 	unsigned int *cache = map->cache;
 
-	*value = cache[reg];
+	*value = cache[index];
 
 	return 0;
 }
@@ -55,9 +58,10 @@ static int regcache_flat_read(struct regmap *map,
 static int regcache_flat_write(struct regmap *map, unsigned int reg,
 			       unsigned int value)
 {
+	unsigned int index = reg / map->reg_stride;
 	unsigned int *cache = map->cache;
 
-	cache[reg] = value;
+	cache[index] = value;
 
 	return 0;
 }
-- 
1.8.3.1



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