regmap: cache: Fall back to register by register read for cache defaults

If we are unable to read the cache defaults for a regmap then fall back
on attempting to read them word by word. This is going to be painfully
slow for large regmaps but might be adequate for smaller ones.
Signed-off-by: NMark Brown <broonie@kernel.org>
[maciej: Use cache_bypass around read and skipping of unreadable regs]
Signed-off-by: NMaciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Tested-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/base/regmap/regcache.c

@@ -30,7 +30,7 @@ static int regcache_hw_init(struct regmap *map)
 	int i, j;
 	int ret;
 	int count;
-	unsigned int val;
+	unsigned int reg, val;
 	void *tmp_buf;
 
 	if (!map->num_reg_defaults_raw)
@@ -67,27 +67,46 @@ static int regcache_hw_init(struct regmap *map)
 		ret = regmap_raw_read(map, 0, tmp_buf,
 				      map->num_reg_defaults_raw);
 		map->cache_bypass = cache_bypass;
-		if (ret < 0)
-			goto err_cache_free;
-
+		if (ret == 0) {
 			map->reg_defaults_raw = tmp_buf;
 			map->cache_free = 1;
+		} else {
+			kfree(tmp_buf);
+		}
 	}
 
 	/* fill the reg_defaults */
 	for (i = 0, j = 0; i < map->num_reg_defaults_raw; i++) {
-		if (regmap_volatile(map, i * map->reg_stride))
+		reg = i * map->reg_stride;
+
+		if (!regmap_readable(map, reg))
+			continue;
+
+		if (regmap_volatile(map, reg))
 			continue;
+
+		if (map->reg_defaults_raw) {
 			val = regcache_get_val(map, map->reg_defaults_raw, i);
-		map->reg_defaults[j].reg = i * map->reg_stride;
+		} else {
+			bool cache_bypass = map->cache_bypass;
+
+			map->cache_bypass = true;
+			ret = regmap_read(map, reg, &val);
+			map->cache_bypass = cache_bypass;
+			if (ret != 0) {
+				dev_err(map->dev, "Failed to read %d: %d\n",
+					reg, ret);
+				goto err_free;
+			}
+		}
+
+		map->reg_defaults[j].reg = reg;
 		map->reg_defaults[j].def = val;
 		j++;
 	}
 
 	return 0;
 
-err_cache_free:
-	kfree(tmp_buf);
 err_free:
 	kfree(map->reg_defaults);