mtd: map_rom: Support UBI on ROM

UBI needs to know the physical erase block size, even on read-only
devices, since it defines the on-device layout. Use a device-tree
provided value to support previously written UBI on read-only NOR.

UBI also needs a non-zero writebufsize, so we set it to one.

Note: This was implemented because hardware write-protected CFI
NOR cannot be probed for the physical erase block size.

Signed-off-by: Joe Schultz <[e-mail]>
Signed-off-by: Aaron Sierra <[e-mail]>
[Brian: removed unneeded #ifdef, note 'optional' erase-size property]
Signed-off-by: Brian Norris <[e-mail]>

Documentation/devicetree/bindings/mtd/mtd-physmap.txt

···
  - vendor-id : Contains the flash chip's vendor id (1 byte).
  - device-id : Contains the flash chip's device id (1 byte).
 
+For ROM compatible devices (and ROM fallback from cfi-flash), the following
+additional (optional) property is defined:
+
+ - erase-size : The chip's physical erase block size in bytes.
+
 The device tree may optionally contain sub-nodes describing partitions of the
 address space. See partition.txt for more detail.
 

drivers/mtd/chips/map_rom.c

···
 #include <linux/errno.h>
 #include <linux/slab.h>
 #include <linux/init.h>
+#include <linux/of.h>
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/map.h>
 
···
 	.name	= "map_rom",
 	.module	= THIS_MODULE
 };
+
+static unsigned int default_erasesize(struct map_info *map)
+{
+	const __be32 *erase_size = NULL;
+
+	erase_size = of_get_property(map->device_node, "erase-size", NULL);
+
+	return !erase_size ? map->size : be32_to_cpu(*erase_size);
+}
 
 static struct mtd_info *map_rom_probe(struct map_info *map)
 {
···
 	mtd->_sync = maprom_nop;
 	mtd->_erase = maprom_erase;
 	mtd->flags = MTD_CAP_ROM;
-	mtd->erasesize = map->size;
+	mtd->erasesize = default_erasesize(map);
 	mtd->writesize = 1;
+	mtd->writebufsize = 1;
 
 	__module_get(THIS_MODULE);
 	return mtd;