drivers: spi_nor: improve documentation and validation on flash layout size

The smallest region that can be erashed is one sector, so setting
SPI_NOR_FLASH_LAYOUT_PAGE_SIZE=2048 will fail at runtime when the
flash page API is used to erase a single (or misaligned) page.  Add a
compile-time check that the requested layout page size is erasable.

Signed-off-by: Peter A. Bigot <[email protected]>

Author: pabigot

drivers/flash/Kconfig.nor

@@ -29,7 +29,8 @@ config SPI_NOR_FLASH_LAYOUT_PAGE_SIZE
 	  When CONFIG_FLASH_PAGE_LAYOUT is used this driver will support
 	  that API.  By default the page size corresponds to the block
 	  size (65536).  Other options include the 32K-byte erase size
-	  (32768), and the sector size (4096).
+	  (32768), the sector size (4096), or any non-zero multiple of the
+	  sector size.
 
 config SPI_NOR_IDLE_IN_DPD
 	bool "Use Deep Power-Down mode when flash is not being accessed."

drivers/flash/spi_nor.c

@@ -528,6 +528,9 @@ static int spi_nor_init(struct device *dev)
 /* instance 0 size in bytes */
 #define INST_0_BYTES (DT_INST_0_JEDEC_SPI_NOR_SIZE / 8)
 
+BUILD_ASSERT_MSG(SPI_NOR_IS_SECTOR_ALIGNED(CONFIG_SPI_NOR_FLASH_LAYOUT_PAGE_SIZE),
+		 "SPI_NOR_FLASH_LAYOUT_PAGE_SIZE must be multiple of 4096");
+
 /* instance 0 page count */
 #define LAYOUT_PAGES_COUNT (INST_0_BYTES / CONFIG_SPI_NOR_FLASH_LAYOUT_PAGE_SIZE)