drivers: spi: allow limiting reads

For some SPI controllers it's not possible to keep the CS active between
transfers and they are limited to a known number of bytes.
This splits spi_flash reads into different iterations in order to respect
the SPI controller limits.
Signed-off-by: NÁlvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NDaniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: NJagan Teki <jagan@openedev.com>

drivers/mtd/spi/spi_flash.c

@@ -516,6 +516,9 @@ int spi_flash_cmd_read_ops(struct spi_flash *flash, u32 offset,
 		else
 			read_len = remain_len;
 
+		if (spi->max_read_size)
+			read_len = min(read_len, spi->max_read_size);
+
 		spi_flash_addr(read_addr, cmd);
 
 		ret = spi_flash_read_common(flash, cmd, cmdsz, data, read_len);

include/spi.h

@@ -86,6 +86,8 @@ struct dm_spi_slave_platdata {
  * @cs:			ID of the chip select connected to the slave.
  * @mode:		SPI mode to use for this slave (see SPI mode flags)
  * @wordlen:		Size of SPI word in number of bits
+ * @max_read_size:	If non-zero, the maximum number of bytes which can
+ *			be read at once.
  * @max_write_size:	If non-zero, the maximum number of bytes which can
  *			be written at once, excluding command bytes.
  * @memory_map:		Address of read-only SPI flash access.
@@ -102,6 +104,7 @@ struct spi_slave {
 #endif
 	uint mode;
 	unsigned int wordlen;
+	unsigned int max_read_size;
 	unsigned int max_write_size;
 	void *memory_map;