mtd: bcm47xxsflash: support reading flash out of mapping window

For reading flash content we use MMIO but it's possible to read only
first 16 MiB this way. It's simply an arch design/limitation.
To support flash sizes bigger than 16 MiB implement indirect access
using ChipCommon registers.
This has been tested using MX25L25635F.

Signed-off-by: Rafał Miłecki <[e-mail]>
Acked-by: Marek Vasut <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/devices/bcm47xxsflash.c

···
 			      size_t *retlen, u_char *buf)
 {
 	struct bcm47xxsflash *b47s = mtd->priv;
+	size_t orig_len = len;
 
 	/* Check address range */
 	if ((from + len) > mtd->size)
 		return -EINVAL;
 
-	memcpy_fromio(buf, b47s->window + from, len);
-	*retlen = len;
+	/* Read as much as possible using fast MMIO window */
+	if (from < BCM47XXSFLASH_WINDOW_SZ) {
+		size_t memcpy_len;
 
-	return len;
+		memcpy_len = min(len, (size_t)(BCM47XXSFLASH_WINDOW_SZ - from));
+		memcpy_fromio(buf, b47s->window + from, memcpy_len);
+		from += memcpy_len;
+		len -= memcpy_len;
+		buf += memcpy_len;
+	}
+
+	/* Use indirect access for content out of the window */
+	for (; len; len--) {
+		b47s->cc_write(b47s, BCMA_CC_FLASHADDR, from++);
+		bcm47xxsflash_cmd(b47s, OPCODE_ST_READ4B);
+		*buf++ = b47s->cc_read(b47s, BCMA_CC_FLASHDATA);
+	}
+
+	*retlen = orig_len;
+
+	return orig_len;
 }
 
 static int bcm47xxsflash_write_st(struct mtd_info *mtd, u32 offset, size_t len,

drivers/mtd/devices/bcm47xxsflash.h

···
 
 #include <linux/mtd/mtd.h>
 
+#define BCM47XXSFLASH_WINDOW_SZ			SZ_16M
+
 /* Used for ST flashes only. */
 #define OPCODE_ST_WREN		0x0006		/* Write Enable */
 #define OPCODE_ST_WRDIS		0x0004		/* Write Disable */
···
 #define OPCODE_ST_RES		0x03ab		/* Read Electronic Signature */
 #define OPCODE_ST_CSA		0x1000		/* Keep chip select asserted */
 #define OPCODE_ST_SSE		0x0220		/* Sub-sector Erase */
+#define OPCODE_ST_READ4B	0x6313		/* Read Data Bytes in 4Byte addressing mode */
 
 /* Used for Atmel flashes only. */
 #define OPCODE_AT_READ				0x07e8