ar71xx: allow to specify max read size for m25p80

Reading from the flash chip on the TL-WR2543ND seems buggy.
If the SPI flash driver tries to read too much data in one
SPI transfer, the flash chip returns bogus values. This can
be caused by a buggy flash chip on my board, or it can
be a bug in our SPI driver.

Add a workaround to the m25p80 driver until I find out the
root cause of the problem. The patch allows to specify the
maximum numner of bytes which can be read safely withint
one SPI transfer.

SVN-Revision: 29679

target/linux/ar71xx/patches-2.6.39/113-mtd-m25p80-allow-to-specify-max-read-size.patch

@@ -0,0 +1,112 @@
+--- a/drivers/mtd/devices/m25p80.c
++++ b/drivers/mtd/devices/m25p80.c
+@@ -94,6 +94,7 @@ struct m25p {
+ 	u16			addr_width;
+ 	u8			erase_opcode;
+ 	u8			*command;
++	size_t			max_read_len;
+ };
+ 
+ static inline struct m25p *mtd_to_m25p(struct mtd_info *mtd)
+@@ -341,6 +342,7 @@ static int m25p80_read(struct mtd_info *
+ 	struct m25p *flash = mtd_to_m25p(mtd);
+ 	struct spi_transfer t[2];
+ 	struct spi_message m;
++	loff_t ofs;
+ 
+ 	DEBUG(MTD_DEBUG_LEVEL2, "%s: %s %s 0x%08x, len %zd\n",
+ 			dev_name(&flash->spi->dev), __func__, "from",
+@@ -364,8 +366,6 @@ static int m25p80_read(struct mtd_info *
+ 	t[0].len = m25p_cmdsz(flash) + FAST_READ_DUMMY_BYTE;
+ 	spi_message_add_tail(&t[0], &m);
+ 
+-	t[1].rx_buf = buf;
+-	t[1].len = len;
+ 	spi_message_add_tail(&t[1], &m);
+ 
+ 	/* Byte count starts at zero. */
+@@ -373,13 +373,6 @@ static int m25p80_read(struct mtd_info *
+ 
+ 	mutex_lock(&flash->lock);
+ 
+-	/* Wait till previous write/erase is done. */
+-	if (wait_till_ready(flash)) {
+-		/* REVISIT status return?? */
+-		mutex_unlock(&flash->lock);
+-		return 1;
+-	}
+-
+ 	/* FIXME switch to OPCODE_FAST_READ.  It's required for higher
+ 	 * clocks; and at this writing, every chip this driver handles
+ 	 * supports that opcode.
+@@ -387,11 +380,44 @@ static int m25p80_read(struct mtd_info *
+ 
+ 	/* Set up the write data buffer. */
+ 	flash->command[0] = OPCODE_READ;
+-	m25p_addr2cmd(flash, from, flash->command);
+ 
+-	spi_sync(flash->spi, &m);
++	ofs = 0;
++	while (len) {
++		size_t readlen;
++		size_t done;
++		int ret;
++
++		ret = wait_till_ready(flash);
++		if (ret) {
++			mutex_unlock(&flash->lock);
++			return 1;
++		}
++
++		if (flash->max_read_len > 0 &&
++		    flash->max_read_len < len)
++			readlen = flash->max_read_len;
++		else
++			readlen = len;
++
++		t[1].rx_buf = buf + ofs;
++		t[1].len = readlen;
++
++		m25p_addr2cmd(flash, from + ofs, flash->command);
++
++		spi_sync(flash->spi, &m);
+ 
+-	*retlen = m.actual_length - m25p_cmdsz(flash) - FAST_READ_DUMMY_BYTE;
++		done = m.actual_length - m25p_cmdsz(flash) -
++		       FAST_READ_DUMMY_BYTE;
++		if (done != readlen) {
++			mutex_unlock(&flash->lock);
++			return 1;
++		}
++
++		ofs += done;
++		len -= done;
++	}
++
++	*retlen = ofs;
+ 
+ 	mutex_unlock(&flash->lock);
+ 
+@@ -901,6 +927,12 @@ static int __devinit m25p_probe(struct s
+ 	flash->mtd.erase = m25p80_erase;
+ 	flash->mtd.read = m25p80_read;
+ 
++	if (data && data->max_read_len) {
++		flash->max_read_len = data->max_read_len;
++		dev_warn(&spi->dev, "max_read_len set to %d bytes\n",
++			flash->max_read_len);
++	}
++
+ 	/* sst flash chips use AAI word program */
+ 	if (info->jedec_id >> 16 == 0xbf)
+ 		flash->mtd.write = sst_write;
+--- a/include/linux/spi/flash.h
++++ b/include/linux/spi/flash.h
+@@ -26,6 +26,7 @@ struct flash_platform_data {
+ 	char		*type;
+ 	const char	**part_probes;
+ 
++	size_t		max_read_len;
+ 	/* we'll likely add more ... use JEDEC IDs, etc */
+ };
+