[PATCH 7/7] eeprom: at25: Split reads into chunks and cap write size

[Eddie James <eajames@linux.ibm.com>]

From: Brad Bishop <bradleyb@fuzziesquirrel.com>

Make use of spi_max_transfer_size to avoid requesting transfers that are
too large for some spi controllers.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
 drivers/misc/eeprom/at25.c | 94 ++++++++++++++++++++++----------------
 1 file changed, 54 insertions(+), 40 deletions(-)

diff --git a/drivers/misc/eeprom/at25.c b/drivers/misc/eeprom/at25.c
index ed8d38b09925..30f0704ab2d4 100644
--- a/drivers/misc/eeprom/at25.c
+++ b/drivers/misc/eeprom/at25.c
@@ -64,12 +64,17 @@ static int at25_ee_read(void *priv, unsigned int offset,
 {
 	struct at25_data *at25 = priv;
 	char *buf = val;
+	size_t max_chunk = spi_max_transfer_size(at25->spi);
+	size_t num_msgs = DIV_ROUND_UP(count, max_chunk);
+	size_t			nr_bytes = 0;
 	u8			command[EE_MAXADDRLEN + 1];
 	u8			*cp;
 	ssize_t			status;
 	struct spi_transfer	t[2];
 	struct spi_message	m;
 	u8			instr;
+	unsigned int		msg_offset;
+	size_t			msg_count;
 
 	if (unlikely(offset >= at25->chip.byte_len))
 		return -EINVAL;
@@ -78,57 +83,64 @@ static int at25_ee_read(void *priv, unsigned int offset,
 	if (unlikely(!count))
 		return -EINVAL;
 
-	cp = command;
-
-	instr = AT25_READ;
-	if (at25->chip.flags & EE_INSTR_BIT3_IS_ADDR)
-		if (offset >= (1U << (at25->addrlen * 8)))
-			instr |= AT25_INSTR_BIT3;
-	*cp++ = instr;
-
-	/* 8/16/24-bit address is written MSB first */
-	switch (at25->addrlen) {
-	default:	/* case 3 */
-		*cp++ = offset >> 16;
-		fallthrough;
-	case 2:
-		*cp++ = offset >> 8;
-		fallthrough;
-	case 1:
-	case 0:	/* can't happen: for better codegen */
-		*cp++ = offset >> 0;
-	}
+	msg_offset = (unsigned int)offset;
+	msg_count = min(count, max_chunk);
+	while (num_msgs) {
+		cp = command;
+
+		instr = AT25_READ;
+		if (at25->chip.flags & EE_INSTR_BIT3_IS_ADDR)
+			if (msg_offset >= (1U << (at25->addrlen * 8)))
+				instr |= AT25_INSTR_BIT3;
+		*cp++ = instr;
+
+		/* 8/16/24-bit address is written MSB first */
+		switch (at25->addrlen) {
+		default:	/* case 3 */
+			*cp++ = msg_offset >> 16;
+			fallthrough;
+		case 2:
+			*cp++ = msg_offset >> 8;
+			fallthrough;
+		case 1:
+		case 0:	/* can't happen: for better codegen */
+			*cp++ = msg_offset >> 0;
+		}
 
-	spi_message_init(&m);
-	memset(t, 0, sizeof(t));
+		spi_message_init(&m);
+		memset(t, 0, sizeof(t));
 
-	t[0].tx_buf = command;
-	t[0].len = at25->addrlen + 1;
-	spi_message_add_tail(&t[0], &m);
+		t[0].tx_buf = command;
+		t[0].len = at25->addrlen + 1;
+		spi_message_add_tail(&t[0], &m);
 
-	t[1].rx_buf = buf;
-	t[1].len = count;
-	spi_message_add_tail(&t[1], &m);
+		t[1].rx_buf = buf + nr_bytes;
+		t[1].len = msg_count;
+		spi_message_add_tail(&t[1], &m);
 
-	mutex_lock(&at25->lock);
+		mutex_lock(&at25->lock);
 
-	/* Read it all at once.
-	 *
-	 * REVISIT that's potentially a problem with large chips, if
-	 * other devices on the bus need to be accessed regularly or
-	 * this chip is clocked very slowly
-	 */
-	status = spi_sync(at25->spi, &m);
-	dev_dbg(&at25->spi->dev, "read %zu bytes at %d --> %zd\n",
-		count, offset, status);
+		status = spi_sync(at25->spi, &m);
 
-	mutex_unlock(&at25->lock);
-	return status;
+		mutex_unlock(&at25->lock);
+
+		if (status)
+			return status;
+
+		--num_msgs;
+		msg_offset += msg_count;
+		nr_bytes += msg_count;
+	}
+
+	dev_dbg(&at25->spi->dev, "read %zu bytes at %d\n",
+		count, offset);
+	return 0;
 }
 
 static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
 {
 	struct at25_data *at25 = priv;
+	size_t maxsz = spi_max_transfer_size(at25->spi);
 	const char *buf = val;
 	int			status = 0;
 	unsigned		buf_size;
@@ -191,6 +203,8 @@ static int at25_ee_write(void *priv, unsigned int off, void *val, size_t count)
 		segment = buf_size - (offset % buf_size);
 		if (segment > count)
 			segment = count;
+		if (segment > maxsz)
+			segment = maxsz;
 		memcpy(cp, buf, segment);
 		status = spi_write(at25->spi, bounce,
 				segment + at25->addrlen + 1);
-- 
2.26.2