[PATCH v2] i2c: designware: preserve invalid SMBus block count

[Baek Kyu, Kim]

When a slave reports an invalid SMBus block count, i2c_dw_read() uses
one extra read to reach STOP. It currently replaces the received count
with 1 for scheduling and also stores 1 in the message buffer. A count
above 32 is therefore exposed as a valid one-byte response.

Keep the received byte in the buffer and limit only the count passed to
i2c_dw_recv_len(). This preserves the extra read for STOP while retaining
the invalid count for the caller to reject.

The DesignWare receive functions were exercised with stubbed register
input for block counts 0, 1, 32, 33 and 255, with PEC both off and on.
The original code replaced invalid counts in the message buffer with 1;
v2 retains the received byte while scheduling the same extra read.
The arm64 i2c-designware-master.o build passed with W=1.
No physical controller or end-to-end SMBus result was measured.

Fixes: 69f035c480d7 ("i2c: designware: Handle invalid SMBus block data response length value")
Signed-off-by: Baek Kyu, Kim <pocj8ur4in@xxxxxxxxx>
---
Changes in v2:
  - Use a separate count variable for the extra read, as Mika Westerberg suggested.

Link to v1: https://marc.info/?l=linux-i2c&m=179344386454087&w=2

 drivers/i2c/busses/i2c-designware-master.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
index a1bcc3797e4ff..49e9e36340c84 100644
--- a/drivers/i2c/busses/i2c-designware-master.c
+++ b/drivers/i2c/busses/i2c-designware-master.c
@@ -542,8 +542,9 @@ i2c_dw_read(struct dw_i2c_dev *dev)
 		for (; len > 0 && rx_valid > 0; len--, rx_valid--) {
 			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
 			tmp &= DW_IC_DATA_CMD_DAT;
-			/* Ensure length byte is a valid value */
 			if (flags & I2C_M_RECV_LEN) {
+				unsigned int count = tmp;
+
 				/*
 				 * if IC_EMPTYFIFO_HOLD_MASTER_EN is set, which cannot be
 				 * detected from the registers, the controller can be
@@ -553,10 +554,10 @@ i2c_dw_read(struct dw_i2c_dev *dev)
 				 * another byte with STOP bit set when the block data
 				 * response length is invalid to complete the transaction.
 				 */
-				if (!tmp || tmp > I2C_SMBUS_BLOCK_MAX)
-					tmp = 1;
+				if (!count || count > I2C_SMBUS_BLOCK_MAX)
+					count = 1;
 
-				len = i2c_dw_recv_len(dev, tmp);
+				len = i2c_dw_recv_len(dev, count);
 			}
 			*buf++ = tmp;
 			dev->rx_outstanding--;