linux-kernel - [PATCH 1/2] i2c: ismt: Don't duplicate the receive length for block reads

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Date:   Mon,  7 Aug 2017 17:10:59 -0400
From:   Stephen Douthit <stephend@...engineering.com>
To:     seth.heasley@...el.com, nhorman@...driver.com
Cc:     wsa@...-dreams.de, danp@...engineering.com,
        stephend@...engineering.com, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] i2c: ismt: Don't duplicate the receive length for block reads

According to Table 15-14 of the C2000 EDS (Intel doc #510524) the
rx data pointed to by the descriptor dptr contains the byte count.

desc->rxbytes reports all bytes read on the wire, including the
"byte count" byte.  So if a device sends 4 bytes in response to a
block read, on the wire and in the DMA buffer we see:

count data1 data2 data3 data4
 0x04  0xde  0xad  0xbe  0xef

That's what we want to return in data->block to the next level.

Instead we were actually prefixing that with desc->rxbytes:

bad
count count data1 data2 data3 data4
 0x05  0x04  0xde  0xad  0xbe  0xef

This was discovered while developing a BMC solution relying on the
ipmi_ssif.c driver which was trying to interpret the bogus length
field as part of the IPMI response.

Signed-off-by: Stephen Douthit <stephend@...engineering.com>
Tested-by: Dan Priamo <danp@...engineering.com>
---
 drivers/i2c/busses/i2c-ismt.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-ismt.c b/drivers/i2c/busses/i2c-ismt.c
index e98e44e..9af2337 100644
--- a/drivers/i2c/busses/i2c-ismt.c
+++ b/drivers/i2c/busses/i2c-ismt.c
@@ -341,8 +341,8 @@ static int ismt_process_desc(const struct ismt_desc *desc,
 			break;
 		case I2C_SMBUS_BLOCK_DATA:
 		case I2C_SMBUS_I2C_BLOCK_DATA:
-			memcpy(&data->block[1], dma_buffer, desc->rxbytes);
-			data->block[0] = desc->rxbytes;
+			memcpy(data->block, dma_buffer, desc->rxbytes);
+			data->block[0] = desc->rxbytes - 1;
 			break;
 		}
 		return 0;
-- 
2.7.5