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From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     linux-i2c@vger.kernel.org
Cc:     jikos@kernel.org, aaron.ma@canonical.com, admin@kryma.net,
        andriy.shevchenko@linux.intel.com, benjamin.tissoires@redhat.com,
        hdegoede@redhat.com, hn.chen@weidahitech.com,
        jarkko.nikula@linux.intel.com, kai.heng.feng@canonical.com,
        linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        mika.westerberg@linux.intel.com, vicamo.yang@canonical.com,
        wsa@kernel.org, Sultan Alsawaf <sultan@kerneltoast.com>
Subject: [PATCH v2 2/4] i2c: designware: Ensure tx_buf_len is nonzero for SMBus block reads
Date:   Wed, 16 Sep 2020 22:22:54 -0700
Message-Id: <20200917052256.5770-3-sultan@kerneltoast.com>
In-Reply-To: <20200917052256.5770-1-sultan@kerneltoast.com>
References: <20200917052256.5770-1-sultan@kerneltoast.com>
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From: Sultan Alsawaf <sultan@kerneltoast.com>

The point of adding a byte to len in i2c_dw_recv_len() is to make sure
that tx_buf_len is nonzero, so that i2c_dw_xfer_msg() can let the i2c
controller know that the i2c transaction can end. Otherwise, the i2c
controller will think that the transaction can never end for block
reads, which results in the stop-detection bit never being set and thus
the transaction timing out.

Adding a byte to len is not a reliable way to do this though; sometimes
it lets tx_buf_len become zero, which results in the scenario described
above. Therefore, just directly ensure tx_buf_len cannot be zero to fix
the issue.

Fixes: c3ae106050b9 ("i2c: designware: Implement support for SMBus block read and write")
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 drivers/i2c/busses/i2c-designware-master.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
index d78f48ca4886..22f28516bca7 100644
--- a/drivers/i2c/busses/i2c-designware-master.c
+++ b/drivers/i2c/busses/i2c-designware-master.c
@@ -395,8 +395,9 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
 	 * Adjust the buffer length and mask the flag
 	 * after receiving the first byte.
 	 */
-	len += (flags & I2C_CLIENT_PEC) ? 2 : 1;
-	dev->tx_buf_len = len - min_t(u8, len, dev->rx_outstanding);
+	if (flags & I2C_CLIENT_PEC)
+		len++;
+	dev->tx_buf_len = len - min_t(u8, len - 1, dev->rx_outstanding);
 	msgs[dev->msg_read_idx].len = len;
 	msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
 
-- 
2.28.0