Re: [PATCH v3] i2c: Squash of SMBus block read patchset to save power

From: Jarkko Nikula
Date: Wed Sep 16 2020 - 13:51:26 EST

On 9/15/20 8:48 PM, Sultan Alsawaf wrote:
On Tue, Sep 15, 2020 at 02:55:48PM +0300, Jarkko Nikula wrote:
I tested this on top of fc4f28bb3daf ("Merge tag 'for-5.9-rc5-tag' of
git://") and seems to be
working fine. What was the key change compared to previous version that was
regressing for me?

This change fixed your issue (and my issue with 5.8):
--- 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;

I've attributed this change with the following commit message:
"i2c: designware: Ensure tx_buf_len is nonzero for SMBus block reads

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."

Ok, nice that you found it.

Does the patch series look good to submit?

Yes, go ahead.