[PATCH v2 2/2] i2c: microchip-core: fix "ghost" detections

[Conor Dooley <conor@kernel.org>]

From: Conor Dooley <conor.dooley@microchip.com>

Running i2c-detect currently produces an output akin to:
    0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         08 -- 0a -- 0c -- 0e --
10: 10 -- 12 -- 14 -- 16 -- UU 19 -- 1b -- 1d -- 1f
20: -- 21 -- 23 -- 25 -- 27 -- 29 -- 2b -- 2d -- 2f
30: -- -- -- -- -- -- -- -- 38 -- 3a -- 3c -- 3e --
40: 40 -- 42 -- 44 -- 46 -- 48 -- 4a -- 4c -- 4e --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: 60 -- 62 -- 64 -- 66 -- 68 -- 6a -- 6c -- 6e --
70: 70 -- 72 -- 74 -- 76 --

This happens because for an i2c_msg with a len of 0 the driver will
mark the transmission of the message as a success once the START has
been sent, without waiting for the devices on the bus to respond with an
ACK/NAK. Since i2cdetect seems to run in a tight loop over all addresses
the NAK is treated as part of the next test for the next address.

Delete the fast path that marks a message as complete when idev->msg_len
is zero after sending a START/RESTART since this isn't a valid scenario.

CC: stable@vger.kernel.org
Fixes: 64a6f1c4987e ("i2c: add support for microchip fpga i2c controllers")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
My original tests with KASAN/UBSAN/PREEMPT_RT enabled saw far fewer of
these "ghost" detections and the skip caused by the occupied address at
0x18 on this bus is part of my attribution of the problem. Unless I'm
mistaken there's no scenario that you consider a message complete after
sending a START/RESTART without waiting for the ACK/NAK and this code
path I deleted is useless? Looking out of tree, it predates my involvement
with the code so I don't know where it came from, nor is there anything
like it in the bare-metal driver the linux one was based on.
---
 drivers/i2c/busses/i2c-microchip-corei2c.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-microchip-corei2c.c b/drivers/i2c/busses/i2c-microchip-corei2c.c
index e5af38dfaa81..b0a51695138a 100644
--- a/drivers/i2c/busses/i2c-microchip-corei2c.c
+++ b/drivers/i2c/busses/i2c-microchip-corei2c.c
@@ -287,8 +287,6 @@ static irqreturn_t mchp_corei2c_handle_isr(struct mchp_corei2c_dev *idev)
 		ctrl &= ~CTRL_STA;
 		writeb(idev->addr, idev->base + CORE_I2C_DATA);
 		writeb(ctrl, idev->base + CORE_I2C_CTRL);
-		if (idev->msg_len == 0)
-			finished = true;
 		break;
 	case STATUS_M_ARB_LOST:
 		idev->msg_err = -EAGAIN;
-- 
2.45.2