linux-kernel - [PATCH 4.14 24/39] drm/dp_mst: correct the shifting in DP_REMOTE_I2C

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Date:   Tue, 14 Jan 2020 11:01:58 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Harry Wentland <harry.wentland@....com>,
        Wayne Lin <Wayne.Lin@....com>, Lyude Paul <lyude@...hat.com>
Subject: [PATCH 4.14 24/39] drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

From: Wayne Lin <Wayne.Lin@....com>

commit c4e4fccc5d52d881afaac11d3353265ef4eccb8b upstream.

[Why]
According to DP spec, it should shift left 4 digits for NO_STOP_BIT
in REMOTE_I2C_READ message. Not 5 digits.

In current code, NO_STOP_BIT is always set to zero which means I2C
master is always generating a I2C stop at the end of each I2C write
transaction while handling REMOTE_I2C_READ sideband message. This issue
might have the generated I2C signal not meeting the requirement. Take
random read in I2C for instance, I2C master should generate a repeat
start to start to read data after writing the read address. This issue
will cause the I2C master to generate a stop-start rather than a
re-start which is not expected in I2C random read.

[How]
Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
drm_dp_encode_sideband_req().

Changes since v1:(https://patchwork.kernel.org/patch/11312667/)
* Add more descriptions in commit and cc to stable

Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Reviewed-by: Harry Wentland <harry.wentland@....com>
Signed-off-by: Wayne Lin <Wayne.Lin@....com>
Cc: stable@...r.kernel.org
Signed-off-by: Lyude Paul <lyude@...hat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103055001.10287-1-Wayne.Lin@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/gpu/drm/drm_dp_mst_topology.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -274,7 +274,7 @@ static void drm_dp_encode_sideband_req(s
 			memcpy(&buf[idx], req->u.i2c_read.transactions[i].bytes, req->u.i2c_read.transactions[i].num_bytes);
 			idx += req->u.i2c_read.transactions[i].num_bytes;

-			buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 5;
+			buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 4;
 			buf[idx] |= (req->u.i2c_read.transactions[i].i2c_transaction_delay & 0xf);
 			idx++;
 		}