linux-kernel - [PATCH 4.4 068/168] drm/dp/mst: Configure no_stop

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Message-Id: <20190424170927.789312806@linuxfoundation.org>
Date:   Wed, 24 Apr 2019 19:08:32 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Brian Vincent <brainn@...il.com>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Dhinakaran Pandiyan <dhinakaran.pandiyan@...el.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.4 068/168] drm/dp/mst: Configure no_stop_bit correctly for remote i2c xfers

[ Upstream commit c978ae9bde582e82a04c63a4071701691dd8b35c ]

We aren't supposed to force a stop+start between every i2c msg
when performing multi message transfers. This should eg. cause
the DDC segment address to be reset back to 0 between writing
the segment address and reading the actual EDID extension block.

To quote the E-DDC spec:
"... this standard requires that the segment pointer be
 reset to 00h when a NO ACK or a STOP condition is received."

Since we're going to touch this might as well consult the
I2C_M_STOP flag to determine whether we want to force the stop
or not.

Cc: Brian Vincent <brainn@...il.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928180403.22499-1-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@...el.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 5a1bafb5ecbb..ff12d926eb65 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3019,6 +3019,7 @@ static int drm_dp_mst_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs
 		msg.u.i2c_read.transactions[i].i2c_dev_id = msgs[i].addr;
 		msg.u.i2c_read.transactions[i].num_bytes = msgs[i].len;
 		msg.u.i2c_read.transactions[i].bytes = msgs[i].buf;
+		msg.u.i2c_read.transactions[i].no_stop_bit = !(msgs[i].flags & I2C_M_STOP);
 	}
 	msg.u.i2c_read.read_i2c_device_id = msgs[num - 1].addr;
 	msg.u.i2c_read.num_bytes_read = msgs[num - 1].len;
-- 
2.19.1