linux-kernel - [PATCH v2] drm/bridge: Clear the DP_AUX_I2C

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Date:   Thu, 17 Feb 2022 16:22:25 +0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Xin Ji <xji@...logixsemi.com>, Robert Foss <robert.foss@...aro.org>
Cc:     Neil Armstrong <narmstrong@...libre.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Daniel Vetter <daniel@...ll.ch>,
        Andrzej Hajda <andrzej.hajda@...el.com>
Subject: [PATCH v2] drm/bridge: Clear the DP_AUX_I2C_MOT bit passed in aux read command.

If the previous transfer didn't end with a command without DP_AUX_I2C_MOT,
the next read trasnfer will miss the first byte. But if the command in
previous transfer is requested with length 0, it's a no-op to anx7625
since it can't process this command. anx7625 requires the last command
to be read command with length > 0.

It's observed that if we clear the DP_AUX_I2C_MOT in read transfer, we
can still get correct data. Clear the read commands with DP_AUX_I2C_MOT
bit to fix this issue.

Fixes: adca62ec370c ("drm/bridge: anx7625: Support reading edid through aux channel")
Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
---
v1->v2: Offline discussed with Xin Ji, it's better to clear the bit on
read commands only.
---
 drivers/gpu/drm/bridge/analogix/anx7625.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
index 633618bafd75d3..2805e9bed2c2f4 100644
--- a/drivers/gpu/drm/bridge/analogix/anx7625.c
+++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
@@ -253,6 +253,8 @@ static int anx7625_aux_trans(struct anx7625_data *ctx, u8 op, u32 address,
 	addrm = (address >> 8) & 0xFF;
 	addrh = (address >> 16) & 0xFF;
 
+	if (!is_write)
+		op &= ~DP_AUX_I2C_MOT;
 	cmd = DPCD_CMD(len, op);
 
 	/* Set command and length */
-- 
2.35.1.265.g69c8d7142f-goog