Re: [PATCH 1/5] drm/dp/mst: Configure no_stop_bit correctly for remote i2c xfers

[Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>]

On Fri, 2018-09-28 at 21:03 +0300, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> 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."
Related question, do you know why the segment and ddc addresses are
defined as 0x30 and 0x50? The E-DDC spec says they should be at 0x60
and 0xA0/0xA1.

> 
> 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.
Reviewing this took a lot of spec reading than I expected.

Setting the no_stop_bit after writing the segment address makes sense.
I have one concern though. drm_do_probe_ddc_edid does not make use of
the I2C_M_STOP flag, which in turn means we won't reset the no_stop_bit
at the end of edid read. Pass the i2c stop flag from the caller?


> 
> Cc: Brian Vincent <brainn@gmail.com>
> References: https://bugs.freedesktop.org/show_bug.cgi?id=108081
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  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 5ff1d79b86c4..3b400eab18a2 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3276,6 +3276,7 @@ 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;

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