Re: [PATCH 2/4] drm/dp/i2c: send bare addresses to properly reset i2c connections (v2)

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Apr 04, 2014 at 01:52:04PM -0400, Alex Deucher wrote:
> We need bare address packets at the start and end of
> each i2c over aux transaction to properly reset the connection
> between transactions.  This mirrors what the existing dp i2c
> over aux algo currently does.
> 
> This fixes EDID fetches on certain monitors especially with
> dp bridges.
> 
> v2: update as per Ville's comments
>     - Set buffer to NULL for zero sized packets
>     - abort the entre transaction if one of the messages fails
> 
> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Thierry Reding <treding@nvidia.com>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 54 +++++++++++++++++++++++------------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 74724aa..125f84d 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -664,12 +664,25 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
>  			   int num)
>  {
>  	struct drm_dp_aux *aux = adapter->algo_data;
> -	unsigned int i, j;
> +	unsigned int m, b;
> +	struct drm_dp_aux_msg msg;
> +	int err = 0;
>  
> -	for (i = 0; i < num; i++) {
> -		struct drm_dp_aux_msg msg;
> -		int err;
> +	memset(&msg, 0, sizeof(msg));
>  
> +	for (m = 0; m < num; m++) {
> +		msg.address = msgs[m].addr;
> +		msg.request = (msgs[m].flags & I2C_M_RD) ?
> +			DP_AUX_I2C_READ :
> +			DP_AUX_I2C_WRITE;
> +		msg.request |= DP_AUX_I2C_MOT;
> +		msg.buffer = NULL;
> +		msg.size = 0;
> +		err = drm_dp_i2c_do_msg(aux, &msg);
> +		if (err < 0) {
> +			printk("error %d in bare address write\n", err);

I guess this printk was some leftover debug thing? Either should be
dropped or converted to some more appropriate DRM_ thing I suppose.

But otherwise the patch looks good:
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> +			break;
> +		}
>  		/*
>  		 * Many hardware implementations support FIFOs larger than a
>  		 * single byte, but it has been empirically determined that
> @@ -677,31 +690,26 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
>  		 * decreased performance. Therefore each message is simply
>  		 * transferred byte-by-byte.
>  		 */
> -		for (j = 0; j < msgs[i].len; j++) {
> -			memset(&msg, 0, sizeof(msg));
> -			msg.address = msgs[i].addr;
> -
> -			msg.request = (msgs[i].flags & I2C_M_RD) ?
> -					DP_AUX_I2C_READ :
> -					DP_AUX_I2C_WRITE;
> -
> -			/*
> -			 * All messages except the last one are middle-of-
> -			 * transfer messages.
> -			 */
> -			if ((i < num - 1) || (j < msgs[i].len - 1))
> -				msg.request |= DP_AUX_I2C_MOT;
> -
> -			msg.buffer = msgs[i].buf + j;
> +		for (b = 0; b < msgs[m].len; b++) {
> +			msg.buffer = msgs[m].buf + b;
>  			msg.size = 1;
>  
>  			err = drm_dp_i2c_do_msg(aux, &msg);
>  			if (err < 0)
> -				return err;
> +				break;
>  		}
> +		if (err < 0)
> +			break;
>  	}
> -
> -	return num;
> +	if (err >= 0)
> +		err = num;
> +	/* send a bare address packet to close out the connection */
> +	msg.request &= ~DP_AUX_I2C_MOT;
> +	msg.buffer = NULL;
> +	msg.size = 0;
> +	(void)drm_dp_i2c_do_msg(aux, &msg);
> +
> +	return err;
>  }
>  
>  static const struct i2c_algorithm drm_dp_i2c_algo = {
> -- 
> 1.8.3.1

-- 
Ville Syrjälä
Intel OTC