[PATCH V3] drm: edid: add support for E-DDC

[Daniel Vetter]

On Fri, Aug 24, 2012 at 02:56:50PM +0530, Shirish S wrote:
> The current logic for probing ddc is limited to
> 2 blocks (256 bytes), this patch adds support
> for the 4 block (512) data.
> 
> To do this, a single 8-bit segment index is
> passed to the display via the I2C address 30h.
> Data from the selected segment is then immediately
> read via the regular DDC2 address using a repeated
> I2C 'START' signal.
> 
> Signed-off-by: Shirish S <s.shirish at samsung.com>
> ---
>  drivers/gpu/drm/drm_edid.c |   83 ++++++++++++++++++++++++++++++++-----------
>  1 files changed, 62 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index a8743c3..a10a130 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -254,6 +254,8 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
>  		      int block, int len)
>  {
>  	unsigned char start = block * EDID_LENGTH;
> +	unsigned char segment = block >> 1;
> +	unsigned short segFlags = segment ? 0 : I2C_M_IGNORE_NAK;

You can inline segFlags to the only place it's used.

>  	int ret, retries = 5;
>  
>  	/* The core i2c driver will automatically retry the transfer if the
> @@ -262,29 +264,68 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
>  	 * generate spurious NAKs and timeouts. Retrying the transfer
>  	 * of the individual block a few times seems to overcome this.
>  	 */
> -	do {
> -		struct i2c_msg msgs[] = {
> -			{
> -				.addr	= DDC_ADDR,
> -				.flags	= 0,
> -				.len	= 1,
> -				.buf	= &start,
> -			}, {
> -				.addr	= DDC_ADDR,
> -				.flags	= I2C_M_RD,
> -				.len	= len,
> -				.buf	= buf,
> +	switch (segment) {
> +		/* To reduce traffic on I2C, we request the i2c xfer
> +		 * for segment addr only if 4 block edid data is
> +		 * present.
> +		 * 0 : DDC
> +		 * 1 : E-DDC
> +		 */

This comment is a bit misleading: We don't avoid the segment xfer to
reduce i2c traffic, but to avoid upsetting any non-compliant screens.

> +	case 0:
> +		do {

Keeping the loop as the outermost would imo be clearer and avoid
duplicating it (it also would make the patch a bit smaller). You could
even do a trick with the msgs array like this:

	msgs[] = { full i2c xfer including segment addr}
	xfers = 3;

	/* Avoid sending the segment addr to not upset non-compliant ddc
	 * monitors. */
	if (!segments)
		msgs++, len--;

	i2c_transefer(adapter, msgs, len);

Also, I think it'd be good to set the IGNORE_NAK on the start addre
transfer, too (but in a separate patch). At least I think we've seen
screens out there that need this :(

Yours, Daniel

> +			struct i2c_msg msgs[] = {
> +				{
> +					.addr	= DDC_ADDR,
> +					.flags	= 0,
> +					.len	= 1,
> +					.buf	= &start,
> +				}, {
> +					.addr	= DDC_ADDR,
> +					.flags	= I2C_M_RD,
> +					.len	= len,
> +					.buf	= buf,
> +				}
> +			};
> +			ret = i2c_transfer(adapter, msgs, 2);
> +			if (ret == -ENXIO) {
> +				DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
> +							adapter->name);
> +				break;
>  			}
> -		};
> -		ret = i2c_transfer(adapter, msgs, 2);
> -		if (ret == -ENXIO) {
> -			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
> -					adapter->name);
> -			break;
> -		}
> -	} while (ret != 2 && --retries);
> +		} while (ret != 2 && --retries);
> +
> +		return ret == 2 ? 0 : -1;
> +	case 1:
> +		do {
> +			struct i2c_msg msgs[] = {
> +				{ /*set segment pointer */
> +					.addr	= DDC_SEGMENT_ADDR,
> +					.flags	= segFlags,
> +					.len	= 1,
> +					.buf	= &segment,
> +				}, { /*set offset */
> +					.addr	= DDC_ADDR,
> +					.flags	= 0,
> +					.len	= 1,
> +					.buf	= &start,
> +				}, { /*set data */
> +					.addr	= DDC_ADDR,
> +					.flags	= I2C_M_RD,
> +					.len	= len,
> +					.buf	= buf,
> +				}
> +			};
> +			ret = i2c_transfer(adapter, msgs, 3);
> +			if (ret == -ENXIO) {
> +				DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
> +							adapter->name);
> +				break;
> +			}
> +		} while (ret != 3 && --retries);
>  
> -	return ret == 2 ? 0 : -1;
> +		return ret == 3 ? 0 : -1;
> +	}
> +	return -1;
>  }
>  
>  static bool drm_edid_is_zero(u8 *in_edid, int length)
> -- 
> 1.7.0.4
> 
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-- 
Daniel Vetter
Mail: daniel at ffwll.ch
Mobile: +41 (0)79 365 57 48