Subject: [PATCH v4 2/5] drm/dp: fix link probing for devices supporting DP 1.4+

From: [email-masked] (Manasi Navare)
Date: Fri, 21 Sep 2018 00:59:30 -0700

Thanks for the patch. Verified with the DP 1.4 spec and looks good to me.
Also look at the related patch that makes use of the correct extended capabilities:

https://patchwork.freedesktop.org/patch/249400/

Reviewed-by: Manasi Navare <[email-masked]>

Manasi

On Thu, Sep 20, 2018 at 03:54:37PM +0100, Damian Kos wrote:
> From: Quentin Schulz <[email-masked]>
> 
> DP 1.4 introduced a DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit in
> DP_TRAINING_AUX_RD_INTERVAL register. If set, DPCD registers from
> DP_DPCD_REV to DP_ADAPTER_CAP should be retrieved starting from
> DP_DPCD_REV_EXTENDED. All registers are copied except DP_DPCD_REV,
> DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT which represent the
> "true capabilities" of DPRX device.
> 
> Original DP_DPCD_REV, DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT
> might falsely return lower capabilities to "avoid interoperability
> issues with some of the existing DP Source devices that malfunction
> when they discover the higher capabilities within those three
> registers.".
> 
> Before DP 1.4, DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit was reserved
> and read 0 so it's safe to check against it even if DP revision is
> <1.4
> 
> Signed-off-by: Quentin Schulz <[email-masked]>
> Signed-off-by: Damian Kos <[email-masked]>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 8c6b9fd89f8a..735ebde5c2f0 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -370,10 +370,38 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link)
>  {
>  	u8 values[3];
>  	int err;
> +	unsigned int addr;
>  
>  	memset(link, 0, sizeof(*link));
>  
> -	err = drm_dp_dpcd_read(aux, DP_DPCD_REV, values, sizeof(values));
> +	/*
> +	 * DP 1.4 introduced a DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit in
> +	 * DP_TRAINING_AUX_RD_INTERVAL register. If set, DPCD registers from
> +	 * DP_DPCD_REV to DP_ADAPTER_CAP should be retrieved starting from
> +	 * DP_DPCD_REV_EXTENDED. All registers are copied except DP_DPCD_REV,
> +	 * DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT which represent the
> +	 * "true capabilities" of DPRX device.
> +	 *
> +	 * Original DP_DPCD_REV, DP_MAX_LINK_RATE and DP_DOWNSTREAMPORT_PRESENT
> +	 * might falsely return lower capabilities to "avoid interoperability
> +	 * issues with some of the existing DP Source devices that malfunction
> +	 * when they discover the higher capabilities within those three
> +	 * registers.".
> +	 *
> +	 * Before DP 1.4, DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT bit was reserved
> +	 * and read 0 so it's safe to check against it even if DP revision is
> +	 * <1.4
> +	 */
> +	err = drm_dp_dpcd_readb(aux, DP_TRAINING_AUX_RD_INTERVAL, values);
> +	if (err < 0)
> +		return err;
> +
> +	if (values[0] & DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT)
> +		addr = DP_DP13_DPCD_REV;
> +	else
> +		addr = DP_DPCD_REV;
> +
> +	err = drm_dp_dpcd_read(aux, addr, values, sizeof(values));
>  	if (err < 0)
>  		return err;
>  
> -- 
> 2.17.1
> 
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