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

Manasi Navare
2018-09-21T07:59:30Z

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 <manasi.d.navare@intel.com>

Manasi

On Thu, Sep 20, 2018 at 03:54:37PM +0100, Damian Kos wrote:
> From: Quentin Schulz <quentin.schulz@free-electrons.com>
>
> 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 <quentin.schulz@free-electrons.com>
> Signed-off-by: Damian Kos <dkos@cadence.com>
> ---
> 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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