From: Quentin Schulz quentin.schulz@free-electrons.com
DP 1.4 introduced a DP_EXTENDED_RCVR_CAPA_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_RCVR_CAPA_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 files changed, 29 insertions(+), 1 deletions(-)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c index 7f5d568..b6a27ab 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_RCVR_CAPA_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_RCVR_CAPA_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_RCVR_CAPA_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;