Re: [PATCH] drm/amd/display:Check dp_128b_132b_supported_link_rates.raw before link_rate setting

[George Zhang <george.zhang@amd.com>]

Hi Jackie,

This patch causes a regression when used with LG UltraGear 4K144 (DP), 
connected to Realtek DP2 USB-C hub. It is expected to be connecting at 
UHBR link rate, but we are observing HBR3 instead.

Thanks,
George

cc: Mark.Broadworth@amd.com

On 6/30/26 10:59, Jackie Dong wrote:
> [WHY]
> In the evolution of displays from DP 1.4 to DP 2.0, due to considerations
> some factors,such as cost, some displays support 128b/132b encoding but
> do not support UHBR rate transmission. The values can be read from DPCD
> register address as below.
> 
> 0x00006 is 03
> 0x02215 is 00
> 
> For such displays, the current kernel reports the following error message.
> 
> [drm:dp_retrieve_lttpr_cap [amdgpu]] *ERROR* retrieve_link_cap: Invalid RX
> 128b_132b_supported_link_rates
> 
> [HOW]
> Add new judgment: if all bits in dp_128b_132b_supported_link_rates.raw
> are non-zero, then further determine the specific UHBR type; otherwise,
> determine that the display does not support UHBR.
> 
> Signed-off-by: Jackie Dong <xy-jackie@139.com>
> ---
>   .../gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
> index 47abb4066709..58cbb7ac8f9e 100644
> --- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
> +++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
> @@ -2079,7 +2079,8 @@ static bool retrieve_link_cap(struct dc_link *link)
>   	if (!dpcd_read_sink_ext_caps(link))
>   		link->dpcd_sink_ext_caps.raw = 0;
>   
> -	if (link->dpcd_caps.channel_coding_cap.bits.DP_128b_132b_SUPPORTED) {
> +	if (link->dpcd_caps.channel_coding_cap.bits.DP_128b_132b_SUPPORTED &&
> +			link->dpcd_caps.dp_128b_132b_supported_link_rates.raw) {
>   		DC_LOG_DP2("128b/132b encoding is supported at link %d", link->link_index);
>   
>   		/* Read 128b/132b suppoerted link rates */