On Tue, 2026-06-30 at 15:11 +0000, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider:
> - [High] Checking `dp_128b_132b_supported_link_rates.raw` before it
> is read from the DPCD register completely disables DP 2.0 UHBR
> support.
> --
>
> commit 11d06153280ec5dae6682a402606818b4f84b79d
> Author: Jackie Dong <[email protected]>
>
> drm/amd/display:Check dp_128b_132b_supported_link_rates.raw before
> link_rate setting
>
> This patch modifies the DP 128b/132b capability check to also verify
> that
> dp_128b_132b_supported_link_rates.raw is non-zero before attempting
> to read
> and parse the specific UHBR link rates.
>
> > 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
> > ---
> > 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) {
>
> [Severity: High]
> Will this change completely disable DP 2.0 UHBR support?
>
> In retrieve_link_cap() in
> drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c,
> dp_128b_132b_supported_link_rates.raw is now checked before it is
> actually
> read from the DPCD register. Since the dc_link struct is zero-
> initialized and
> the read hasn't occurred yet, won't this raw value always evaluate to
> 0 upon
> connection of any DP 2.0 monitor?
>
> This would cause the if condition to always fail, bypassing the
> actual read
> block below and permanently preventing UHBR feature detection.
>
Hi Sashiko,
I have verified it as below. To add
dp_128b_132b_supported_link_rates.raw as a new judgment doesn's impact
the value of dp_128b_132b_supported_link_rates.raw. If the monitor can
support UHBR and it can be into feature detection and assigned the
right UHBR rate.
root@yoga-Yoga-Pro-7-15ASH11:/media/yoga/Data/git/linux-502d801f0ab03-
20260622# grep -B23 "Invalid RX 128b_132b_supported_link_rates"
drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c
//if (link-
>dpcd_caps.channel_coding_cap.bits.DP_128b_132b_SUPPORTED) {
link->dpcd_caps.dp_128b_132b_supported_link_rates.raw = 0x10;
pr_info("Jackie %x is SUPPORTED, %x: RX
128b_132b_supported_link_rates.raw\n", link-
>dpcd_caps.channel_coding_cap.bits.DP_128b_132b_SUPPORTED, link-
>dpcd_caps.dp_128b_132b_supported_link_rates.raw) ;
//pr_info("Jackie %x: RX
128b_132b_supported_link_rates.raw\n", link-
>dpcd_caps.dp_128b_132b_supported_link_rates.raw) ;
//pr_info("Jackie RX 128b_132b_supported_link_rates.raw\n") ;
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 */
core_link_read_dpcd(link,
DP_128B132B_SUPPORTED_LINK_RATES,
&link->dpcd_caps.dp_128b_132b_supported_link_rates.raw,
sizeof(link-
>dpcd_caps.dp_128b_132b_supported_link_rates.raw));
if (link-
>dpcd_caps.dp_128b_132b_supported_link_rates.bits.UHBR20) {
pr_info("Jackie RX
128b_132b_supported_link_rates is UHBR20\n") ;
link->reported_link_cap.link_rate =
LINK_RATE_UHBR20;
} else if (link-
>dpcd_caps.dp_128b_132b_supported_link_rates.bits.UHBR13_5)
link->reported_link_cap.link_rate =
LINK_RATE_UHBR13_5;
else if (link-
>dpcd_caps.dp_128b_132b_supported_link_rates.bits.UHBR10)
link->reported_link_cap.link_rate =
LINK_RATE_UHBR10;
else
//link->reported_link_cap.link_rate =
LINK_RATE_UHBR10;//Jackie Add
dm_error("%s: Invalid RX
128b_132b_supported_link_rates\n", __func__);
root@yoga-Yoga-Pro-7-15ASH11:/media/yoga/Data/git/linux-502d801f0ab03-
20260622# dmesg |grep -i "fail\|error\|Jackie"
[ 0.025954] KHO: Failed to reserve lowmem scratch buffer
[ 0.025956] KHO: Failed to reserve scratch area, disabling kexec
handover
[ 1.731523] RAS: Correctable Errors collector initialized.
[ 7.629667] Jackie 1 is SUPPORTED, 10: RX
128b_132b_supported_link_rates.raw
[ 7.630067] [drm:retrieve_link_cap [amdgpu]] *ERROR*
retrieve_link_cap: Invalid RX 128b_132b_supported_link_rates
Above is my verified source code and test result, FYR.
Thanks for your comments.
> > DC_LOG_DP2("128b/132b encoding is supported at
> > link %d", link->link_index);
> >
> > /* Read 128b/132b suppoerted link rates */
> > core_link_read_dpcd(link,
> > DP_128B132B_SUPPORTED_LINK_RATES,
> > &link-
> > >dpcd_caps.dp_128b_132b_supported_link_rates.raw,
>
