Re: [PATCH RFC] drm/dp_tunnel: Allow BWA when the DP IN adapter does not advertise it

[Diego C N]

Hi Mika,

Oh, I see. Well, thanks anyway for taking time to check this patch and
for answering it.
I'll keep it as my personal patch since it's been working very well
for the past weeks.

On Mon, Nov 2, 2026 at 2:02 PM Mika Westerberg
<mika.westerberg@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Sat, Oct 31, 2026 at 01:18:18AM +0100, Diego Noguês via B4 Relay wrote:
> > From: Diego Noguês <diego.sure@xxxxxxxxx>
> >
> > On an Alder Lake host (i7-1265U) with a Thunderbolt 4 dock (Dell WD22TB4,
> > Goshen Ridge) driving two 4K@60 monitors through two DP tunnels, the
> > monitor whose tunnel is created second is limited to 4K@30. Which monitor
> > is affected depends only on the order in which they are powered on.
> >
> > The thunderbolt driver enables the BW allocation mode (BWA) on both DP IN
> > adapters: it sets the CM BWA support (DP_USB4_DRIVER_BW_CAPABILITY), a
> > group ID and the estimated BW. The DP IN adapter, however, never sets
> > DP_IN_BW_ALLOCATION_MODE_SUPPORT in DP_TUNNELING_CAPABILITIES (0xe000d
> > reads 0x01), so tunnel_regs_are_valid() rejects the tunnel with:
>
> Alder Lake firmware really does not support this and by forcing it to be
> enabled you may end up in untested code paths and interesting behavior :)
> BWA was only officially supported starting from Meteor Lake.



-- 
Diego C. N.