Re: thunderbolt: second DP tunnel starved at boot while first tunnel's DPRX read is pending

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi,

On Sat, Nov 07, 2026 at 04:11:22PM +0100, Mykola Dimura wrote:
> Hi,
> 
> With two monitors on a Thunderbolt 4 dock, one of them comes up limited
> to 4K@30 after every boot (or dock re-plug). Which one gets 60 Hz is
> effectively random. Plugging the monitors in one at a time, in the right
> order, gives 4K@60 on both until the next boot. The cause is
> the order-dependent DP tunnel bandwidth reservation in tb_tunnel_one_dp().

Yes, this is by design. It's now called the "legacy" mode because it works
exactly like the firmware CM: first monitor gets all, second gets what is
left. So plugging them one by one is the correct workaround. The solution
to this is the bandwidth allocation mode but unfortunately it's not
supported until Meteor Lake.

We don't really want to add any new command line parameters. I have an
internal patch series that makes it possible from userspace to affect the
monitor selection via eBPF but that's not yet ready to be upstreamed. It
would help in your case though.