Re: [PATCH] usb: typec: tbt: Correct swapped TBT adapter type values

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 8/18/26 6:20 PM, Sven Peter wrote:
> Table F-10 of the USB Type-C Cable and Connector Specification R2.5 on
> page 412 defines bit 16 of the TBT3 Device Discover Mode VDO as 0 = TBT3
> Adapter and 1 = TBT2 Legacy Adapter. Linux has those two swapped since
> their original introduction in commit ca469c292edc ("usb: typec: Add
> definitions for Thunderbolt 3 Alternate Mode").
> 
> ChromiumOS EC's include/usb_pd_tbt.h has them the correct way around
> and references the USB Type-C ECN "Thunderbolt 3 Compatibility Updates"
> as fixing an error where they were originally swapped which is presumably
> where the wrong order originally came from.
> 
> I've also confirmed the correct mapping with an Apple Thunderbolt 3
> to Thunderbolt 2 adapter which does set bit 16 in that VDO.
> 
> Swap the two values and update all users. Also rename the old defines
> so that no user accidentally ends up with an inverted value.
> No functional change.
> 
> Link: https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-release-25
> Link: https://chromium.googlesource.com/chromiumos/platform/ec/+/db93814b6e73c8545d23714fe0674c10814d901a/include/usb_pd_tbt.h#90
> Signed-off-by: Sven Peter <sven@kernel.org>
> ---
> I ran into this when bringing up thunderbolt for Apple Silicon SoCs and
> was very confused why the condition for "tbt2 adapter" vs "tbt3 adapter"
> seemed to be backwards.
> I think the qcom pmic_glink_altmode.c actually has a bug there that was
> hidden and/or caused by the wrong values: It used to always set
> TBT_ADAPTER_TBT3 which sounds plausible but actually claims to be a
> legacy TBT2 adapter on the wire. I don't have the hardware or know the
> intention there though.
> The other users look correct to me since they just pass the value
> through.

I ran into this in the past

https://lore.kernel.org/lkml/20250926-topic-tbt_header_bit_fix-v1-1-122238af5c82@oss.qualcomm.com/

where seems like back then I misinterpreted that the intel driver
doesn't need changes.

For the pg-altmode driver, looking at the FW interface I don't see
any info being passed to the OS regarding tbt2 vs 3, so we should
presumably always assume the latter (i.e. just fixing the header
is enough).

Konrad