Re: [PATCH] thunderbolt: debugfs: Don't stop reading SB registers if just one fails

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

On Fri, Apr 10, 2026 at 04:29:43PM +0200, Konrad Dybcio wrote:
> On 4/9/26 4:32 PM, Mika Westerberg wrote:
> > On Thu, Apr 09, 2026 at 02:59:22PM +0200, Konrad Dybcio wrote:
> >> On 4/9/26 2:04 PM, Mika Westerberg wrote:
> 
> [...]
> 
> >>> I assume you have tested this on a hardware that supports this too, right?
> >>
> >> Hardware that exposes that register this does not exercise the altered
> >> code path.
> > 
> > Well it may happen now that previously we got -EIO from some other register
> > and we stopped there, now this changes and we actually continue reading so
> > this definitely should be tested.
> 
> The only register before USB4_SB_GEN4_TXFFE that isn't in-spec for
> both retimers in v1.0 and v2.0 is USB4_SB_LRD_TUNING (0x07). The PS8830
> interestingly reports all zeroes (not a bounce).
> 
> The registers following USB4_SB_GEN4_TXFFE in the array are
> USB4_SB_VERSION and USB4_SB_DATA. The former is not accessed anywhere
> else in the code, at first glance. The latter is, during NVM r/w and
> in margining ops, which have definitely been in use for a long time.
> 
> Plus both of them are the v1.0 spec. The USB4_SB_GEN4_TXFFE specifically
> isn't (the retimer supplement pdf lists it as Rsvd, the main spec pdf
> omits it in the SB register table), as it wasn't previously useful (since
> Gen4 came about in v2.0).
> 
> 
> I don't think there's an easy way to limit the reading of this register
> since the bit indicating Gen4 capability is in USB4_SB_LINK_CONF (0x0c),
> which is Rsvd on retimers regardless of the spec revision. A connected
> port could easily have higher/lower capabilities, too.

Agree.

> So all in all, my understanding is that any bugs caused by this patch
> (which would have to be in the form of "reading a register causes a
> wrongful change in behavior") would really surface spec non-compliance
> from a retimer, which should be quirked out explicitly if that's the
> case.

Right. Also given the fact that this is debugfs access so not anything we
would do during normal operations, likelihood of this causing issues for
regular user should be pretty low.

> I only have hardware with various Parade retimers, none of which claim
> Gen4 support.

Okay I can give the next version a try on my test systems to make sure
there are no surprises. I should have a pretty extensive variation of
retimers (v1, v2 and pre-USB4).