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

[Mika Westerberg]

On Fri, Apr 10, 2026 at 04:43:54PM +0200, Konrad Dybcio wrote:
> On 4/10/26 4:29 PM, 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.
> 
> Checked again, the USB4_SB_FW_VERSION (0x02) register's lowest 8 bytes
> are 0/1 for retimers implementing USB4v1 and 2 for v2, so we may go this
> path too

I also checked from Retimer 1.0 spec and there it is still "Reserved. May
have non-zero value". Probably not good to rely on that.