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

[Konrad Dybcio]

On 4/10/26 5:10 PM, Mika Westerberg wrote:
> 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.

In Table 4-3 below that definition, it says:

"""
Shall be set to 00h or 01h

It is recommended that this field be set to 01h.
"""

But we can revisit limiting those reads another day

Konrad