Re: [PATCH RFC] thunderbolt: Skip reading Rsvd SB registers on older-gen hardware

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

On 4/13/26 1:40 PM, Mika Westerberg wrote:
> On Mon, Apr 13, 2026 at 12:54:41PM +0200, Konrad Dybcio wrote:
>> From: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
>>
>> The Linux TBT subsystem supports hardware aligned with the latest
>> USB4 v2.0 specification. In some places though, it assumes registers
>> only defined in that specification version (previously marked as
>> Reserved) are always accessible.
>>
>> Every version of the spec states:
>>
>> """
>> Unless specified otherwise, the Connection Manager shall not write a
>> register with a value that is marked as “Rsvd”. Writing a register with
>> a value that is marked as “Rsvd” results in undefined behavior.
>> """
>>
>> The effective list of SB registers that need this guarding currently
>> exclusively contains ones that aren't used outside the debugfs dump
>> logic, so just add the required checks there.
>>
>> Fixes: 54e418106c76 ("thunderbolt: Add debugfs interface")
>> Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
>> ---
>>  drivers/thunderbolt/debugfs.c | 20 ++++++++++++--------
>>  drivers/thunderbolt/retimer.c | 11 ++++++++++-
>>  drivers/thunderbolt/sb_regs.h | 11 ++++++-----
>>  drivers/thunderbolt/tb.h      |  2 ++
>>  4 files changed, 30 insertions(+), 14 deletions(-)
> 
> This is alternative for the v2 patch you sent earlier, right? I prefer that
> one over this.

I think they're complementary. This patch ensures compliance with the
quoted part of the spec, while the other one improves the UX and aids
debugging.

Konrad