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

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

On Mon, Apr 13, 2026 at 01:43:49PM +0200, Konrad Dybcio wrote:
> 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.

This adds a lot of code for just debugfs access so I think we are better
without.