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 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.