[PATCH v2] thunderbolt: debugfs: Don't stop reading SB registers if just one fails

[Konrad Dybcio <konradybcio@kernel.org>]

From: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>

The GEN4 TxFFE register is not part of the USB4 v1.0 specification, so
understandably some pre-USB4v2 retimers (like the Parade PS8830) don't
seem to implement it.

The immediate idea to counter this would be to introduce a version
check for that specific register, but on a second thought, the current
flow only returns a quiet -EIO if there's any failures, without hinting
at what the actual problem is.

To take care of both of these issues, simply print an error line for
each SB register read that fails and go on with attempting to read the
others.

Note that this is not quite in-spec behavior ("The SB Register Space
registers shall have the structure and fields described in Table 4-17.
Registers not listed in Table 4-20 are undefined and shall not be
used."), but it's the easiest fix that shouldn't have real-world bad
side effects.

Fixes: 6d241fa00159 ("thunderbolt: Add sideband register access to debugfs")
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
---
Changes in v2:
- Don't print the error retval, replace the message with '<not
  accessible>' to mimic other error paths in the sys/debugfs TBT code
- Slightly reword the commit message
- Link to v1: https://lore.kernel.org/r/20260409-topic-tbt_sb_debugfs-v1-1-131540e0cc2b@oss.qualcomm.com
---
 drivers/thunderbolt/debugfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/thunderbolt/debugfs.c b/drivers/thunderbolt/debugfs.c
index 042f6a0d0f7f..d089d2decedd 100644
--- a/drivers/thunderbolt/debugfs.c
+++ b/drivers/thunderbolt/debugfs.c
@@ -2361,8 +2361,10 @@ static int sb_regs_show(struct tb_port *port, const struct sb_reg *sb_regs,
 		memset(data, 0, sizeof(data));
 		ret = usb4_port_sb_read(port, target, index, regs->reg, data,
 					regs->size);
-		if (ret)
-			return ret;
+		if (ret) {
+			seq_printf(s, "0x%02x <not accessible>\n", regs->reg);
+			continue;
+		}
 
 		seq_printf(s, "0x%02x", regs->reg);
 		for (j = 0; j < regs->size; j++)

---
base-commit: db7efce4ae23ad5e42f5f55428f529ff62b86fab
change-id: 20260409-topic-tbt_sb_debugfs-2e500fee9706

Best regards,
-- 
Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>