[PATCH RFC] thunderbolt: Skip reading Rsvd SB registers on older-gen hardware
From: Konrad Dybcio
Date: Mon Apr 13 2026 - 06:57:15 EST
From: Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxx>
---
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(-)
diff --git a/drivers/thunderbolt/debugfs.c b/drivers/thunderbolt/debugfs.c
index 042f6a0d0f7f..b74685bcd2a8 100644
--- a/drivers/thunderbolt/debugfs.c
+++ b/drivers/thunderbolt/debugfs.c
@@ -67,6 +67,7 @@ enum usb4_margin_cap_time_indp {
struct sb_reg {
unsigned int reg;
unsigned int size;
+ u8 min_spec_ver;
};
#define SB_MAX_SIZE 64
@@ -75,9 +76,9 @@ struct sb_reg {
static const struct sb_reg port_sb_regs[] = {
{ USB4_SB_VENDOR_ID, 4 },
{ USB4_SB_PRODUCT_ID, 4 },
- { USB4_SB_DEBUG_CONF, 4 },
- { USB4_SB_DEBUG, 54 },
- { USB4_SB_LRD_TUNING, 4 },
+ { USB4_SB_DEBUG_CONF, 4, .min_spec_ver = 2 },
+ { USB4_SB_DEBUG, 54, .min_spec_ver = 2 },
+ { USB4_SB_LRD_TUNING, 4, .min_spec_ver = 2 },
{ USB4_SB_OPCODE, 4 },
{ USB4_SB_METADATA, 4 },
{ USB4_SB_LINK_CONF, 3 },
@@ -92,11 +93,11 @@ static const struct sb_reg retimer_sb_regs[] = {
{ USB4_SB_VENDOR_ID, 4 },
{ USB4_SB_PRODUCT_ID, 4 },
{ USB4_SB_FW_VERSION, 4 },
- { USB4_SB_LRD_TUNING, 4 },
+ { USB4_SB_LRD_TUNING, 4, .min_spec_ver = 2 },
{ USB4_SB_OPCODE, 4 },
{ USB4_SB_METADATA, 4 },
{ USB4_SB_GEN23_TXFFE, 4 },
- { USB4_SB_GEN4_TXFFE, 4 },
+ { USB4_SB_GEN4_TXFFE, 4, .min_spec_ver = 2 },
{ USB4_SB_VERSION, 4 },
{ USB4_SB_DATA, 64 },
};
@@ -2347,7 +2348,7 @@ DEBUGFS_ATTR_RW(counters);
static int sb_regs_show(struct tb_port *port, const struct sb_reg *sb_regs,
size_t size, enum usb4_sb_target target, u8 index,
- struct seq_file *s)
+ u8 spec_version, struct seq_file *s)
{
int ret, i;
@@ -2358,6 +2359,9 @@ static int sb_regs_show(struct tb_port *port, const struct sb_reg *sb_regs,
u8 data[64];
int j;
+ if (regs->min_spec_ver > spec_version)
+ continue;
+
memset(data, 0, sizeof(data));
ret = usb4_port_sb_read(port, target, index, regs->reg, data,
regs->size);
@@ -2388,7 +2392,7 @@ static int port_sb_regs_show(struct seq_file *s, void *not_used)
}
ret = sb_regs_show(port, port_sb_regs, ARRAY_SIZE(port_sb_regs),
- USB4_SB_TARGET_ROUTER, 0, s);
+ USB4_SB_TARGET_ROUTER, 0, port->config.thunderbolt_version, s);
mutex_unlock(&tb->lock);
out_rpm_put:
@@ -2503,7 +2507,7 @@ static int retimer_sb_regs_show(struct seq_file *s, void *not_used)
}
ret = sb_regs_show(rt->port, retimer_sb_regs, ARRAY_SIZE(retimer_sb_regs),
- USB4_SB_TARGET_RETIMER, rt->index, s);
+ USB4_SB_TARGET_RETIMER, rt->index, rt->spec_version, s);
mutex_unlock(&tb->lock);
out_rpm_put:
diff --git a/drivers/thunderbolt/retimer.c b/drivers/thunderbolt/retimer.c
index b1e8d9d6454d..b27a34fe83ea 100644
--- a/drivers/thunderbolt/retimer.c
+++ b/drivers/thunderbolt/retimer.c
@@ -389,8 +389,8 @@ const struct device_type tb_retimer_type = {
static int tb_retimer_add(struct tb_port *port, u8 index, u32 auth_status,
bool on_board)
{
+ u32 vendor, device, version;
struct tb_retimer *rt;
- u32 vendor, device;
int ret;
ret = usb4_port_sb_read(port, USB4_SB_TARGET_RETIMER, index,
@@ -409,6 +409,14 @@ static int tb_retimer_add(struct tb_port *port, u8 index, u32 auth_status,
return ret;
}
+ ret = usb4_port_sb_read(port, USB4_SB_TARGET_RETIMER, index,
+ USB4_SB_FW_VERSION, &version, sizeof(version));
+ if (ret) {
+ if (ret != -ENODEV)
+ tb_port_warn(port, "failed read retimer Version: %d\n", ret);
+ return ret;
+ }
+
rt = kzalloc_obj(*rt);
if (!rt)
@@ -417,6 +425,7 @@ static int tb_retimer_add(struct tb_port *port, u8 index, u32 auth_status,
rt->index = index;
rt->vendor = vendor;
rt->device = device;
+ rt->spec_version = FIELD_GET(USB4_SB_FW_VERSION_VERSION, version);
rt->auth_status = auth_status;
rt->port = port;
rt->tb = port->sw->tb;
diff --git a/drivers/thunderbolt/sb_regs.h b/drivers/thunderbolt/sb_regs.h
index 5391502a4b87..a9c2d22ff69f 100644
--- a/drivers/thunderbolt/sb_regs.h
+++ b/drivers/thunderbolt/sb_regs.h
@@ -12,10 +12,11 @@
#define USB4_SB_VENDOR_ID 0x00
#define USB4_SB_PRODUCT_ID 0x01
-#define USB4_SB_FW_VERSION 0x02
-#define USB4_SB_DEBUG_CONF 0x05
-#define USB4_SB_DEBUG 0x06
-#define USB4_SB_LRD_TUNING 0x07
+#define USB4_SB_FW_VERSION 0x02 /* Retimer-only */
+#define USB4_SB_FW_VERSION_VERSION GENMASK(7, 0)
+#define USB4_SB_DEBUG_CONF 0x05 /* Port-only, USB4 v2.0 */
+#define USB4_SB_DEBUG 0x06 /* Port-only, USB4 v2.0 */
+#define USB4_SB_LRD_TUNING 0x07 /* USB4 v2.0 */
#define USB4_SB_OPCODE 0x08
enum usb4_sb_opcode {
@@ -42,7 +43,7 @@ enum usb4_sb_opcode {
#define USB4_SB_METADATA_NVM_AUTH_WRITE_MASK GENMASK(5, 0)
#define USB4_SB_LINK_CONF 0x0c
#define USB4_SB_GEN23_TXFFE 0x0d
-#define USB4_SB_GEN4_TXFFE 0x0e
+#define USB4_SB_GEN4_TXFFE 0x0e /* USB4 v2.0 */
#define USB4_SB_VERSION 0x0f
#define USB4_SB_DATA 0x12
diff --git a/drivers/thunderbolt/tb.h b/drivers/thunderbolt/tb.h
index 217c3114bec8..7eba058e9c2b 100644
--- a/drivers/thunderbolt/tb.h
+++ b/drivers/thunderbolt/tb.h
@@ -335,6 +335,7 @@ struct usb4_port {
* @no_nvm_upgrade: Prevent NVM upgrade of this retimer
* @auth_status: Status of last NVM authentication
* @margining: Pointer to margining structure if enabled
+ * @spec_version: The implemented USB4 Retimer Specification version
*/
struct tb_retimer {
struct device dev;
@@ -346,6 +347,7 @@ struct tb_retimer {
struct tb_nvm *nvm;
bool no_nvm_upgrade;
u32 auth_status;
+ u8 spec_version;
#ifdef CONFIG_USB4_DEBUGFS_MARGINING
struct tb_margining *margining;
#endif
---
base-commit: db7efce4ae23ad5e42f5f55428f529ff62b86fab
change-id: 20260413-topic-usb4_limit_sb_reads-965ed26524a1
Best regards,
--
Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx>