ptp: ocp: Document sysfs output format for backward compatibility

Add a comment to ptp_ocp_tty_show() explaining that the sysfs output
intentionally does not include a trailing newline. This is required for
backward compatibility with existing userspace software that reads the
sysfs attribute and uses the value directly as a device path.

A previous attempt to add a newline to align with common kernel
conventions broke userspace applications that were opening device paths
like "/dev/ttyS4\n" instead of "/dev/ttyS4", resulting in ENOENT errors.

This comment prevents future attempts to "fix" this behavior, which would
break existing userspace applications.

Link: https://lore.kernel.org/netdev/20251030124519.1828058-1-zhongqiu.han@oss.qualcomm.com/
Link: https://lore.kernel.org/netdev/aef3b850-5f38-4c28-a018-3b0006dc2f08@linux.dev/
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251107074533.416048-1-zhongqiu.han@oss.qualcomm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/ptp/ptp_ocp.c b/drivers/ptp/ptp_ocp.c
index a5c3632529862f3019c0dbcfd8f65ea857edfab4..eeebe4d149f71ed4b5a64b107086cff1c3be54bd 100644 (file)
--- a/drivers/ptp/ptp_ocp.c
+++ b/drivers/ptp/ptp_ocp.c
@@ -3430,6 +3430,12 @@ ptp_ocp_tty_show(struct device *dev, struct device_attribute *attr, char *buf)
        struct dev_ext_attribute *ea = to_ext_attr(attr);
        struct ptp_ocp *bp = dev_get_drvdata(dev);
 
+       /*
+        * NOTE: This output does not include a trailing newline for backward
+        * compatibility. Existing userspace software uses this value directly
+        * as a device path (e.g., "/dev/ttyS4"), and adding a newline would
+        * break those applications. Do not add a newline to this output.
+        */
        return sysfs_emit(buf, "ttyS%d", bp->port[(uintptr_t)ea->var].line);
 }