Commit 5b9192c2 authored by Zhongqiu Han's avatar Zhongqiu Han Committed by Jakub Kicinski
Browse files

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: default avatarJakub Kicinski <kuba@kernel.org>
Signed-off-by: default avatarZhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Reviewed-by: default avatarVadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251107074533.416048-1-zhongqiu.han@oss.qualcomm.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 73edb26b

drivers/ptp/ptp_ocp.c

+6 −0
Original line number Diff line number Diff line
@@ -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);

}