linux-kernel - [PATCH] ptp: ocp: Document sysfs output format for backward compatibility

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Message-ID: <20251107074533.416048-1-zhongqiu.han@oss.qualcomm.com>
Date: Fri,  7 Nov 2025 15:45:33 +0800
From: Zhongqiu Han <zhongqiu.han@....qualcomm.com>
To: jonathan.lemon@...il.com, vadim.fedorenko@...ux.dev,
        richardcochran@...il.com, andrew+netdev@...n.ch, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhongqiu.han@....qualcomm.com
Subject: [PATCH] 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@...nel.org>
Signed-off-by: Zhongqiu Han <zhongqiu.han@....qualcomm.com>
---
 drivers/ptp/ptp_ocp.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/ptp/ptp_ocp.c b/drivers/ptp/ptp_ocp.c
index a5c363252986..eeebe4d149f7 100644
--- 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);
 }
 
-- 
2.43.0