[PATCH] staging: greybus: uart: document c_cflag handling in set_termios

[PATCH] staging: greybus: uart: document c_cflag handling in set_termios

[Debjeet Banerjee]

gb_tty_set_termios() derives UART line configuration from a subset of
termios->c_cflag bits, namely CSIZE, CSTOPB, PARENB, PARODD, CMSPAR,
CRTSCTS, CLOCAL and CBAUD. Other c_cflag bits are not interpreted by
the driver and are not represented in the Greybus UART protocol
messages.

The existing FIXME suggests clearing unsupported bits from termios.
However, the driver already limits its behavior to the supported subset
when constructing line coding, and unused bits are effectively ignored.
No invalid or unsupported values are propagated to the hardware.

Replace the FIXME with a comment documenting which c_cflag bits are
consumed by the driver and clarifying that other bits are ignored.

No functional change intended.

Signed-off-by: Debjeet Banerjee <debjeetbanerjee48@gmail.com>
---
 drivers/staging/greybus/uart.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 7d060b4cd33d..49d685a6ad8c 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -494,8 +494,20 @@ static void gb_tty_set_termios(struct tty_struct *tty,
 				(termios->c_cflag & CMSPAR ? 2 : 0) : 0;
 
 	newline.data_bits = tty_get_char_size(termios->c_cflag);
-
-	/* FIXME: needs to clear unsupported bits in the termios */
+	/*
+	 * The Greybus UART driver only interprets a subset of termios
+	 * c_cflag bits when configuring line settings:
+	 *
+	 *   - CSIZE via tty_get_char_size() for data bits
+	 *   - CSTOPB for stop-bit format
+	 *   - PARENB, PARODD, CMSPAR for parity encoding
+	 *   - CRTSCTS for hardware flow control
+	 *   - CLOCAL for modem control handling
+	 *   - CBAUD via C_BAUD() for baud rate and B0 semantics
+	 *
+	 * Other c_cflag bits are ignored as they are not represented in
+	 * the Greybus UART protocol.
+	 */
 	gb_tty->clocal = ((termios->c_cflag & CLOCAL) != 0);
 
 	if (C_BAUD(tty) == B0) {
-- 
2.53.0

Re: [PATCH] staging: greybus: uart: document c_cflag handling in set_termios

[Greg KH]

On Sun, Apr 19, 2026 at 08:56:32AM +0530, Debjeet Banerjee wrote:
> gb_tty_set_termios() derives UART line configuration from a subset of
> termios->c_cflag bits, namely CSIZE, CSTOPB, PARENB, PARODD, CMSPAR,
> CRTSCTS, CLOCAL and CBAUD. Other c_cflag bits are not interpreted by
> the driver and are not represented in the Greybus UART protocol
> messages.
> 
> The existing FIXME suggests clearing unsupported bits from termios.
> However, the driver already limits its behavior to the supported subset
> when constructing line coding, and unused bits are effectively ignored.
> No invalid or unsupported values are propagated to the hardware.
> 
> Replace the FIXME with a comment documenting which c_cflag bits are
> consumed by the driver and clarifying that other bits are ignored.

Ignoring is fine, but shouldn't the bits be cleared to let userspace
know about this?  I think that's why we wrote that FIXME all so long
ago.

thanks,

greg k-h

Re: [PATCH] staging: greybus: uart: document c_cflag handling in set_termios

[Debjeet Banerjee]

On Sun, 26 Apr 2026 21:16:05 +0200, Greg Kroah-Hartman wrote:
> Ignoring is fine, but shouldn't the bits be cleared to let userspace
> know about this?  I think that's why we wrote that FIXME all so long
> ago.

Thanks for the feedback.

That makes sense. I had considered clearing unsupported bits earlier, but held off to
avoid modifying termios and left them ignored instead. Your point about userspace
visibility makes sense.

I'll send a v2 that clears unsupported c_cflag bits accordingly.

Regards,
Debjeet

[PATCH v2] staging: greybus: uart: clear unsupported c_cflag bits in set_termios

[Debjeet Banerjee]

gb_tty_set_termios() derives UART line configuration from a subset of
termios->c_cflag bits (CSIZE, CSTOPB, PARENB, PARODD, CMSPAR, CRTSCTS,
CLOCAL and CBAUD). Other bits are not interpreted by the driver and are
not represented in the Greybus UART protocol.

Mask unsupported c_cflag bits so that userspace-visible termios
reflects the supported bits implemented by the driver.

This addresses the existing FIXME.

Signed-off-by: Debjeet Banerjee <debjeetbanerjee48@gmail.com>
---
v2:
  - Clear unsupported c_cflag bits as suggested
  - Update comment to mention the change
---
 drivers/staging/greybus/uart.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 7d060b4cd33d..9c10e6baa7e0 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -495,7 +495,24 @@ static void gb_tty_set_termios(struct tty_struct *tty,
 
 	newline.data_bits = tty_get_char_size(termios->c_cflag);
 
-	/* FIXME: needs to clear unsupported bits in the termios */
+	/*
+	 * The Greybus UART driver only interprets a subset of termios
+	 * c_cflag bits when configuring line settings:
+	 *
+	 *   - CSIZE via tty_get_char_size() for data bits
+	 *   - CSTOPB for stop-bit format
+	 *   - PARENB, PARODD, CMSPAR for parity encoding
+	 *   - CRTSCTS for hardware flow control
+	 *   - CLOCAL for modem control handling
+	 *   - CBAUD via C_BAUD() for baud rate and B0 semantics
+	 *
+	 * Mask unsupported c_cflag bits.
+	 */
+	termios->c_cflag &= (CSIZE | CSTOPB |
+			     PARENB | PARODD | CMSPAR |
+			     CLOCAL | CRTSCTS |
+			     CBAUD);
+
 	gb_tty->clocal = ((termios->c_cflag & CLOCAL) != 0);
 
 	if (C_BAUD(tty) == B0) {
-- 
2.53.0