Re: [PATCH v5] USB: serial: xr: Add TIOCGRS485 and TIOCSRS485 ioctls

[Johan Hovold <johan@kernel.org>]

On Sat, Jul 08, 2023 at 05:56:50PM +0300, Jarkko Sonninen wrote:
> Exar devices like XR21B1411 can control an RS485 transceiver by
> automatically asserting the RTS#/RS485 pin before sending data
> and deasserting it when the last stop bit has been transmitted.
> The polarity of the RST#/RS485 signal is configurable and the
> hardware also supports half-duplex turn-around delay and
> address matching mode.
> 
> Add support for enabling and disabling RS-485 mode and
> configuring the RST#/RS485 signal polarity using the TIOCGRS485
> and TIOCSRS485 ioctls. Support for half-duplex turn-around delay
> and address matching mode are left unimplemented for now.
> 
> User enables RS-485 mode by setting SER_RS485_ENABLED flag in
> struct serial_rs485 flags. User should also set either
> SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND to select the
> behaviour of the RTS#/RS485 pin. Setting SER_RS485_RTS_AFTER_SEND
> will drive RTS#/RS485 high during transmission. As this is the
> typical application described by Exar, it is selected when
> user sets neither or both flags.
> 
> Signed-off-by: Jarkko Sonninen <kasper@iki.fi>
> ---
> Changes in v3:
>  - In this version only rs485.flags are stored to state.
>  - There is no locking as only one bit of the flags is used.
>  - ioctl returns -ENOIOCTLCMD as the actual error handling is in tty code.
> Changes in v4:
>  - Store struct rs485 to data
>  - Add mutex to protect data->rs485.
>  - Implement SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND flags
>  - SER_RS485_RTS_ON_SEND is the default like in serial_core.c
> Change in v5:
>  - Use tty->termios_rwsem semaphore instead of own mutex
>  - Set SER_RS485_RTS_AFTER_SEND as the default
>  - Fix XR_GPIO_MODE_RS485_TX_H setting with SER_RS485_RTS_ON_SEND
>  - Add missing __user directives

I've applied this one now but with the small change I just mentioned
that makes SER_RS485_RTS_ON_SEND determine SER_RS485_RTS_AFTER_SEND.

Since this sets SER_RS485_RTS_AFTER_SEND when neither flag is set, I
believe you get the behaviour you preferred (even if
SER_RS485_RTS_AFTER_SEND is now cleared when both flags are set).

Let me know otherwise and we'll discuss it. Here's the result:

	https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/commit/?h=usb-next&id=974e2f6a0554685493cc44406bc7d8ba0a3b0e33

Thanks for sticking with. I think the end result looks really good.

Johan