linux-kernel - Re: [PATCH tty v1 01/74] serial: core: Provide port lock wrappers

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Date:   Tue, 19 Sep 2023 16:24:48 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH tty v1 01/74] serial: core: Provide port lock wrappers

On Thu 2023-09-14 20:43:18, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> When a serial port is used for kernel console output, then all
> modifications to the UART registers which are done from other contexts,
> e.g. getty, termios, are interference points for the kernel console.
> 
> So far this has been ignored and the printk output is based on the
> principle of hope. The rework of the console infrastructure which aims to
> support threaded and atomic consoles, requires to mark sections which
> modify the UART registers as unsafe. This allows the atomic write function
> to make informed decisions and eventually to restore operational state. It
> also allows to prevent the regular UART code from modifying UART registers
> while printk output is in progress.
> 
> All modifications of UART registers are guarded by the UART port lock,
> which provides an obvious synchronization point with the console
> infrastructure.
> 
> Provide wrapper functions for spin_[un]lock*(port->lock) invocations so
> that the console mechanics can be applied later on at a single place and
> does not require to copy the same logic all over the drivers.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  include/linux/serial_core.h | 79 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 79 insertions(+)
> 
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index bb6f073bc159..f1d5c0d1568c 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> +/**
> + * uart_port_lock_irqsave - Lock the UART port, save and disable interrupts
> + * @up:		Pointer to UART port structure
> + * @flags:	Pointer to interrupt flags storage
> + */
> +static inline void uart_port_lock_irqsave(struct uart_port *up, unsigned long *flags)
> +{
> +	spin_lock_irqsave(&up->lock, *flags);
> +}

IMHO, it would have been better to pass the flags variable directly
via a macro as it is done in most *_lock_*_irqsafe() APIs. I mean
something like:

/**
 * uart_port_trylock_irqsave - Try to lock the UART port, save and disable interrupts
 * @up:		Pointer to UART port structure
 * @flags:	Interrupt flags storage
 *
 * Returns: True if lock was acquired, false otherwise
 */
#define uart_port_lock_irqsave(up, flags)		\
({							\
	local_irq_save(flags);				\
	uart_port_lock(lock)				\
})

> +
> +/**
> + * uart_port_trylock - Try to lock the UART port
> + * @up:		Pointer to UART port structure
> + *
> + * Returns: True if lock was acquired, false otherwise
> + */
> +static inline bool uart_port_trylock(struct uart_port *up)
> +{
> +	return spin_trylock(&up->lock);
> +}
> +
> +/**
> + * uart_port_trylock_irqsave - Try to lock the UART port, save and disable interrupts
> + * @up:		Pointer to UART port structure
> + * @flags:	Pointer to interrupt flags storage
> + *
> + * Returns: True if lock was acquired, false otherwise
> + */
> +static inline bool uart_port_trylock_irqsave(struct uart_port *up, unsigned long *flags)
> +{
> +	return spin_trylock_irqsave(&up->lock, *flags);
> +}

Similar here:

/**
 * uart_port_trylock_irqsave - Try to lock the UART port, save and disable interrupts
 * @up:		Pointer to UART port structure
 * @flags:	Interrupt flags storage
 *
 * Returns: True if lock was acquired, false otherwise
 */
#define uart_port_trylock_irqsave(up, flags)			\
({								\
	bool __ret;						\
								\
	local_irq_save(flags);					\
	__ret = uart_port_trylock(lock)				\
	if (!__ret)						\
		local_irq_restore(flags);			\
	__ret;							\
})

I do not resist on this rather cosmetic change. The current code seems
to be doing what is expected. Feel free to keep it and use:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr

PS: I am sorry for the late review. I have made a quick look on Monday
    and it looked straightforward. I have got this idea today when
    having a closer look.