Re: [PATCH 1/2] printk: nbcon: Introduce Braille helpers

[John Ogness]

On 2026-09-22, Petr Mladek <pmladek@xxxxxxxx> wrote:
> These helpers will be used when calling console->write_atomic
> in the Braille console driver.
>
> The Braille console is not registered as a standard printk console.
> Instead, it is integrated with the virtual terminal (VT) and writes
> data using the legacy con->write() callback of the associated serial
> console driver.
>
> When the underlying console driver is converted to the NBCON API, the
> Braille console needs to use the con->write_atomic() callback instead.
> This callback must be synchronized by acquiring the nbcon console context.
>
> Fixes: d3539347022a ("serial: 8250: Switch to nbcon console, take 2")

This is not 8250 specific. But I am not sure which commit you want to
mark as Fixes.

For 62627bf0cadf ("kdb: Adapt kdb_msg_write to work with NBCON
consoles") there was no Fixes tag.

> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  include/linux/console.h |  8 ++++++
>  kernel/printk/nbcon.c   | 59 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 67 insertions(+)
>
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 502d1abe3f50..d780f6de303a 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -615,6 +615,10 @@ extern bool nbcon_allow_unsafe_takeover(void);
>  extern bool nbcon_kdb_try_acquire(struct console *con,
>  				  struct nbcon_write_context *wctxt);
>  extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
> +extern bool nbcon_is_braille(struct nbcon_write_context *wctxt);
> +extern bool nbcon_braille_try_acquire(struct console *con,
> +				      struct nbcon_write_context *wctxt);
> +extern void nbcon_braille_release(struct nbcon_write_context *wctxt);
>  
>  /*
>   * Check if the given console is currently capable and allowed to print
> @@ -678,8 +682,12 @@ static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
>  static inline bool nbcon_kdb_try_acquire(struct console *con,
>  					 struct nbcon_write_context *wctxt) { return false; }
>  static inline void nbcon_kdb_release(struct nbcon_write_context *wctxt) { }
> +static inline bool nbcon_is_braille(struct nbcon_write_context *wctxt) { return false; }
>  static inline bool console_is_usable(struct console *con, short flags,
>  				     bool use_atomic) { return false; }
> +static inline bool nbcon_braille_try_acquire(struct console *con,
> +					     struct nbcon_write_context *wctxt) { return false; }
> +static inline void nbcon_braille_release(struct nbcon_write_context *wctxt) { }
>  #endif
>  
>  extern int console_set_on_cmdline;
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index d17704fe93ae..a5e053ffe4da 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -2002,3 +2002,62 @@ void nbcon_kdb_release(struct nbcon_write_context *wctxt)
>  	 */
>  	__nbcon_atomic_flush_pending_con(ctxt->console, prb_next_reserve_seq(prb));
>  }
> +
> +/**
> + * nbcon_is_braille - Checks whether the nbcon write context is using Braille console
> + *
> + * @wctxt:	checked nbcon write context
> + *
> + * Return: True when the write context is associated with a Braille console.
> + *	   Othrewise, return false.
> + *
> + * Context: Can be called in any context but only when Braille console is
> + *	registered and the struct console could not disappear.
> + */
> +bool nbcon_is_braille(struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +	struct console *con = ctxt->console;
> +
> +	return con && con->flags & CON_BRL;
> +}
> +
> +/**
> + * nbcon_braille_try_acquire - Try to acquire nbcon console for braille_write()
> + *
> + * @con:	The nbcon console to acquire
> + * @wctxt:	The nbcon write context to be used on success
> + *
> + * Context:	braille_write() for emitting a single buffer on Braille console.
> + *
> + * Return:	True if the console was acquired. False otherwise.
> + *
> + * Braille console is not registered as a proper printk consoles. Instead,
> + * it is integrated with the graphical virtual terminal.
> + *
> + * This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY.
> + */
> +bool nbcon_braille_try_acquire(struct console *con,
> +			   struct nbcon_write_context *wctxt)
> +{
> +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	memset(ctxt, 0, sizeof(*ctxt));
> +	ctxt->console = con;
> +	ctxt->prio    = NBCON_PRIO_EMERGENCY;
> +
> +	return nbcon_context_try_acquire(ctxt, false);
> +}

This is really the same thing as nbcon_kdb_try_acquire().

As you mentioned in your cover letter, it is a bug that
nbcon_kdb_try_acquire() enters an unsafe section because @unsafe is a
bool rather than a usage counter.

Since we have limited bits for struct nbcon_state, we need to stop
entering unsafe within nbcon_kdb_try_acquire(). As I mentioned in the
response to your cover letter, all the other CPUs should be quiesced
anyway.

John