Re: [patch V2 04/13] USB: serial: digi_acceleport: Remove in_interrupt() usage

From: Johan Hovold <johan@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Johan Hovold <johan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-usb@vger.kernel.org,
	Thomas Winischhofer <thomas@winischhofer.net>,
	Mathias Nyman <mathias.nyman@intel.com>,
	Valentina Manea <valentina.manea.m@gmail.com>,
	Shuah Khan <shuah@kernel.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	linux-omap@vger.kernel.org, Kukjin Kim <kgene@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org,
	Felipe Balbi <balbi@kernel.org>,
	Duncan Sands <duncan.sands@free.fr>
Subject: Re: [patch V2 04/13] USB: serial: digi_acceleport: Remove in_interrupt() usage
Date: Sun, 25 Oct 2020 18:16:13 +0100	[thread overview]
Message-ID: <20201025171613.GT26280@localhost> (raw)
In-Reply-To: <20201019101110.019266389@linutronix.de>

On Mon, Oct 19, 2020 at 12:06:33PM +0200, Thomas Gleixner wrote:
> From: Ahmed S. Darwish <a.darwish@linutronix.de>
> 
> The usage of in_interrupt() in drivers is phased out and Linus clearly
> requested that code which changes behaviour depending on context should
> either be separated or the context be conveyed in an argument passed by the
> caller, which usually knows the context.
> 
> The debug printk() in digi_write() prints in_interrupt() as context
> information. TTY writes happen always in preemptible task context and
> console writes can happen from almost any context, so in_interrupt() is not
> really helpful.

The above statement is not correct, TTY writes can and do happen from
other contexts, including soft IRQ (e.g. PPP).

> Aside of that issuing a printk() from a console->write() callback is not a
> really brilliant idea for obvious reasons.

True, but we don't need to sprinkle conditionals for the benefit of
people trying to debug USB serial drivers using a USB serial console.
They get what they deserve. ;)

> Remove the in_interrupt() printout and make the printk() depend on tty.
> 
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Johan Hovold <johan@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: linux-usb@vger.kernel.org
> 
> ---
>  drivers/usb/serial/digi_acceleport.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> --- a/drivers/usb/serial/digi_acceleport.c
> +++ b/drivers/usb/serial/digi_acceleport.c
> @@ -911,9 +911,10 @@ static int digi_write(struct tty_struct
>  	unsigned char *data = port->write_urb->transfer_buffer;
>  	unsigned long flags = 0;
>  
> -	dev_dbg(&port->dev,
> -		"digi_write: TOP: port=%d, count=%d, in_interrupt=%ld\n",
> -		priv->dp_port_num, count, in_interrupt());
> +	if (tty) {
> +		dev_dbg(&port->dev, "digi_write: TOP: port=%d, count=%d\n",
> +			priv->dp_port_num, count);
> +	}

So just drop the in_interrupt() here.

Also note that we already have another unconditional dev_dbg() at the
end of this function.

>  
>  	/* copy user data (which can sleep) before getting spin lock */
>  	count = min(count, port->bulk_out_size-2);
> 

Johan