linux-kernel - Re: [PATCH 17/18] serial: sc16is7xx: refactor EFR lock

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Message-ID: <ZYMQRv1Td7FjH5Mh@smile.fi.intel.com>
Date: Wed, 20 Dec 2023 18:03:18 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Hugo Villeneuve <hugo@...ovil.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, jringle@...dpoint.com,
	kubakici@...pl, phil@...pberrypi.org, bo.svangard@...eddedart.se,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	Hugo Villeneuve <hvilleneuve@...onoff.com>
Subject: Re: [PATCH 17/18] serial: sc16is7xx: refactor EFR lock

On Tue, Dec 19, 2023 at 12:19:01PM -0500, Hugo Villeneuve wrote:
> From: Hugo Villeneuve <hvilleneuve@...onoff.com>
> 
> Move common code for EFR lock/unlock of mutex into functions for code reuse
> and clarity.

...

> @@ -333,6 +333,7 @@ struct sc16is7xx_one {
>  	struct sc16is7xx_one_config	config;
>  	bool				irda_mode;
>  	unsigned int			old_mctrl;
> +	u8				old_lcr; /* Value before EFR access. */
>  };

Have you run `pahole`?
I believe with

	unsigned int			old_mctrl;
	u8				old_lcr; /* Value before EFR access. */
	bool				irda_mode;

layout it will take less memory.

...

> +/* In an amazing feat of design, the Enhanced Features Register (EFR)

/*
 * This is NOT the style we use for multi-line
 * comments in the serial subsystem. On contrary
 * this comment can be used as a proper example.
 * (Yes, I noticed it's an old comment, but take
 *  a chance to fix it.)
 */

> + * shares the address of the Interrupt Identification Register (IIR).
> + * Access to EFR is switched on by writing a magic value (0xbf) to the
> + * Line Control Register (LCR). Any interrupt firing during this time will
> + * see the EFR where it expects the IIR to be, leading to
> + * "Unexpected interrupt" messages.
> + *
> + * Prevent this possibility by claiming a mutex while accessing the EFR,
> + * and claiming the same mutex from within the interrupt handler. This is
> + * similar to disabling the interrupt, but that doesn't work because the
> + * bulk of the interrupt processing is run as a workqueue job in thread
> + * context.
> + */

...

> +	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
> +			     SC16IS7XX_LCR_CONF_MODE_B);

One line. (Yes, 81 character, but readability is as good as before.

-- 
With Best Regards,
Andy Shevchenko