linux-kernel - Re: [PATCH v7 5/6] serial: Support for RS-485 multipoint addresses

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Date:   Wed, 15 Jun 2022 17:13:13 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-serial@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Lukas Wunner <lukas@...ner.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v7 5/6] serial: Support for RS-485 multipoint addresses

On Wed, Jun 15, 2022 at 03:48:28PM +0300, Ilpo Järvinen wrote:
> Add support for RS-485 multipoint addressing using 9th bit [*]. The
> addressing mode is configured through .rs485_config().
> 
> ADDRB in termios indicates 9th bit addressing mode is enabled. In this
> mode, 9th bit is used to indicate an address (byte) within the
> communication line. ADDRB can only be enabled/disabled through
> .rs485_config() that is also responsible for setting the destination and
> receiver (filter) addresses.
> 
> [*] Technically, RS485 is just an electronic spec and does not itself
> specify the 9th bit addressing mode but 9th bit seems at least
> "semi-standard" way to do addressing with RS485.
> 
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: linux-api@...r.kernel.org
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-arch@...r.kernel.org

Hmm... In order to reduce commit messages you can move these Cc:s after the
cutter line ('---').

...

> -	__u32	padding[5];		/* Memory is cheap, new structs
> -					   are a royal PITA .. */
> +	__u8	addr_recv;
> +	__u8	addr_dest;
> +	__u8	padding[2 + 4 * sizeof(__u32)];		/* Memory is cheap, new structs
> +							 * are a royal PITA .. */

I'm not sure it's an equivalent. I would leave u32 members  untouched, so
something like

	__u8	addr_recv;
	__u8	addr_dest;
	__u8	padding0[2];		/* Memory is cheap, new structs
	__u32	padding1[4];		 * are a royal PITA .. */

And repeating about `pahole` tool which may be useful here to check for ABI
potential changes.

-- 
With Best Regards,
Andy Shevchenko