linux-kernel - Re: [PATCH v8 5/6] serial: Support for RS-485 multipoint addresses

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Message-ID: <YrBS03ymAWVajy7e@smile.fi.intel.com>
Date:   Mon, 20 Jun 2022 13:58:27 +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 v8 5/6] serial: Support for RS-485 multipoint addresses

On Mon, Jun 20, 2022 at 09:40:29AM +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.

> The changes to serial_rs485 struct were test built with a few traps to
> detect mislayouting on archs lkp/0day builts for (all went fine):
>   BUILD_BUG_ON(((&rs485.delay_rts_after_send) + 1) != &rs485.padding[0]);
>   BUILD_BUG_ON(&rs485.padding[1] != &rs485.padding1[0]);
>   BUILD_BUG_ON(sizeof(rs485) != ((u8 *)(&rs485.padding[4]) -
> 				 ((u8 *)&rs485.flags) + sizeof(__u32)));

You may add static_asserts() for the above mentioned cases.

> [*] 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.

...

> -	__u32	padding[5];		/* Memory is cheap, new structs
> -					   are a royal PITA .. */
> +	union {
> +		/* v1 */
> +		__u32	padding[5];		/* Memory is cheap, new structs are a pain */
> +
> +		/* v2 (adds addressing mode fields) */

How user space will inform a kernel that it's trying v2?

Usually when we have a union, it should be accompanied with the enum or version
or something to tell which part of it is in use. I can imagine that in this case
it's implied by the IOCTL parameters that never should be used on a garbage.

Either add a commit message / UAPI comment or add a version field or ...?

> +		struct {
> +			__u8	addr_recv;
> +			__u8	addr_dest;
> +			__u8	padding0[2];
> +			__u32	padding1[4];
> +		};
> +	};

-- 
With Best Regards,
Andy Shevchenko