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

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Message-ID: <a9b8ec3a-4f40-c0f5-e1a-bb577d5937ff@linux.intel.com>
Date:   Mon, 20 Jun 2022 14:26:17 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc:     linux-serial <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,
        LKML <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, 20 Jun 2022, Andy Shevchenko wrote:

> 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.

I'll add into the end of serial_core.h but in a cleaned up form
using offsetof(). Those above look rather ugly :-).

> > [*] 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;

The flags in .flags indicate when these two new fields are in use. Do you 
think I need something beyond that. Maybe I should remove those comments 
so they don't mislead you to think it's a "version" for real?


-- 
 i.