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

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Message-ID: <c0996d75-070e-21e6-eb51-a10a358dbb46@kernel.org>
Date:   Thu, 16 Jun 2022 07:48:58 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-serial <linux-serial@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.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 v7 5/6] serial: Support for RS-485 multipoint addresses

On 16. 06. 22, 7:04, Ilpo Järvinen wrote:
> On Wed, 15 Jun 2022, Andy Shevchenko wrote:
> 
>> 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 ('---').
> 
> Ok, although the toolchain I use didn't support preserving --- content
> so I had to create hack to preserve them, hopefully nothing backfires due
> to the hack. :-)
> 
>>> -	__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.
> 
> I cannot take __u32 padding[] away like that, this is an uapi header.

Yeah, but it's padding after all. I would personally break it for 
example as Andy suggests (if pahole shows no differences in size on both 
32/64 bit) and wait if something breaks. To be honest, I'd not expect 
anyone to touch it. And if someone does, we would fix it somehow and 
they should too...

thanks,
-- 
js
suse labs