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

[Ilpo Järvinen]

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@xxxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: linux-api@xxxxxxxxxxxxxxx
> > Cc: linux-doc@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: linux-arch@xxxxxxxxxxxxxxx
> 
> 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. Or 
do you mean I should create anonymous union? ...I'm skeptical that can be 
pulled off w/o breaking user-space compile in some circumstances. Anon 
unions were only introduced by C11 but is it ok to rely on C11 in uapi/ 
headers?

Even making padding smaller has some unwanted consequences if somebody is 
clearing just .padding. In retrospect, having padding as a direct member 
doesn't seem a good idea. That padding[5] should have been within an union 
right from the start to make this easily extendable.

Maybe create a copy of that struct under another name which is just equal 
sized, that would give more freedom on member naming. But can I change 
ioctl's param type to another struct (in _IOR/_IOWR) w/o breaking 
something?

-- 
 i.