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

On Wed, 15 Jun 2022, Andy Shevchenko wrote:

On Wed, Jun 15, 2022 at 03:48:28PM +0300, Ilpo Järvinen wrote:

quoted

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@arndb.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-api@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.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. :-)

quoted

-	__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.