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
