On Tue, 30 Aug 2022, Greg Kroah-Hartman wrote:
> On Tue, Aug 30, 2022 at 10:29:56AM +0300, Ilpo Järvinen wrote:
> > The struct serial_rs485 has a .padding field to make uapi updates
> > easier. It wastes space, however. Create struct kserial_rs485 which is
> > a kerner counterpart w/o padding.
>
> "kernel"?
>
> And what is the size difference now?
Roughly 16B of padding is eliminated. That saving is then multiplied by
the times it appears in in-kernel structs (2x per uart_port, 1x per rs485
supporting driver). As said in my other reply, if you feel it's too
little gained by eliminating the padding, I can drop this patch, just let
me know.
> > +/**
> > + * struct kserial_rs485 - kernel-side struct for controlling RS485 settings.
> > + * @flags: RS485 feature flags
> > + * @delay_rts_before_send: Delay before send (milliseconds)
> > + * @delay_rts_after_send: Delay after send (milliseconds)
> > + * @addr_recv: Receive filter for RS485 addressing mode
> > + * (used only when %SER_RS485_ADDR_RECV is set).
> > + * @addr_dest: Destination address for RS485 addressing mode
> > + * (used only when %SER_RS485_ADDR_DEST is set).
> > + *
> > + * Must match with struct serial_rs485 in include/uapi/linux/serial.h excluding
> > + * the padding.
>
> Why must this match?
Because serial_rs485_from_user() and serial_rs485_to_user() just copy
things over from one struct type to another w/o considering the fields
individually. If that's not acceptable, I could make it copy field by
field but it didn't feel necessary to allow "real" fields to differ to
achieve padding elimination...
> And how is that going to be enforced?
With static_assert()s in serial_core.c. I'll add a note about that into
the comment.
> > + */
> > +struct kserial_rs485 {
> > + __u32 flags;
> > + __u32 delay_rts_before_send;
> > + __u32 delay_rts_after_send;
> > + struct {
> > + __u8 addr_recv;
> > + __u8 addr_dest;
> > + };
>
> As this is an in-kernel structure, this should be "u32" and "u8" now.
Right, I'll change those.
--
i.
