Re: [PATCH v2 4/4] serial: Add kserial_rs485 to avoid wasted space due to .padding

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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?

> +/**
> + * 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?  And how is that going to be enforced?

> + */
> +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.

thanks,

greg k-h