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

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Message-ID: <Yw3PHg0imhJyb9sf@kroah.com>
Date:   Tue, 30 Aug 2022 10:49:34 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Russell King <linux@...linux.org.uk>,
        Richard Genoud <richard.genoud@...il.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        Lino Sanfilippo <LinoSanfilippo@....de>
Subject: Re: [PATCH v2 4/4] serial: Add kserial_rs485 to avoid wasted space
 due to .padding

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