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

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Date:   Tue, 30 Aug 2022 11:44:59 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Jiri Slaby <jirislaby@...nel.org>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial <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, LKML <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, 30 Aug 2022, Jiri Slaby wrote:

> On 30. 08. 22, 9:29, Ilpo Järvinen wrote:
> > --- a/include/linux/serial_core.h
> > +++ b/include/linux/serial_core.h
> > @@ -31,6 +31,29 @@ struct serial_struct;
> ...
> > + * Must match with struct serial_rs485 in include/uapi/linux/serial.h
> > excluding
> > + * the padding.
> 
> Have you considered BUILD_BUG_ON() checks with few offset_of()s to enforce the
> above?

It's all enforced at the end of serial_core.c already. Would you perhaps 
prefer I'd put them elsewhere or is it fine as is?

/*
 * Compile-time asserts for struct kserial_rs485 and struct serial_rs485 equality
 * (except padding).
 */
static_assert(offsetof(struct kserial_rs485, flags) ==
              offsetof(struct serial_rs485, flags));
static_assert(offsetof(struct kserial_rs485, delay_rts_before_send) ==
              offsetof(struct serial_rs485, delay_rts_before_send));
static_assert(offsetof(struct kserial_rs485, delay_rts_after_send) ==
              offsetof(struct serial_rs485, delay_rts_after_send));
static_assert(offsetof(struct kserial_rs485, addr_recv) ==
              offsetof(struct serial_rs485, addr_recv));
static_assert(offsetof(struct kserial_rs485, addr_dest) ==
              offsetof(struct serial_rs485, addr_dest));
static_assert(sizeof(struct kserial_rs485) <= sizeof(struct serial_rs485));


-- 
 i.

> > +struct kserial_rs485 {
> > +	__u32	flags;
> > +	__u32	delay_rts_before_send;
> > +	__u32	delay_rts_after_send;
> > +	struct {
> > +		__u8    addr_recv;
> > +		__u8    addr_dest;
> > +	};
> > +};