netdev - RE: [PATCH v3] Renesas Ethernet AVB driver

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Date:	Tue, 14 Apr 2015 11:31:07 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Lino Sanfilippo' <LinoSanfilippo@....de>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"pawel.moll@....com" <pawel.moll@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"richardcochran@...il.com" <richardcochran@...il.com>
CC:	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>
Subject: RE: [PATCH v3] Renesas Ethernet AVB driver

From: Lino Sanfilippo
> On 14.04.2015 00:07, Sergei Shtylyov wrote:
> 
...
> > +#ifdef __LITTLE_ENDIAN
> > +	u32 res: 16;	/* Reserved bits */
> > +	u32 ts_sh: 16;	/* Timestamp high */
> > +#else
> > +	u32 ts_sh: 16;	/* Timestamp high */
> > +	u32 res: 16;	/* Reserved bits */
> > +#endif
> > +};
> 
> I recall a thread in which the use of bitfields for structs that are
> shared with the hardware was considered a bad idea (because the compiler
> is free to reorder the fields). Shift operations are probably a better
> choice here.

The compiler itself isn't free to reorder the fields, but the order
is an implementation decision for the compiler/ABI.
An ABI will probably define a bit order, but it doesn't have to match
the endianness.

Shifting and masking also tends to generate better code.

	David

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