linux-kernel - Re: [RFC] Normalizing byteorder/unaligned access API

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Date:	Wed, 8 Oct 2008 09:13:09 +0200 (CEST)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Harvey Harrison <harvey.harrison@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-arch <linux-arch@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Matthew Wilcox <matthew@....cx>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Boaz Harrosh <bharrosh@...asas.com>
Subject: Re: [RFC] Normalizing byteorder/unaligned access API

On Tue, 7 Oct 2008, Harvey Harrison wrote:
> [related question regarding the SCSI-private endian helper needs at the end]
> 
> Currently on the read side, we have (le16 as an example endianness)
> 
> le16_to_cpup(__le16 *)
> get_unaligned_le16(void *)
> 
> And on the write side:
> 
> *(__le16)ptr = cpu_to_le16(u16)
> put_unaligned_le16(u16, void *);
> 
> On the read side, Al said he would have preferred the unaligned version
> take the same types as the aligned, rather than void *.  AKPM didn't think

As I said before, me too (take the same types as the aligned). I like to
rely on sparse for:

    struct {
	...
	__le32 x;
	...
    } s __attribute__ ((packed));

get_unaligned_le16(&s.x);

> the use of get_ was that great as get/put generally implies some kind of reference
> taking in the kernel.

OK.

> As the le16_to_cpup has been around for so long and is more recognizable, let's
> make it the same for the unaligned case and typesafe:
> 
> le16_to_cpup(__le16 *)
> unaligned_le16_to_cpup(__le16 *)

I always hated that naming...

> On the write side, the above get/put and type issues are still there, in addition AKPM felt
> that the ordering of the put_unaligned parameters was opposite what was intuitive and that
> the pointer should come first.
> 
> In this case, as there is currently no aligned helper (other than in some drivers defining macros)
> define the api thusly:
> 
> Aligned:
> write_le16(__le16 *ptr, u16 val)
> 
> Unaligned:
> unaligned_write_le16(__le16 *ptr, u16 val)

Does it write to MMIO I/O space? No? Then please don't use write (like
in writeb()).

What about load_{unaligned_,}le16() and store_{unaligned_,}le16()?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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