netdev - Re: [PATCH 2/14] nes: device structures and defines

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Date:	Wed, 8 Aug 2007 15:48:25 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jeff Garzik <jeff@...zik.org>, ggrundstrom@...effect.com,
	rdreier@...co.com, ewg@...ts.openfabrics.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 2/14] nes: device structures and defines

On Wednesday 08 August 2007 15:38:50 Andi Kleen wrote:
> On Wed, Aug 08, 2007 at 03:28:33PM +0200, Michael Buesch wrote:
> > On Wednesday 08 August 2007 15:08:28 Andi Kleen wrote:
> > > On Wed, Aug 08, 2007 at 03:02:35PM +0200, Michael Buesch wrote:
> > > > On Wednesday 08 August 2007 14:55:11 Andi Kleen wrote:
> > > > > On Wed, Aug 08, 2007 at 01:50:35PM +0200, Michael Buesch wrote:
> > > > > > On Wednesday 08 August 2007 14:38:10 Andi Kleen wrote:
> > > > > > > Jeff Garzik <jeff@...zik.org> writes:
> > > > > > > > > +			val, reg_index, addr, addr+4); */
> > > > > > > > > +	writel(cpu_to_le32(reg_index), addr);
> > > > > > > > > +	writel(cpu_to_le32(val),(u8 *)addr + 4);
> > > > > > > > 
> > > > > > > > wrong -- endian conversion macros not needed with writel()
> > 
> > > > Fact is that we do _not_ need cpu_to_le32 with writel.
> > > 
> > > We do on a big endian platform if the register is LE. I assume that's the case 
> > > on this hardware.
> > 
> > That is not true.
> > writeX does automatically convert to bus-endian.
> > Which, in case of the PCI bus, is little endian.
> > So if your register is LE (which it is most likely), you don't
> > need any conversion. If your register is BE (which I very much doubt),
> > then you need swab32().
> > In _no_ case you need cpu_to_xx().
> 
> Hmm, I checked a couple of BE architectures and none seem to convert explicitely.

That depends on the arch.
Look at this from ppc, for example:

184 static inline void writel(__u32 b, volatile void __iomem *addr)
185 {
186         out_le32(addr, b);
187 }

out_le32 writes a little endian value. So it converts it.
Also note that b is __u32. Sparse would complain, if someone used cpu_to_xx
on it.

> But ok it's possible that their PCI bridges convert. I'll take your
> word for it although it sounds somewhat dubious.
>
> However if that's true then there are quite some drivers wrong:
> 
> % grep -r write[bwl]\(cpu_to *   | wc -l
> 57

Yeah, seems so.
Here comes an example (with 16bit values)

Little endian register
	LittleEndian arch		BigEngian arch
value	0xBBAA				0xAABB
writew	0xBBAA				0xAABB
on bus	0xBBAA				0xBBAA
on dev	0xBBAA				0xBBAA

Big endian register
	LittleEndian arch		BigEngian arch
value	0xBBAA				0xAABB
swab16	0xAABB				0xBBAA
writew	0xAABB				0xBBAA
on bus	0xAABB				0xAABB
on dev	0xAABB				0xAABB

I hope I got it right. :)
Same result on every arch.

Most hardware has little endian registers. Some can switch
endianess based on some bit, too. So in case of a BE register that
bit has to be flipped, or if not possible swabX() has to be used.

-- 
Greetings Michael.
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