linux-kernel - Re: [PATCH] ARM: Gemini: Add support for PCI BUS

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Date:	Tue, 30 Nov 2010 09:15:05 +0100 (CET)
From:	Hans Ulli Kroll <ulli.kroll@...glemail.com>
To:	Paulius Zaleckas <paulius.zaleckas@...il.com>
cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Hans Ulli Kroll <ulli.kroll@...glemail.com>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: Gemini: Add support for PCI BUS



On Mon, 29 Nov 2010, Paulius Zaleckas wrote:

> On 11/29/2010 10:02 PM, Arnd Bergmann wrote:
> > On Monday 29 November 2010 19:52:55 Paulius Zaleckas wrote:
> > > On 11/29/2010 06:45 PM, Arnd Bergmann wrote:
> > > > There are many differences between readl and __raw_readl, including
> > > >
> > > > * __raw_readl does not have barriers and does not serialize with
> > > >     spinlocks, so it breaks on out-of-order CPUs.
> > > > * __raw_readl does not have a specific endianess, while readl is
> > > >     fixed little-endian, just as the hardware is in this case.
> > > >     The endian-conversion is a NOP on little-endian ARM, but required
> > > >     if you actually run on a big-endian ARM (you don't).
> > > > * __raw_readl may not be atomic, gcc is free to split the access
> > > >     into byte wise reads (it normally does not, unless you mark
> > > >     the pointer __attribute__((packed))).
> > > >
> > > > In essence, it is almost never a good idea to use __raw_readl, and
> > > > the double underscores should tell you so.
> > >
> > > You are wrong:
> > >
> > > Since CONFIG_ARM_DMA_MEM_BUFFERABLE is NOT defined for FA526 core,
> > > no barriers are in use when using readl. It just translates into
> > > le32_to_cpu(__raw_readl(x)). Now this CPU has physical pin for endianess
> > > configuration and if you will chose big-endian you will fail to read
> > > internal registers, because they ALSO change endianess and le32_to_cpu()
> > > will screw it. However it is different when accessing registers through
> > > PCI bus, then you need to use readl().
> >
> > Ok, I only checked that the platform does not support big-endian Linux
> > kernel, not if the HW designers screwed up their registers, sorry about
> > that.
> >
> > The other points are of course still valid: If the code ever gets
> > used on an out of order CPU, it is broken. More importantly, if someone
> > looks at the code as an example for writing another PCI support code,
> > it may end up getting copied to some place where it ends up causing
> > trouble.
> >
> > The typical way to deal with mixed-endian hardware reliably is to have
> > a header file containing code like
> >
> > #ifdef CONFIG_GEMINI_BIG_ENDIAN_IO
> > #define gemini_readl(x) __swab32(readl(x))
> > #define ...
> > #else
> > #define gemini_readl(x) readl(x))
> > #endif
> >
> > This also takes care of the (not as unlikely as you'd hope) case that
> > the next person reusing the PCI hardware wires its endianess different
> > from the CPU endianess.
> 
> Actually I am not very sure how CPU works in big endian mode :)
> I have never tried it and I think only some guys who made it did that.
> So readl will work for 99.99% of cases. In datasheet they say that:
> "All registers in Gemini use Little Endian and must be accessed by aligned
> 32-bit word operations. The bus connection interface logic provides an Endian
> Conversion function."
> For me it looks like it can mean whatever you want :)
> 

I think the endianes pin switched only the cpu, not the hardware 
registers.

Here is some sample code from the ethernet devive on Gemini
typedef union
{
	unsigned int bits32;
	struct bit
	{
#if (BIG_ENDIAN==1)
		unsigned int reserved		: 15;	// bit 31:17
		unsigned int v_bit_mode		: 1;	// bit 16
		unsigned int device_id		: 12;	// bit 15:4
		unsigned int revision_id	: 4;	// bit  3:0
#else
		unsigned int revision_id	: 4;	// bit  3:0
		unsigned int device_id		: 12;	// bit 15:4
		unsigned int v_bit_mode		: 1;	// bit 16
		unsigned int reserved		: 15;	// bit 31:17
#endif
	} bits;
} TOE_VERSION_T;


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