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Date: Wed, 1 Dec 2010 12:52:28 +0100 (CET) From: Hans Ulli Kroll <ulli.kroll@...glemail.com> To: Paulius Zaleckas <paulius.zaleckas@...il.com> cc: Hans Ulli Kroll <ulli.kroll@...glemail.com>, Arnd Bergmann <arnd@...db.de>, linux-arm-kernel@...ts.infradead.org, Russell King <linux@....linux.org.uk>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] ARM: Gemini: Add support for PCI BUS On Tue, 30 Nov 2010, Paulius Zaleckas wrote: > On Tue, Nov 30, 2010 at 10:15 AM, Hans Ulli Kroll > <ulli.kroll@...glemail.com> wrote: > > > > > > 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. > > Yes, but original driver used readl/writel and it does le32_to_cpu, > so that structure definition is just reversing it. > If you will use __raw_readl/__raw_writel than there will be no need > for this redefinition. > > > 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 > > The other thing is that this endianess redefinition is very starnge since > it should swap bytes and not bits inside this struct. So I assume that > big endian was never tested on this driver and it will not work. > But ofcouse I can be wrong here :) > At this momment my brain restarts in very slow motion mode ;-) This can't work. The definition Storlinksemi uses for swapping bits and bytes are totaly wrong. They never _even_ testet this, or understand little endian or big endian. Take this simple sample typedef union { unsigned int bits32; struct bit { #if (BIG_ENDIAN==0) unsigned int a : 1; unsigned int b : 31; #else unsigned int b : 31; unsigned int a : 1; #endif }; } TEST; They swaped the bits inside one byte > > } bits; > > } TOE_VERSION_T; > > > > > > >
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