linux-kernel - Re: [RESEND PATCH 2/4] asm-generic: io: don't perform swab during {in,out} string functions

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Message-ID: <CAMuHMdXJaUD6u+m1ax1_QYVXDrdm-W7VDUq0VN6a_v9QAgP4og@mail.gmail.com>
Date:	Sun, 28 Oct 2012 10:28:30 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Mike Frysinger <vapier@...too.org>
Subject: Re: [RESEND PATCH 2/4] asm-generic: io: don't perform swab during
 {in,out} string functions

Hi Ben,

On Tue, Oct 23, 2012 at 3:25 AM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Thu, 2012-10-18 at 07:48 +0200, Geert Uytterhoeven wrote:
>> So assume you have the bytestream "Hello, world!\n" in memory on the
>> PCI device.I.e.
>>
>> 00000000  48 65 6c 6c 6f 2c 20 77  6f 72 6c 64 21 0a        |Hello, world!.|
>>
>> You want to copy it to system RAM using readsl(), which does:
>>
>>                u32 *buf = buffer;
>>                do {
>>                        u32 x = __raw_readl(addr + PCI_IOBASE);
>>                        *buf++ = x;
>>                 } while (--count);
>>
>> On little endian, the first __raw_readl() should return "0x6c6c6548", so
>> it is stored correctly by "*buf = x ".
>
> Right.
>
>> On big endian, the first __raw_readl() should return "0x48656c6c" instead,
>> else it's stored incorrectly by "*buf = x ".
>> But the PCI bus is little endian, so I expect __raw_readl() would return
>> "0x6c6c6548", and thus needs swapping?
>
> No. The PCI bus will return 0x48656c6c.
>
> This is due to how the PCI bus is wired to the CPU bus, which is called
> "byte address invariant". When doing a read of your byte 0, the CPU will
> effectively read 0 with byte enables picking 48. Since the CPU wants
> the first byte in the MSB, the bus must be wired up to the CPU such that
> the MSB is the first byte in address order.

According to
https://developer.apple.com/library/mac/#documentation/DeviceDrivers/Conceptual/WritingPCIDrivers/endianness/endianness.html

"Byte-invariant addressing is a property of the bus bridge itself."

and

"From the software designer’s perspective, this means that the
hardware does not byte swap the data. However, from the hardware
designer’s perspective, the hardware must byte swap all data."

So this depends on a correct hardware implementation in the PCI
host bridge?

> This is how a BE CPU is normally wired to a LE bus. The fact that a
> register needs to be swapped comes from the fact that the device will
> put the MSB in the higher address byte, which then corresponds to the
> LSB on the BE CPU -> needs swapping.
>
> But for a byte stream, as you can see, no swapping is required.
>
> To some extent, it looks as if the wiring is byte swapped, in order to
> provide the byte address invariance (and thus causes registers to look
> byteswapped) but it's really not, it's just a convention.
>
>> On m68k (classic with MMU), the situation is a bit more complicated, as some ISA
>> (or PCMCIA) busses are physically wired swapped. But inw() and insw() always do
>> either both swapping, or both no swapping. The state of support for ISA drivers
>> is a bit vague, though.
>
> Then something is horribly wrong in those m68k setups :-) Either in the
> way the busses are wired or in your implementation of either inw or
> insw.

On (classic) m68k all of this is not about PCI (Atari Hades PCI is no
more), but about ISA and PCMCIA. I.e. no PCI host bridge with a modern
understanding of how it should be wired correctly on a big endian platform.

>> Now, on m68k without MMU, inw() doesn't do swapping, but insw() does.
>> This agrees more or less with you and Will, except that the bus seems to be
>> physically swapped?
>
> Yeah that's possible. In the early days people incorrectly thought it
> would be smart to do that, thus avoiding a byteswap for register
> accesses .... and causing all data (DMA or FIFO) to be all backward.
>
> That's for example why the disks in the Tivo are byteswapped 16-bit by
> 16-bit on the platter (ie, all the data is).
>
> I wouldn't be surprised if some 68k suffered from similar HW designer
> stupidity.

Yep, Atari IDE suffers from this, too.

> But the "proper" generic rule is byte address invariance, and that
> implies swap on register access and no swap on byte streams.

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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