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[Qemu-devel] Re: [PATCH 00/15] [RFC] MMIO endianness cleanup
From: |
Paul Brook |
Subject: |
[Qemu-devel] Re: [PATCH 00/15] [RFC] MMIO endianness cleanup |
Date: |
Thu, 25 Nov 2010 12:14:46 +0000 |
User-agent: |
KMail/1.13.5 (Linux/2.6.36-trunk-amd64; KDE/4.4.5; x86_64; ; ) |
> The way mmio endianness is currently implemented is horrifying.
Agreed.
> #ifdef TARGET_WORDS_BIGENDIAN
> val = bswap32(val);
> #endif
>
> With the move to get device code only compiled once, this has
> become harder and harder to justify though, since we don't know
> the target endianness during compile time.
Not just that, it's wrong to start with. I've used machines with both native
and cross-endian 16550 based UARTs.
> So my solution to the issue is to make every device define if
> it's a little, big or native (target) endianness device. This
> basically tells the layers below what endianness the device
> expects mmio to occur in. Little endian devices on little endian
> hosts don't swap. On big endian hosts they do. Same the other
> way around.
>
> The only reason I added "native" endianness is that we have some
> PV devices like the fw_cfg that expect qemu's broken behavior.
> These devices are the minority though. In the long run I'd expect
> to see most code be committed with either of the two endianness
> choices.
I'd prefer to avoid this, or at least document it as a temporary hack that
should be removed. If a device can exist in either endian, then we really
want to push this decision down to the board-level code.
One of the reasons I haven't bothered fixing this yet is that this feels like
something that should be a device/bus property. e.g. PCI devices/busses are
always little-endian[1], as as mentioned above some devices come in both
flavors. I guess we can go with your approach for now, and make sure we fix
this properly when we introduce bus-specific registration functions.
Paul
[1] Ignoring magical byteswapping cpu-pci bridges, but they're broken by
design, and thankfully quite rare.
- Re: [Qemu-devel] [PATCH 08/15] prep: Declare as little endian, (continued)
[Qemu-devel] [PATCH 10/15] ppc4xx_pci: Declare as little endian, Alexander Graf, 2010/11/25
[Qemu-devel] [PATCH 09/15] versatile_pci: Declare as little endian, Alexander Graf, 2010/11/25
[Qemu-devel] [PATCH 14/15] isa_mmio: Always use little endian, Alexander Graf, 2010/11/25
[Qemu-devel] [PATCH 02/15] Add endianness as io mem parameter, Alexander Graf, 2010/11/25
Re: [Qemu-devel] [PATCH 00/15] [RFC] MMIO endianness cleanup, Gerd Hoffmann, 2010/11/25
[Qemu-devel] Re: [PATCH 00/15] [RFC] MMIO endianness cleanup,
Paul Brook <=
[Qemu-devel] Re: [PATCH 00/15] [RFC] MMIO endianness cleanup, Blue Swirl, 2010/11/26