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    SubjectRe: [Linux-fbdev-devel] [PATCH 1/2] fb: add support for foreign endianness
    Benjamin Herrenschmidt schrieb:
    > On Tue, 2008-02-19 at 00:35 +0100, Clemens Koller wrote:
    >> Valdis.Kletnieks@vt.edu schrieb:
    >>> On Mon, 18 Feb 2008 08:18:47 +0100, Krzysztof Helt said:
    >>>> I know two fb drivers which use endianess information (pm2fb and s3c2410fb).
    >>>> Both resolve endianess at driver level. Actually, both handle it by setting special
    >>>> bits so the graphics chip itself reorder bytes to transform foreign endianess.
    >>>> I understand that this patch is for chips which cannot reorder bytes by themselves.
    >>> Does anybody know of such a chip that's actually available in the wild? Or are
    >>> we writing drivers for speculative possible chips?
    >>>
    >> I had troubles with the Silicon Motion SM501/SM502 endianess on PowerPC PCI vs. LocalBus.
    >> The chip also has a register to swap endianess, but that seems to only affect some
    >> LocalBus modes.
    >> The current fb and X drivers are working, but when it comes to font
    >> aliasing and hw-acceleration, the problems start to rise again...
    >
    > Most "sane" gfx chips nowadays provide configurable surfaces that allow
    > to perform the swap when writing/reading from regions of the
    > framebuffer, with the ability to set a different swapper setting (based
    > on bit depth) per region.

    Most! But not the SM50x. I still hope I would be wrong here. :-(

    > Then there is also the risk that your PCI<->Localbus has been wired
    > improperly :-)

    That's not an issue in my case. The SM50x can be connected to
    either an PCI or some Local/CPU-whateverbus IF.
    I.e. on the MPC85xx PowerPC, PCI and LocalBus are separate bussses.
    If the sm501 is attached to the MPC85xx' PCI like any other video card,
    the PCI config-space is can be accessed as usual, whereas the framebuffer
    memory area is byte-swapped compared to other common video cards.

    So, to get back on topic:
    I would welcome endianess swapping in SW. Some architectures (PowerPC)
    should also be able to do swapped-endian mmapping. I just haven't
    had time for a closer look but it looks also interesting way to do it
    that way.

    Regards,

    Clemens


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