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    SubjectRe: [tip:x86/urgent] x86: Remove usedac in feature-removal-schedule.txt
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    On Mon, 14 Dec 2009 09:57:55 -0500
    Valdis.Kletnieks@vt.edu wrote:

    > On Mon, 14 Dec 2009 07:58:01 GMT, tip-bot for FUJITA Tomonori said:
    > > Commit-ID: 06f8bda8324fa8bf39eed81d8b3df08063a37696
    >
    > > x86: Remove usedac in feature-removal-schedule.txt
    >
    > > The usedac option enables us to stop via_no_dac() setting
    > > forbid_dac to 1. That is, someone who uses VIA bridges can use
    > > DAC with this option even if some of VIA bridges seem to be
    > > broken about DAC.
    >
    > Does there exist real hardware where this makes sense? If the chipset
    > detects as "broken-DAC", is it in fact safe to use?

    Not safe. Probably, you would see data corruption.

    arch/x86/kernel/pci-dma.c says:

    /* Many VIA bridges seem to corrupt data for DAC. Disable it here */


    Seems that some of VIA bridges were fine. So this option made sense
    with some hardware. I'm not sure if there are still users of this
    option now.


    > Or is it similar to
    > the 'force-enable HPET' code for some older boxes, where the HPET was in
    > fact there but simply not advertised, so going ahead and using it was
    > in fact perfectly safe? Allowing the use of "working but not advertised"
    > is probably a good thing, allowing the use of known-broken probably isn't.
    >
    > If it's just unadvertised, I wonder if if there's a way to write a quirk
    > for VIA systems that will detect the situation and enable the support?

    I guess that we could however it doesn't worth adding tricks for old
    hardware.


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