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    SubjectRe: [PATCH] barrier patch set
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    On Saturday 20 of March 2004 01:42, Jeff Garzik wrote:
    > Bartlomiej Zolnierkiewicz wrote:
    > > On Saturday 20 of March 2004 01:14, Jeff Garzik wrote:
    > >>Bartlomiej Zolnierkiewicz wrote:
    > >>>The fact that spec says "supported" not "enabled" in description of
    > >>>word86 makes me wonder - can they be disabled? (FLUSH CACHE is mandatory
    > >>>for General feature set and FLUSH CACHE EXT is mandatory if 48-bit LBA
    > >>> is supported)
    > >>
    > >>Yes, that's why there are separate 'supported' and 'enabled' bits for
    > >>each feature.
    > >>
    > >>Words 82-84 are 'supported' bits. Words 85-87 are 'enabled' bits.
    > >>These bits mirror each other, i.e. Word 83 and Word 86 have basically
    > >>the same bits, except that Word 86 definitions change _slightly_ since
    > >>the only bits that are relevant are the ones for features that can be
    > >>disabled/enabled.
    > >>
    > >>You use set-features command to enable and disable these features, and
    > >>then the result shows up in subsequent identify-device command output.
    > >>
    > >>If the driver is testing for a capability but does not enable it, then
    > >>always use the 'enabled' set of bits, not the 'supported' set of bits.
    > >
    > > This is quite obvious but I am talking about confusing wording in
    > > description of word86 - for some features 'enabled' is used and for
    > > others 'supported'
    >
    > Yeah, mainly the difference is communicating in the description of each
    > word.
    >
    > Anyway, what I described is how things work :) For example, features
    > that are always enabled in the drive are listed with both support and
    > enabled bits set. The driver sees that, and does not issue a
    > set-features command, because it does not need to.

    Yep but in your original mail you suggested that we should explicitly enable
    FLUSH CACHE and FLUSH CACHE EXT features - there are even no subcommands
    to do this. ;-)

    > >>>IMO to test if FLUSH CACHE works we should just issue it during disk
    > >>>setup and check result. This way we can use FLUSH CACHE also on < ATA-6
    > >>>devices (there is a lot of them).
    > >>
    > >>I disagree. "just issue it" is how those LG cdrom drives got cooked.
    > >
    > > I'm aware of LG fun. Jens already stated that current barrier
    > > implementation is disk-only and I'm talking about disks only.
    > >
    > > If anybody reused CACHE FLUSH opcode for disk drive he/she deserves to
    > > loose. 8)
    >
    > Well... If you don't check the proper feature bits found in the spec, I
    > blame the driver for ignoring the spec... :)

    Please point me to these bits in ATA-4 or ATA-5 spec.
    Have you checked them as I asked?

    > >>All drives that support flush-cache list the relevant bits in
    > >>identify-device, even on pre-ATA-6 devices. Whether the feature was
    > >>optional or mandantory, we can check the feature bits.
    > >
    > > Hm. so this is undocumented in the spec?
    >
    > ? When it was optional, there was a feature bit to test. When it
    > became mandantory, the feature bit to test stayed in there. The feature
    > bit is zero, otherwise. Makes it possible to use "just test the bit"
    > and have things Just Work(tm). :)

    I wish it was so simple. Here is an example to make it clear:

    model: WDC WD800JB-00CRA1 firmware: 17.07W77
    word 0x83 is 4b01, word 0x86 is 0x0801

    and drive of course supports CACHE FLUSH command.

    Cheers,
    Bartlomiej

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