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    SubjectRe: [PATCH v5 10/10] x86, efi: EFI boot stub support
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    On Thu, 2011-11-24 at 15:43 +0100, Maarten Lankhorst wrote:
    > Hey Matt,
    >
    > On 11/23/2011 08:27 PM, Matt Fleming wrote:
    >
    > > On Wed, 2011-11-23 at 01:44 +0100, Maarten Lankhorst wrote:
    > >> When I tested this with v3.2-rc2 it didn't boot, it hung before it
    > >> initialized the kernel.
    > >> Without initrd it works fine, though.
    > > Bah, so this change actually makes booting worse? You said before that
    > > you almost made it to userspace but this seems to hang much earlier now.
    > > Is that correct?
    > >
    > > ... back to the drawing board.
    > I was looking at why grub2 could boot, seems to be it reads in chunks of
    > 256 kilobytes. I seem to be able to get it to boot with chunks of 4 mb
    > as well, but didn't test beyond that.
    >
    > So the fix is to simply read the file in parts, otherwise efi hangs..
    > As a nice side effect, short reads are also handled, but the efi
    > firmware seems to choke over huge reads and dies.

    Urrggghh.....

    Thanks a lot for diagnosing this, Maarten! The copy of grub that I have
    (fedora's version) looks like it uses the disk_io protocol to read from
    disks, I wonder if there's a reason for this. I Cc'd Peter Jones in case
    he's seen anything like this before.

    Aha, read_file() in the elilo source contains this helpful comment,

    /*
    * We load by chunks rather than a single big read because
    * early versions of EFI had troubles loading files
    * from floppies in a single big request. Breaking
    * the read down into chunks of 4KB fixed that
    * problem. While this problem has been fixed, we still prefer
    * this method because it tells us whether or not we're making
    * forward progress.
    */

    (but reading the code it's clear it uses a chunk size of 16KB, not 4KB)

    So, it looks like it's not just reading from floppies that hits this bug
    in the firmware.

    Maarten, could you send a patch against the 'x86/efi-stub' branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/mfleming/linux.git that
    splits the read up into 16KB chunks and explains,

    - why we need to do the reading in chunks
    - why we chose the 16KB chunk size (because elilo said so)

    That way, the bug fix will be properly documented and attributed to you.

    --
    Matt Fleming, Intel Open Source Technology Center



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