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    SubjectRe: RFC: Put printk buffer in video ram
    On Sun, 2009-11-22 at 14:15 +0200, Maxim Levitsky wrote: 
    > On Sun, 2009-11-22 at 09:25 +0100, Ingo Molnar wrote:
    > > * Frederic Weisbecker <> wrote:
    > >
    > > > On Sun, Nov 22, 2009 at 04:05:06AM +0200, Maxim Levitsky wrote:
    > > > > After doing some successful debugging by placing printk buffer in video
    > > > > ram, here I publish cleaned version of it.
    > > > >
    > > > > I discovered that on my system video ram isn't cleared on reboot, and I
    > > > > took advantage of that by placing printk buffer directly there.
    > > > > This allows to capture oopses/panicks almost from everywhere.
    > > > > It is also very simple to setup.

    Few more thoughts:

    First of all, if I implement this as a console driver, I won't be able
    to capture all kernel log, but only from the point I register the
    My implementation, also isn't set up very early, but I copy already
    written log to the memory buffer.

    Problem of automatic unregistration isn't a problem, its just a feature,
    I could skip using.

    Secondary, I found out that system ram isn't cleared ether on my
    notebook, however I found out that I can't reserve small memory range
    with 'memmap' kernel parameter. System just panics.
    I used for reference 'memmap=2M$0x60000000'
    My system has 2GB of memory.

    I could reserve the memory same way using the code, but it would
    probably fail in same way. Also I could try early reservations, but I am
    not sure I can reserve _any_ address range. I test this.

    I think that patches to put printk buffer at predefined area in system
    ram were once posted on LKML.

    And now, to be honest, lets talk about existing solutions:

    1 - kdump/kexec - not a bad thing, but it has some disadvantages, namely
    I need to compile seperate kernel, there is a probility of new kernel
    not booting up (in fact I once installed kexec-tools, and it made system
    reboot using kexec, and I remember a hang because of that...)
    However, this approach doesn't need any 'help' from BIOS.

    2 - mtdoops + phram - There is a console driver that logs into mtd
    device. I don't know how it selects one.
    It is possible to create a 'fake' mtd device that will span a range in
    physical memory.
    I couldn't make this work, it has problems with early output, etc..

    Best regards,
    Maxim Levitsky

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