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    SubjectDRM-based Oops viewer
    Hello DRM/UEFI maintainers,

    Several years ago, I wrote a set of patches to dump the kernel
    log to disk upon panic -- through BIOS INT 0x13 services. [1]

    The overwhelming response was that it's unsafe to do this in a
    generic manner. Linus proposed a video-based viewer instead: [2]

    If you want to do the BIOS services thing, do it for video: copy the
    oops to low RAM, return to real mode, re-run the graphics card POST
    routines to initialize text-mode, and use the BIOS to print out the
    oops. That is WAY less scary than writing to disk.

    Of course it's 2019 now though, and it's quite known that
    Intel is officially obsoleting the PC/AT BIOS by 2020.. [3]

    Researching whether this can be done from UEFI, it was also clear
    that UEFI "Runtime Services" do not provide any re-initialization
    routines. [4]

    The maximum possible that UEFI can provide is a GOP-provided
    framebuffer that's ready to use by the OS -- even after the UEFI
    boot phase is marked as done through ExitBootServices(). [5]

    Of course, once native drivers like i915 or radeon take over,
    such a framebuffer is toast... [6]

    Thus a possible remaining option, is to display the oops through
    "minimal" DRM drivers provided for each HW variant... Since
    these special drivers will run only and fully under a panic()
    context though, several constraints exist:

    - The code should be fully synchronous (irqs are disabled)
    - It should not allocate any dynamic memory
    - It should make minimal assumptions about HW state
    - It should not chain into any other kernel subsystem
    - It has ample freedom to use delay-based loops and the
    like, the kernel is already dead.

    How feasible is it to have such a special "DRM viewoops"
    framework + its minimal drivers in the kernel?

    The target is to start from i915, since that's what in my
    laptop now, and work from there..

    Some final notes:

    - The NT kernel has a similar concept, but for storage instead.
    They're used to dump core under kernel panic() situations,
    and are called "Minoport storage drivers". [7]

    - Since Windows 7+, a very fancy Blue Screen of Death is
    displayed, with Unicode and whatnot, implying GPU drivers
    involvement. [8]

    - Mac OS X also does something similar [9]

    - On Linux laptops, the current situation is _really_ bad.

    In any graphical session, type "echo c > /proc/sysrq-trigger";
    the screen will just completely freeze...

    Desired first goal: just print the panic() log

    Thanks a lot,

    [1] https://lore.kernel.org/lkml/20110125134748.GA10051@laptop
    [2] https://lore.kernel.org/lkml/AANLkTinU0KYiCd4p=z+=ojbkeEoT2G+CAYvdRU02KJEn@mail.gmail.com

    [3] https://uefi.org/sites/default/files/resources/Brian_Richardson_Intel_Final.pdf

    [4] UEFI v2.7 spec, Chapter 8, "Services — Runtime Services"
    [5] UEFI v2.7 spec, Section 12.9, "Graphics Output Protocol"
    "The Graphics Output Protocol supports this capability by
    providing the EFI OS loader access to a hardware frame buffer
    and enough information to allow the OS to draw directly to
    the graphics output device."

    [6] linux/drivers/gpu/drm/i915/i915_drv.c::i915_kick_out_firmware_fb()
    linux/drivers/gpu/drm/radeon/radeon_drv.c::radeon_pci_probe()

    [7] https://docs.microsoft.com/en-us/windows-hardware/drivers/storage/restrictions-on-miniport-drivers-that-manage-the-boot-drive

    [8] https://upload.wikimedia.org/wikipedia/commons/archive/5/56/20181019151937%21Bsodwindows10.png
    [9] https://upload.wikimedia.org/wikipedia/commons/4/4a/Mac_OS_X_10.2_Kernel_Panic.jpg

    --darwi
    http://darwish.chasingpointers.com

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