Re: DRM-based Oops viewer

["Noralf Trønnes" <noralf@tronnes.org>]

Den 10.03.2019 02.31, skrev Ahmed S. Darwish:
> 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,
> 

I just sent out a patchset I had lying around that tries to solve this:
https://patchwork.freedesktop.org/series/57849/

Noralf.

> [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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