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drm-panic: kernel: Explain DRM panic settings

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Xi Ruoyao 2024-10-30 20:51:59 +08:00
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chapter10/kernel.xml
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@ -272,6 +272,82 @@
</listitem>
</varlistentry>
<varlistentry>
<term>
<parameter>
Display a user-friendly message when a kernel panic occurs
</parameter>
</term>
<listitem>
<para>This will make the kernel correctly display the message
in case a kernel panic happens. Without this, it would be more
difficult to diagnose a panic: if <option>CONFIG_DRM</option>,
<option>CONFIG_DRM_SIMPLEDRM</option>, or etc. are not enabled,
the relevent message lines can be flushed alway because the VGA
console is too small; if <option>CONFIG_DRM</option> etc. are
enabled but this option is not, the display can be completely
disrupted when the kernel panics.</para>
<note>
<para>
The DRM panic handler requires some support from the DRM
driver to work correctly, so it may not work with some DRM
drivers. To ensure it functional before the root file system
is successfully mounted (normally most kernel panic cases to
be diagnosed by a LFS user happen before or during mounting
the root file system, for example the panic caused by an
incorrect <option>root=</option> setting in the kernel command
line of <filename>grub.cfg</filename>), build the DRM driver
for your GPU (for example <option>CONFIG_DRM_AMDGPU</option>)
as a kernel module instead of a part of the kernel so it won't
be loaded before sucessfully mounting the root file system.
Doing so also avoids the necessity of embedding some
<ulink url='&blfs-book;postlfs/firmware.html#video-firmware'>firmware</ulink>
into an initramfs or the kernel image in case the DRM driver
requires the firmware to be available on initialization.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>
<parameter>
Panic screen formatter
</parameter>
</term>
<listitem>
<para>Set this to <literal>kmsg</literal> to make sure the last
kernel messages lines are displayed when a kernel panic happens.
The default, <literal>user</literal>, would make the kernel show
only a <quote>user friendly</quote> panic message which is not
helpful on diagnostic. The third choice,
<literal>qr_code</literal>, would make the kernel to display a
QR code containing the last kernel message lines. The QR code
can be decoded with an external device (for example a smart phone)
into much more message lines than how much
<literal>kmsg</literal> can display, but doing so requires a Rust
compiler that LFS does not provide.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<parameter>
Mark VGA/VBE/EFI FB as generic system framebuffer
</parameter> and
<parameter>Simple framebuffer driver</parameter>
</term>
<listitem>
<para>These allow to use the VESA framebuffer (or the EFI
framebuffer if booting the LFS system via UEFI) as a DRM device.
The VESA framebuffer will be set up by GRUB (or the EFI
framebuffer will be set up by the UEFI firmware), so the DRM panic
handler can function before the GPU-specific DRM driver is
loaded.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<parameter>