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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 +272,71 @@
</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 and a running DRM driver
supports to do so. Without this, it would be more
difficult to diagnose a panic: if no DRM driver is running,
we'd be on the VGA console which can only hold 24 lines and
the relevent kernel message is often flushed away; if a DRM
driver is running, the display is often completely messed up
on panic. As of Linux-6.12, none of the dedicated drivers for
mainstream GPU models really supports this, but it's supported by
the <quote>Simple framebuffer driver</quote> which runs on the
VESA (or EFI) framebuffer before the dedicated GPU driver is
loaded. If the dedicated GPU driver is built as a module
(instead of a part of the kernel image) and no initramfs is
used, this functionality will work just fine before the root
file system is mounted and it's already enough for providing
information about most LFS configuration errors causing a
panic (for example, an incorrect <parameter>root=</parameter>
setting in <xref linkend='ch-bootable-grub'/>).</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<parameter>
Panic screen formatter
</parameter>
</term>
<listitem>
<para>Set this <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 compress
the last kernel message lines into a QR code and display it.
The QR code can hold more message lines than plain text and it
can be decoded with an external device (like a smart phone).
But it 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>