PCI: Add pci_irq_type() to query the allocated interrupt typegrafted

Add a helper that returns PCI_IRQ_MSIX, PCI_IRQ_MSI, or PCI_IRQ_INTX
based on the interrupt type the PCI core selected after
pci_alloc_irq_vectors().

Several drivers already open-code this check against pdev->msix_enabled
and pdev->msi_enabled, or even open code this helper [1].

A common helper avoids the duplication and keeps drivers from accessing
the bitfield directly (see also [2]).

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Link: https://elixir.bootlin.com/linux/v7.1/source/drivers/net/ethernet/aquantia/atlantic/aq_pci_func.c#L196 [1]
Inspired-by: John Hubbard <jhubbard@nvidia.com>
Link: https://lore.kernel.org/all/DKKG2QM3YJYB.Z2H2B2UXJ75N@kernel.org/ [2]
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260813165234.620555-5-dakr@kernel.org
[ Add missing pci_irq_type() stub for CONFIG_PCI=n. ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 48 insertions, 0 deletions

diff --git a/Documentation/admin-guide/init.rst b/Documentation/admin-guide/init.rst
new file mode 100644
index 000000000..41f06a091
--- /dev/null
+++ b/Documentation/admin-guide/init.rst
@@ -0,0 +1,48 @@
+Explaining the "No working init found." boot hang message
+=========================================================
+:Authors: Andreas Mohr <andi at lisas period de>
+          Cristian Souza <cristianmsbr at gmail period com>
+
+This document provides some high-level reasons for failure
+(listed roughly in order of execution) to load the init binary.
+
+1) **Unable to mount root FS**: Set "debug" kernel parameter (in bootloader
+   config file or CONFIG_CMDLINE) to get more detailed kernel messages.
+
+2) **init binary doesn't exist on rootfs**: Make sure you have the correct
+   root FS type (and ``root=`` kernel parameter points to the correct
+   partition), required drivers such as storage hardware (such as SCSI or
+   USB!) and filesystem (ext3, jffs2, etc.) are builtin (alternatively as
+   modules, to be pre-loaded by an initrd).
+
+3) **Broken console device**: Possibly a conflict in ``console= setup``
+   --> initial console unavailable. E.g. some serial consoles are unreliable
+   due to serial IRQ issues (e.g. missing interrupt-based configuration).
+   Try using a different ``console= device`` or e.g. ``netconsole=``.
+
+4) **Binary exists but dependencies not available**: E.g. required library
+   dependencies of the init binary such as ``/lib/ld-linux.so.2`` missing or
+   broken. Use ``readelf -d <INIT>|grep NEEDED`` to find out which libraries
+   are required.
+
+5) **Binary cannot be loaded**: Make sure the binary's architecture matches
+   your hardware. E.g. i386 vs. x86_64 mismatch, or trying to load x86 on ARM
+   hardware. In case you tried loading a non-binary file here (shell script?),
+   you should make sure that the script specifies an interpreter in its
+   shebang header line (``#!/...``) that is fully working (including its
+   library dependencies). And before tackling scripts, better first test a
+   simple non-script binary such as ``/bin/sh`` and confirm its successful
+   execution. To find out more, add code ``to init/main.c`` to display
+   kernel_execve()s return values.
+
+Please extend this explanation whenever you find new failure causes
+(after all loading the init binary is a CRITICAL and hard transition step
+which needs to be made as painless as possible), then submit a patch to LKML.
+Further TODOs:
+
+- Implement the various ``run_init_process()`` invocations via a struct array
+  which can then store the ``kernel_execve()`` result value and on failure
+  log it all by iterating over **all** results (very important usability fix).
+- Try to make the implementation itself more helpful in general, e.g. by
+  providing additional error messages at affected places.
+