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Some firmware supplies PCI host bridge _CRS that includes address space
unusable by PCI devices, e.g., space occupied by host bridge registers or
used by hidden PCI devices.
To avoid this unusable space, Linux currently excludes E820 reserved
regions from _CRS windows; see 4dc2287c1805 ("x86: avoid E820 regions when
allocating address space").
However, this use of E820 reserved regions to clip things out of _CRS is
not supported by ACPI, UEFI, or PCI Firmware specs, and some systems have
E820 reserved regions that cover the entire memory window from _CRS.
4dc2287c1805 clips the entire window, leaving no space for hot-added or
uninitialized PCI devices.
For example, from a Lenovo IdeaPad 3 15IIL 81WE:
BIOS-e820: [mem 0x4bc50000-0xcfffffff] reserved
pci_bus 0000:00: root bus resource [mem 0x65400000-0xbfffffff window]
pci 0000:00:15.0: BAR 0: [mem 0x00000000-0x00000fff 64bit]
pci 0000:00:15.0: BAR 0: no space for [mem size 0x00001000 64bit]
Future patches will add quirks to enable/disable E820 clipping
automatically.
Add a "pci=no_e820" kernel command line option to disable clipping with
E820 reserved regions. Also add a matching "pci=use_e820" option to enable
clipping with E820 reserved regions if that has been disabled by default by
further patches in this patch-set.
Both options taint the kernel because they are intended for debugging and
workaround purposes until a quirk can set them automatically.
[bhelgaas: commit log, add printk]
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1868899 Lenovo IdeaPad 3
Link: https://lore.kernel.org/r/20220519152150.6135-2-hdegoede@redhat.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Benoit Grégoire <benoitg@coeus.ca>
Cc: Hui Wang <hui.wang@canonical.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.