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Bjorn Helgaas ce526e1b29 PCI: Avoid FLR for Mediatek MT7922 WiFi 
commit 81f64e925c upstream.

The Mediatek MT7922 WiFi device advertises FLR support, but it apparently
does not work, and all subsequent config reads return ~0:

  pci 0000:01:00.0: [14c3:0616] type 00 class 0x028000 PCIe Endpoint
  pciback 0000:01:00.0: not ready 65535ms after FLR; giving up

After an FLR, pci_dev_wait() waits for the device to become ready.  Prior
to d591f6804e ("PCI: Wait for device readiness with Configuration RRS"),
it polls PCI_COMMAND until it is something other that PCI_POSSIBLE_ERROR
(~0).  If it times out, pci_dev_wait() returns -ENOTTY and
__pci_reset_function_locked() tries the next available reset method.
Typically this is Secondary Bus Reset, which does work, so the MT7922 is
eventually usable.

After d591f6804e, if Configuration Request Retry Status Software
Visibility (RRS SV) is enabled, pci_dev_wait() polls PCI_VENDOR_ID until it
is something other than the special 0x0001 Vendor ID that indicates a
completion with RRS status.

When RRS SV is enabled, reads of PCI_VENDOR_ID should return either 0x0001,
i.e., the config read was completed with RRS, or a valid Vendor ID.  On the
MT7922, it seems that all config reads after FLR return ~0 indefinitely.
When pci_dev_wait() reads PCI_VENDOR_ID and gets 0xffff, it assumes that's
a valid Vendor ID and the device is now ready, so it returns with success.

After pci_dev_wait() returns success, we restore config space and continue.
Since the MT7922 is not actually ready after the FLR, the restore fails and
the device is unusable.

We considered changing pci_dev_wait() to continue polling if a
PCI_VENDOR_ID read returns either 0x0001 or 0xffff.  This "works" as it did
before d591f6804e, although we have to wait for the timeout and then fall
back to SBR.  But it doesn't work for SR-IOV VFs, which *always* return
0xffff as the Vendor ID.

Mark Mediatek MT7922 WiFi devices to avoid the use of FLR completely.  This
will cause fallback to another reset method, such as SBR.

Link: https://lore.kernel.org/r/20250212193516.88741-1-helgaas@kernel.org
Fixes: d591f6804e ("PCI: Wait for device readiness with Configuration RRS")
Link: https://github.com/QubesOS/qubes-issues/issues/9689#issuecomment-2582927149
Link: https://lore.kernel.org/r/Z4pHll_6GX7OUBzQ@mail-itl
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-02-21 14:01:34 +01:00
arch alpha: make stack 16-byte aligned (most cases) 2025-02-21 14:01:33 +01:00
block block: don't revert iter for -EIOCBQUEUED 2025-02-17 10:05:19 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: api - Fix boot-up self-test race 2025-02-08 09:57:28 +01:00
Documentation Documentation/networking: fix basic node example document ISO 15765-2 2025-02-21 14:01:16 +01:00
drivers PCI: Avoid FLR for Mediatek MT7922 WiFi 2025-02-21 14:01:34 +01:00
fs orangefs: fix a oob in orangefs_debug_write 2025-02-21 14:01:27 +01:00
include kunit: platform: Resolve 'struct completion' warning 2025-02-21 14:01:25 +01:00
init Compiler Attributes: disable __counted_by for clang < 19.1.3 2024-12-05 14:02:45 +01:00
io_uring io_uring/uring_cmd: remove dead req_has_async_data() check 2025-02-21 14:01:20 +01:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-05 14:03:02 +01:00
kernel ring-buffer: Update pages_touched to reflect persistent buffer content 2025-02-21 14:01:29 +01:00
lib maple_tree: simplify split calculation 2025-02-17 10:05:42 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm/compaction: fix UBSAN shift-out-of-bounds warning 2025-02-17 10:05:36 +01:00
net can: j1939: j1939_sk_send_loop(): fix unable to send messages with data length zero 2025-02-21 14:01:33 +01:00
rust rust: init: use explicit ABI to clean warning in future compilers 2025-02-17 10:05:23 +01:00
samples samples/landlock: Fix possible NULL dereference in parse_path() 2025-02-08 09:57:11 +01:00
scripts kbuild: Use -fzero-init-padding-bits=all 2025-02-21 14:01:27 +01:00
security KEYS: trusted: dcp: fix improper sg use with CONFIG_VMAP_STACK=y 2025-02-17 10:05:09 +01:00
sound ASoC: Intel: bytcr_rt5640: Add DMI quirk for Vexia Edu Atla 10 tablet 5V 2025-02-21 14:01:27 +01:00
tools selftests: gpio: gpio-sim: Fix missing chip disablements 2025-02-21 14:01:26 +01:00
usr initramfs: shorten cmd_initfs in usr/Makefile 2024-07-16 01:07:52 +09:00
virt KVM: x86: switch hugepage recovery thread to vhost_task 2024-12-05 14:02:43 +01:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore Kbuild updates for v6.12 2024-09-24 13:02:06 -07:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: update location of media main tree 2024-12-05 14:02:42 +01:00
Makefile tools: fix annoying "mkdir -p ..." logs when building tools in parallel 2025-02-21 14:01:22 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

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 reStructuredText 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.