[Linux-kernel-mentees] [PATCH v4 00/12] PCI: Remove '*val = 0' from pcie_capability_read_*()

Saheed O. Bolarinwa refactormyself at gmail.com
Fri Jul 31 11:02:28 UTC 2020

- Drop uses of pcie_capability_read_*() return value. This related to
  [1] which is pointing towards making the accessors return void.
- Remove patches found to be unnecessary
- Reword some commit messages

- Split previous PATCH 6/13 into two : PATCH 6/14 and PATCH 7/14
- Fix commit message of PATCH 5/14
- Update Patch numbering and Commit messages
- Add 'Acked by Greg KH' to PATCH 2/14
- Add PATCH version

- Fix missing comma, causing the email cc error
- Fix typos and numbering errors in commit messages
- Add commit message to 13/13
- Add two more patches: PATCH 3/13 and PATCH 4/13

- Patch 6/12 depends on Patch 5/12. However Patch 5/12 has no dependency.
  Please, merge PATCH 6/12 only after Patch 5/12.
- Patch 12/12 depends on all preceding patches. Please merge Patch 12/12
  only after other patches in this series have been merged.
- All other patches have no dependencies besides those mentioned above and
  can be merge individually.

PATCH 5/12:
Set the default case in the switch-statement to set status
to "Power On".

PATCH 1/12 to 11/12:
Use the value read by pcie_capability_read_*() to determine success or
failure. This is done by checking if it is ~0, while maintaining the
functions' behaviour. This ensures that the changes in PATCH 12/12 does
not introduce any bug.

PATCH 12/12:
There are several reasons why a PCI capability read may fail whether the
device is present or not. If this happens, pcie_capability_read_*() will
and *val is set to 0.

This behaviour if further ensured by this code inside

 ret = pci_read_config_dword(dev, pci_pcie_cap(dev) + pos, val);
  * Reset *val to 0 if pci_read_config_dword() fails, it may
  * have been written as 0xFFFFFFFF if hardware error happens
  * during pci_read_config_dword().
 if (ret)
	 *val = 0;
 return ret;

a) Since all pci_generic_config_read() does is read a register value,
it may return success after reading a ~0 which *may* have been fabricated
by the PCI host bridge due to a read timeout. Hence pci_read_config_*() 
will return success with a fabricated ~0 in *val, indicating a problem.
In this case, the assumed behaviour of  pcie_capability_read_*() will be
wrong. To avoid error slipping through, more checks are necessary.

b) pci_read_config_*() will return PCIBIOS_DEVICE_NOT_FOUND only if 
dev->error_state = pci_channel_io_perm_failure (i.e. 
pci_dev_is_disconnected()) or if pci_generic_config_read() can't find the
device. In both cases *val is initially set to ~0 but as shown in the code
above pcie_capability_read_*() resets it back to 0. Even with this effort,
drivers still have to perform validation checks more so if 0 is a valid

Most drivers only consider the case (b) and in some cases, there is the 
expectation that on timeout *val has a fabricated value of ~0, which *may*
not always be true as explained in (a).

In any case, checks need to be done to validate the value read and maybe
confirm which error has occurred. It is better left to the drivers to do.

Check the return value of pcie_capability_read_dword() to ensure success
and avoid bug as a result of Patch 14/14.
Remove the reset of *val to 0 when pci_read_config_*() fails.

[1] https://lore.kernel.org/linux-pci/20200714234625.GA428442@bjorn-Precision-5520/

Saheed O. Bolarinwa (12):
  IB/hfi1: Check if pcie_capability_read_*() reads ~0
  misc: rtsx: Check if pcie_capability_read_*() reads ~0
  ath9k: Check if pcie_capability_read_*() reads ~0
  iwlegacy: Check if pcie_capability_read_*() reads ~0
  PCI: pciehp: Set "Power On" as the default get_power_status
  PCI: pciehp: Check if pcie_capability_read_*() reads ~0
  PCI/ACPI: Check if pcie_capability_read_*() reads ~0
  PCI: Check if pcie_capability_read_*() reads ~0
  PCI/PM: Check if pcie_capability_read_*() reads ~0
  PCI/AER: Check if pcie_capability_read_*() reads ~0
  PCI/ASPM: Check if pcie_capability_read_*() reads ~0
  PCI: Remove '*val = 0' from pcie_capability_read_*()

 drivers/infiniband/hw/hfi1/aspm.c            |  6 ++--
 drivers/misc/cardreader/rts5227.c            |  2 +-
 drivers/misc/cardreader/rts5249.c            |  2 +-
 drivers/misc/cardreader/rts5260.c            |  2 +-
 drivers/misc/cardreader/rts5261.c            |  2 +-
 drivers/net/wireless/ath/ath9k/pci.c         |  3 +-
 drivers/net/wireless/intel/iwlegacy/common.c |  2 +-
 drivers/pci/access.c                         | 14 --------
 drivers/pci/hotplug/pciehp_hpc.c             | 13 +++++---
 drivers/pci/pci-acpi.c                       |  4 +--
 drivers/pci/pci.c                            | 34 ++++++++++++++------
 drivers/pci/pcie/aer.c                       |  2 +-
 drivers/pci/pcie/aspm.c                      | 10 +++---
 drivers/pci/probe.c                          | 12 +++----
 14 files changed, 56 insertions(+), 52 deletions(-)


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