[Linux-kernel-mentees] [PATCH 0/13] PCI: Remove '*val = 0' from pcie_capability_read_*()

[Saheed Olayemi Bolarinwa <refactormyself@gmail.com>]

From: Bolarinwa Olayemi Saheed <refactormyself@gmail.com>

MERGING:
Only Patch 13/13 depend on all preceeding patchs. All other patches are
independent of one another. Hence, please merge PATCH 13/13 only after
other patches in this series have been merged.

PATCH 6/13:
Make the function set status to "Power On" by default and only set to
Set "Power Off" only if pcie_capability_read_word() is successful and
(slot_ctrl & PCI_EXP_SLTCTL_PCC) == PCI_EXP_SLTCTL_PWR_OFF. 

PATCH 1/13 to 12/13:
Check the return value of pcie_capability_read_*() to ensure success or
confirm failure. While maintaining these functions, this ensures that the
changes in PATCH 13/13 does not introduce any bug. 

PATCH 13/13:
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
return -EINVAL/PCIBIOS_BAD_REGISTER_NUMBER or PCIBIOS_DEVICE_NOT_FOUND
and *val is set to 0.

This behaviour if further ensured by this code inside
pcie_capability_read_*()

 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
value.

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 13/13.
Remove the reset of *val to 0 when pci_read_config_*() fails.


Bolarinwa Olayemi Saheed (13):
  IB/hfi1: Check the return value of pcie_capability_read_*()
  misc: rtsx: Check the return value of pcie_capability_read_*()
  ath9k: Check the return value of pcie_capability_read_*()
  iwlegacy: Check the return value of pcie_capability_read_*()
  PCI: pciehp: Check the return value of pcie_capability_read_*()
  PCI: pciehp: Make "Power On" the default 
  PCI/ACPI: Check the return value of pcie_capability_read_*()
  PCI: pciehp: Check the return value of pcie_capability_read_*()
  PCI: Check the return value of pcie_capability_read_*()
  PCI/PM: Check return value of pcie_capability_read_*()
  PCI/AER: Check the return value of pcie_capability_read_*()
  PCI/ASPM: Check the return value of pcie_capability_read_*()
  PCI: Remove '*val = 0' from pcie_capability_read_*()

 drivers/net/wireless/ath/ath9k/pci.c         | 5 +++--
 drivers/net/wireless/intel/iwlegacy/common.c | 4 ++--
 drivers/infiniband/hw/hfi1/aspm.c | 7 ++++---
 drivers/misc/cardreader/rts5227.c | 5 +++--
 drivers/misc/cardreader/rts5249.c | 5 +++--
 drivers/misc/cardreader/rts5260.c | 5 +++--
 drivers/misc/cardreader/rts5261.c | 5 +++--
 drivers/pci/pcie/aer.c  |  5 +++--
 drivers/pci/pcie/aspm.c | 33 +++++++++++++++++----------------
 drivers/pci/hotplug/pciehp_hpc.c | 47 ++++++++++++++++----------------
 drivers/pci/pci-acpi.c           | 10 ++++---
 drivers/pci/probe.c              | 29 ++++++++++++--------
 drivers/pci/access.c | 14 --------------
 13 files changed, 87 insertions(+), 87 deletions(-)

-- 
2.18.2

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