On Wed, Jul 15, 2020 at 02:38:29PM +0000, David Laight wrote:

From: Oliver O'Halloran

quoted

Sent: 15 July 2020 05:19

On Wed, Jul 15, 2020 at 8:03 AM Arnd Bergmann [off-list ref] wrote:

...

quoted

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- config space accesses are very rare compared to memory
  space access and on the hardware side the error handling
  would be similar, but readl/writel don't return errors, they just
  access wrong registers or return 0xffffffff.
  arch/powerpc/kernel/eeh.c has a ton extra code written to
  deal with it, but no other architectures do.

TBH the EEH MMIO hooks were probably a mistake to begin with. Errors
detected via MMIO are almost always asynchronous to the error itself
so you usually just wind up with a misleading stack trace rather than
any kind of useful synchronous error reporting. It seems like most
drivers don't bother checking for 0xFFs either and rely on the
asynchronous reporting via .error_detected() instead, so I have to
wonder what the point is. I've been thinking of removing the MMIO
hooks and using a background poller to check for errors on each PHB
periodically (assuming we don't have an EEH interrupt) instead. That
would remove the requirement for eeh_dev_check_failure() to be
interrupt safe too, so it might even let us fix all the godawful races
in EEH.

I've 'played' with PCIe error handling - without much success.
What might be useful is for a driver that has just read ~0u to
be able to ask 'has there been an error signalled for this device?'.

In many cases a driver will know that ~0 is not a valid value for the
register it's reading.  But if ~0 *could* be valid, an interface like
you suggest could be useful.  I don't think we have anything like that
today, but maybe we could.  It would certainly be nice if the PCI core
noticed, logged, and cleared errors.  We have some of that for AER,
but that's an optional feature, and support for the error bits in the
garden-variety PCI_STATUS register is pretty haphazard.  As you note
below, this sort of SERR/PERR reporting is frequently hard-wired in
ways that takes it out of our purview.

Bjorn