Re: [PATCH v8 3/6] pci:host: Add Altera PCIe host controller driver

[Ley Foon Tan]

On Mon, Oct 12, 2015 at 8:03 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Friday 09 October 2015 18:15:40 Bjorn Helgaas wrote:
> >
> > I don't know if this should be a kernel taint, a simple warning in
> > dmesg, or what.  I guess the tainting mechanism is probably too
> > general-purpose for this, and add_taint() doesn't give any dmesg
> > indication.  We wouldn't see the taint unless the problem actually
> > caused an oops or panic.  In this case, I think I want a clue in dmesg
> > so we have a chance of seeing it even if there is no oops.  So
> > probably something like a dev_warn("non-compliant config accesses")
> > would work.
> >
> > You really should double-check with the hardware guys, because it's
> > pretty obvious that the PCI spec requires 1- and 2-byte config
> > accesses to work correctly.  For example, if you read/modify/write to
> > update PCI_COMMAND, you will inadvertently clear the RW1C bits in
> > PCI_STATUS.
>
> Would it help to require a DT property here that flags the device
> as having a broken config space?
>
> Then we could implement both in the driver, and only use the
> RMW based implementation if the firmware describes the device
> as "altera,broken-pci-config-space".
>

I have checked the PCI/TLP specification, the address needs to be
4-byte aligned. But, we can use "byte enable" field to update specific
bytes.
For example, if byte enable is 0x3 (0011b), that mean it only update
lower 2 bytes. By doing this, we can resolve the RW1C issue here.
I will update the driver with this in next revision.

Thanks for reviewing.

Regards
Ley Foon
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