Re: [PATCH v2 2/3] PCI: qcom: Read back PARF_LTSSM register

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Feb 15, 2024 at 11:21:45AM +0100, Konrad Dybcio wrote:
> On 14.02.2024 23:28, Bjorn Helgaas wrote:
> > On Wed, Feb 14, 2024 at 10:35:16PM +0100, Konrad Dybcio wrote:
> >> On 12.02.2024 22:17, Bjorn Helgaas wrote:
> >>> Maybe include the reason in the subject?  "Read back" is literally
> >>> what the diff says.
> >>>
> >>> On Sat, Feb 10, 2024 at 06:10:06PM +0100, Konrad Dybcio wrote:
> >>>> To ensure write completion, read the PARF_LTSSM register after setting
> >>>> the LTSSM enable bit before polling for "link up".
> >>>
> >>> The write will obviously complete *some* time; I assume the point is
> >>> that it's important for it to complete before some other event, and it
> >>> would be nice to know why that's important.
> >>
> >> Right, that's very much meaningful on non-total-store-ordering
> >> architectures, like arm64, where the CPU receives a store instruction,
> >> but that does not necessarily impact the memory/MMIO state immediately.
> > 
> > I was hinting that maybe we could say what the other event is, or what
> > problem this solves?  E.g., maybe it's as simple as "there's no point
> > in polling for link up until after the PARF_LTSSM store completes."
> > 
> > But while the read of PARF_LTSSM might reduce the number of "is the
> > link up" polls, it probably wouldn't speed anything up otherwise, so I
> > suspect there's an actual functional reason for this patch, and that's
> > what I'm getting at.
> 
> So, the register containing the "enable switch" (PARF_LTSSM) can (due
> to the armv8 memory model) be "written" but not "change the value of
> memory/mmio from the perspective of other (non-CPU) memory-readers
> (such as the MMIO-mapped PCI controller itself)".
> 
> In that case, the CPU will happily continue calling qcom_pcie_link_up()
> in a loop, waiting for the PCIe controller to bring the link up, however
> the PCIE controller may have never received the PARF_LTSSM "enable link"
> write by the time we decide to time out on checking the link status.
> 
> It may also never happen for you, but that's exactly like a classic race
> condition, where it may simply not manifest due to the code around the
> problematic lines hiding it. It may also only manifest on certain CPU
> cores that try to be smarter than you and keep reordering/delaying
> instructions if they don't seem immediately necessary.

Does this mean the register is mapped incorrectly, e.g., I see arm64
has many different kinds of mappings for cacheability,
write-buffering, etc?

Or, if it is already mapped correctly, are we confident that none of
the *other* register writes need similar treatment?  Is there a rule
we can apply to know when the read-after-write is needed?

Bjorn