[QUESTION] Early Write Acknowledge for PCIe configuration space

Will Deacon
2017-01-09T11:23:41Z

On Mon, Jan 09, 2017 at 10:59:47AM +0000, John Garry wrote:
> On 06/01/2017 11:24, Arnd Bergmann wrote:
> >On Friday, January 6, 2017 11:15:22 AM CET John Garry wrote:
> >>[apologies if this has been queried before]
> >>
> >>Hi ARM guys,
> >>
> >>I have a question about the device memory attributes we assign for PCIe
> >>config space for arm64. Currently we use ioremap to map in the config
> >>space; this uses nGnRE, which means we enable Early Write Acknowledge.
> >>
> >>The ARMv8 ARM states that "ARM recommend that No Early Write Acknowledge
> >>Hint is used for PCIe configuration writes".
> >>
> >>I understand a problem with using E is in that configuration write is a
> >>non-post operation, which means the RP requires to get the completion
> >>ack from the EP The problem here is if CPU writes data to ECAM by E,
> >>complete will go back to CPU directly, and maybe at this point the write
> >>has not reached the EP.
> >>
> >>I believe that this may cause ordering issues in PCI read/write. In
> >>practice we use non-relaxed readl/writel to access config space, which
> >>include the synchronization barriers, which, *as I understand*, even if
> >>for full system domain, may be negated by the E attribute for PCIe.
> >
> >I don't think the barriers in readl/writel are enough here, in particular
> >the write barrier is *before* the access to synchronize DMAs
> >on RAM with MMIO accesses, which is a bit different from what you
> >have here.
> >
> >>So a question: why is the recommendation in the ARMv8 ARM ignored?
> >
> >Probably nobody thought about this properly in the Linux drivers. The
> >ARMv8 ARM sounds correct here.
> >
> >I/O space may have the same issue, as it also requires non-posted
> >accesses.
>
> Right, so our HW team's recommendation - from ARM's memory model and also
> PCIe order model - is that not only config space but also PCIe memory mapped
> IO has the same attribute (nE).

What's the rationale behind that recommendation?

Will