PCIe: readl()

PCIe: readl()

[Muni Sekhar]

Hi all,

We have a free-running counter on x86 PCIe bus and plan to use it as a
clocksource. The 'read' field of the clocksource structure reads from
this free-running counter and returns this value.

Since PCIe reads are synchronous, is it safe to use readl() API to
read this free-running counter value in struct clocksource.read()? If
not, what's the best way to read the counter value from the struct
clocksource read field?

Is there any other faster method to read from PCIe device memory?


-- 
Thanks,
Sekhar

Re: PCIe: readl()

[Bjorn Helgaas]

On Thu, Jan 06, 2022 at 10:43:52PM +0530, Muni Sekhar wrote:
> Hi all,
> 
> We have a free-running counter on x86 PCIe bus and plan to use it as a
> clocksource. The 'read' field of the clocksource structure reads from
> this free-running counter and returns this value.
> 
> Since PCIe reads are synchronous, is it safe to use readl() API to
> read this free-running counter value in struct clocksource.read()? If
> not, what's the best way to read the counter value from the struct
> clocksource read field?

Yes, you can use readl().  Of course, the driver for your PCIe device
will have to do the usual setup, e.g.,

  probe(struct pci_dev *dev)
  {
    pci_request_regions(dev, "name");
    addr = pci_ioremap_bar(dev, 0);
    readl(addr);

The readl() should give you a single PCIe transaction, which is the
best you can do.

Bjorn