Advice on delaying de-asserting PCIe reset

[Patrick Williams]

On Wed, May 12, 2021 at 06:36:28AM -0700, sainath grandhi wrote:
> On Tue, May 11, 2021 at 2:46 PM Patrick Williams <patrick at stwcx.xyz> wrote:
> >
> > On Tue, May 11, 2021 at 12:50:34PM -0700, sainath grandhi wrote:
> > > We are potentially facing a scenario where de-asserting the PCIe
> > > PERST# should wait until an endpoint in the PCI hierarchy is ready.
> > > Since the endpoint of interest is an FPGA, it takes "some" time to
> > > come out of reset, boot etc. to be ready and participate in Link
> > > training followed by config space requests from Linux.
> >
> > I've worked on devices like this but they are a violation of the PCIe
> > spec.  You have something like 10ms or 100ms by PCIe standards for your
> > device to come out of reset.
> >
> > The cases where I've dealt with this we hacked the BIOS to just delay
> > after the PERST# but before probing.
> Got it.
> When you say BIOS, do you refer to u-boot?

No, this is just my confusion and invalid assumption on what you're
trying to do.

I assumed you were trying to deal with a custom PCIe device attached to
your host processors and trying to sequence the power and PERST# via the
BMC, not that you were attaching this device directly to the BMC.

> >
> > > So we are checking for options on how we can delay de-asserting PERST#
> > > in the Linux PCIe controller driver, if possible in a standard way.
> > >
> > > A simple approach would be to add some time delay or wait for a signal
> > > (via some pin) from the endpoint in the PCIe controller driver before
> > > de-asserting PERST#.
> > > But that would make the change specific to our use-case in an
> > > otherwise generic board controller driver. And maintaining that logic
> > > can become cumbersome.
> > >
> > > How does Linux in general support such PCI endpoints to work fine?
> > > Any advice on how to approach this scenario is appreciated.
> >
> > Are you asking about Linux on the BMC or Linux on the managed host?
> > I'm trying to figure out how your questions are related to OpenBMC.
> >
> I am asking about Linux on BMC.

I don't think many people have PCIe devices attached to the BMC and
especially not custom PCIe devices like what you're describing.

> > One possibility, if you're talking about a PCIe device attached to
> > the managed host, would be to separate the power sequencing of the PCIe
> > card from the host processors.  You can bring up power to the PCIe card
> > independent from the host processors to give it time to come up and
> > be ready to listen to PERST#.  That is another option if you can't
> > modify the BIOS.
> >
> > --
> > Patrick Williams

-- 
Patrick Williams
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