Re: [PATCH v1] drivers: pci: introduce configurable delay for Rockchip PCIe bus scan

From: Vincenzo Palazzo
Date: Mon May 15 2023 - 07:04:43 EST

> >
> > There *is* a way for a PCIe device to say "I need more time". It does
> > this by responding to that Vendor ID config read with Request Retry
> > Status (RRS, aka CRS in older specs), which means "I'm not ready yet,
> > but I will be ready in the future." Adding a delay would definitely
> > make a difference here, so my guess is this is what's happening.
> >
> > Most root complexes return ~0 data to the CPU when a config read
> > terminates with UR or RRS. It sounds like rockchip does this for UR
> > but possibly not for RRS.
> >
> > There is a "RRS Software Visibility" feature, which is supposed to
> > turn the RRS into a special value (Vendor ID == 0x0001), but per [1],
> > rockchip doesn't support it (lspci calls it "CRSVisible").
> >
> > But the CPU load instruction corresponding to the config read has to
> > complete by reading *something* or else be aborted. It sounds like
> > it's aborted in this case. I don't know the arm64 details, but if we
> > could catch that abort and determine that it was an RRS and not a UR,
> > maybe we could fabricate the magic RRS 0x0001 value.
> >
> > imx6q_pcie_abort_handler() does something like that, although I think
> > it's for arm32, not arm64. But obviously we already catch the abort
> > enough to dump the register state and panic, so maybe there's a way to
> > extend that?
> Perhaps a hook mechanism that allows drivers to register with the
> serror handler and offer to handle specific errors before the generic
> code causes the system panic?

This sounds to me a good general solution that also help to handle
future HW like this one.

So this is a Concept Ack for me.