linux-kernel - Re: [PATCH v2] PCI: rockchip: Avoid accessing PCIe registers with clocks gated

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Date:   Thu, 24 Jun 2021 18:28:41 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org,
        Peter Robinson <pbrobinson@...il.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH v2] PCI: rockchip: Avoid accessing PCIe registers with
 clocks gated

On Fri, Jun 25, 2021 at 12:18:48AM +0100, Robin Murphy wrote:
> On 2021-06-24 22:57, Bjorn Helgaas wrote:
> > On Tue, Jun 08, 2021 at 10:04:09AM +0200, Javier Martinez Canillas wrote:
> > > IRQ handlers that are registered for shared interrupts can be called at
> > > any time after have been registered using the request_irq() function.
> > > 
> > > It's up to drivers to ensure that's always safe for these to be called.
> > > 
> > > Both the "pcie-sys" and "pcie-client" interrupts are shared, but since
> > > their handlers are registered very early in the probe function, an error
> > > later can lead to these handlers being executed before all the required
> > > resources have been properly setup.
> > > 
> > > For example, the rockchip_pcie_read() function used by these IRQ handlers
> > > expects that some PCIe clocks will already be enabled, otherwise trying
> > > to access the PCIe registers causes the read to hang and never return.
> > 
> > The read *never* completes?  That might be a bit problematic because
> > it implies that we may not be able to recover from PCIe errors.  Most
> > controllers will timeout eventually, log an error, and either
> > fabricate some data (typically ~0) to complete the CPU's read or cause
> > some kind of abort or machine check.
> > 
> > Just asking in case there's some controller configuration that should
> > be tweaked.
> 
> If I'm following correctly, that'll be a read transaction to the native side
> of the controller itself; it can't complete that read, or do anything else
> either, because it's clock-gated, and thus completely oblivious (it might be
> that if another CPU was able to enable the clocks then everything would
> carry on as normal, or it might end up totally deadlocking the SoC
> interconnect). I think it's safe to assume that in that state nothing of
> importance would be happening on the PCIe side, and even if it was we'd
> never get to know about it.

Oh, right, that makes sense.  I was thinking about the PCIe side, but
if the controller itself isn't working, of course we wouldn't get that
far.

I would expect that the CPU itself would have some kind of timeout for
the read, but that's far outside of the PCI world.

Bjorn