Message-ID: <20160227143637.GA21037@localhost>
Date:	Sat, 27 Feb 2016 08:36:37 -0600
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Jayachandran Chandrashekaran Nair 
	<jayachandran.chandrashekaran@...adcom.com>
Cc:	Jayachandran C <jchandra@...adcom.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, Rob Herring <robh@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [RFC PATCH 2/2] PCI: Handle Broadcom Vulcan quirks

On Sat, Feb 27, 2016 at 01:44:49PM +0530, Jayachandran Chandrashekaran Nair wrote:
> On Tue, Feb 23, 2016 at 8:42 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Tue, Feb 23, 2016 at 08:10:03PM +0530, Jayachandran Chandrashekaran Nair wrote:
> > > On Thu, Feb 18, 2016 at 9:19 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > On Wed, Feb 17, 2016 at 10:36:55PM +0530, Jayachandran Chandrashekaran Nair wrote:
> > > >> On Wed, Feb 17, 2016 at 2:33 AM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > >> > On Tue, Feb 16, 2016 at 11:39:41PM +0530, Jayachandran Chandrashekaran Nair wrote:
> > > >> >> On Tue, Feb 16, 2016 at 10:44 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > >> >> > On Tue, Feb 16, 2016 at 09:47:34PM +0530, Jayachandran Chandrashekaran Nair wrote:
> > > >> >> >> On Tue, Feb 16, 2016 at 12:00 AM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > >> >> >> > On Mon, Feb 15, 2016 at 02:56:38PM +0530, Jayachandran C wrote:
> > > >> >> >> >> Handle two quirks of the PCI controller on Broadcom's Vulcan processor.
> > > >> >> >> >>  - Mark internal bridges so that they are skipped during DMA alias
> > > >> >> >> >>    search.
> > > >> >> >> >>  - Skip BAR0 resource assignment for internal bridges. The BARs
> > > >> >> >> >>    of internal bridges should not be assigned from the mem resource
> > > >> >> >> >>    range.
> > > >> >> >> >> ...
> > > >> >> >> >>  drivers/pci/quirks.c | 21 +++++++++++++++++++++
> > > >> >> >> >>  1 file changed, 21 insertions(+)
> > > >> >> >> >>
> > > >> >> >> >> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> > > >> >> >> >> index 0575a1e..afc186a 100644
> > > >> >> >> >> --- a/drivers/pci/quirks.c
> > > >> >> >> >> +++ b/drivers/pci/quirks.c
> > > >> >> >> >> @@ -3705,6 +3705,27 @@ DECLARE_PCI_FIXUP_HEADER(0x1283, 0x8892, quirk_use_pcie_bridge_dma_alias);
> > > >> >> >> >>  DECLARE_PCI_FIXUP_HEADER(0x8086, 0x244e, quirk_use_pcie_bridge_dma_alias);
> > > >> >> >> >>
> > > >> >> >> >>  /*
> > > >> >> >> >> + * Two levels of bridges in Broadcom Vulcan are not real PCI or PCIe bridges.
> > > >> >> >> >> + * These are internal bridges and should not be used for dma alias
> > > >> >> >> >> + * calculations. Additionally, the BAR0 of thes bridges should not be
> > > >> >> >> >> + * assigned with a mem resource from linux
> > > >> >> >> >> + */
> > > >> >> >> >> +static void quirk_bridge_brcm_vulcan_internal(struct pci_dev *pdev)
> > > >> >> >> >> +{
> > > >> >> >> >> +     struct resource *r = &pdev->resource[0];
> > > >> >> >> >> +
> > > >> >> >> >> +     /* skip from alias search */
> > > >> >> >> >> +     pdev->dev_flags |= PCI_DEV_FLAGS_BRIDGE_SKIP_ALIAS;
> > > >> >> >> >> +
> > > >> >> >> >> +     /* clear BAR0, should not be used from Linux */
> > > >> >> >> >> +     memset(r, 0, sizeof(*r));
> > > >> >> >> >
> > > >> >> >> > This definitely needs some explanation.  The whole point of the
> > > >> >> >> > architected PCI config space header is so that generic OS code can
> > > >> >> >> > manage the device without having to add device-specific code.
> > > >> >> >> >
> > > >> >> >> > Are you saying the register at 0x10 in config space is not actually a
> > > >> >> >> > BAR at all?  Or it is a BAR, but you don't think anybody should need
> > > >> >> >> > to use it?
> > > >> >> >>
> > > >> >> >> These is a proper BAR (64 bit pref) in the bridge's Type 1 header, but:
> > > >> >> >> 1. Assigning a memory resource from the pci memory window to this
> > > >> >> >>    BAR causes the PCIe system to fail (this is a bug). So we cannot
> > > >> >> >>    expose this BAR to standard Linux code without even more quirks
> > > >> >> >>    and hacks.
> > > >> >> >
> > > >> >> > Are you saying assigning space for this BAR exposes a Linux PCI bug?
> > > >> >> > If so, I'd like to know more about that because we should fix it.
> > > >> >> >
> > > >> >> > Or does it expose a hardware bug in the Broadcom device?
> > > >> >>
> > > >> >> This is a hardware bug (or non-compliance). The BAR cannot be
> > > >> >> assigned from the PCI MEM range. To use it we need to program
> > > >> >> an address outside the areas assigned to PCI to the BAR. But like
> > > >> >> I said it is better for us to ignore the BAR in linux and then the
> > > >> >> device acts as a normal bridge.
> > > >> >
> > > >> > Your PCI host bridge has a window of address space that it forwards
> > > >> > from the primary (CPU) side to the secondary (PCI) side.  I assume
> > > >> > that's what you mean by the "PCI MEM range."  Apparently if the BAR is
> > > >> > programmed inside that window, it causes some hardware error.  That
> > > >> > still seems strange; there's no driver for the device, and we know the
> > > >> > range is in use so it won't be assigned to any other device, so Linux
> > > >> > should never *access* the range.
> > > >>
> > > >> This is correct. The write to this bridge BAR with a address from
> > > >> the host bridge window will cause a hardware issue.
> > > >>
> > > >> > Apparently if you program the BAR *outside* the window, the hardware
> > > >> > error does not happen, and the private registers are accessible.  But
> > > >> > Linux currently doesn't know where this space is.
> > > >> >
> > > >> > If we ignore the BAR in Linux, apparently the hardware error does not
> > > >> > happen.  But the BAR still contains some value, so this is really the
> > > >> > same as having the BAR outside the window, presumably because it came
> > > >> > out of reset that way, or maybe firmware programmed it.
> > > >>
> > > >> Yes.
> > > >>
> > > >> > It sounds to me like you should do the following:
> > > >> >
> > > >> > a) Have firmware program this BAR where you want it,
> > > >> >
> > > >> > b) Describe these private registers as internal PCI bridge registers,
> > > >> >    using a DT "reg" property,
> > > >>
> > > >> Two ponts here:
> > > >> - We have to support ACPI as well
> > > >
> > > > ACPI can do something similar.  This register space would be described
> > > > in a _CRS method.  IIRC there is actually a bit in the _CRS descriptor
> > > > that was intended to tell you whether the resource is (a) consumed
> > > > directly by the device or (b) forwarded to downstream devices.  But I
> > > > think BIOSes didn't use that bit correctly, so it's useless, so the
> > > > _CRS method might have to be on a different device.
> > > >
> > > >> - There is no need to use the private registers in linux, so the
> > > >>    whole thing will be pointless.
> > > >>
> > > >> > c) Describe the host bridge window (which doesn't include the above
> > > >> >    registers) using the normal "ranges" property, and
> > > >>
> > > >> This is standard code (and ACPI works as well)..
> > > >>
> > > >> > d) Arrange for config writes to BAR 0 to be dropped and for reads to
> > > >> >    return 0, maybe by checks in your config accessors.
> > > >>
> > > >> Here we are hiding the BAR from linux using checks in config read
> > > >> write (if i understand correctly). This will need custom config accessors
> > > >> for Vulcan both on device tree as well as in ACPI.
> > > >
> > > > Config accessors are usually not specific to DT or ACPI.
> > > >
> > > >> The patch (above) is trying to do it in a much much simpler way by
> > > >> clearing the bar0 resource in DECLARE_PCI_FIXUP_HEADER quirk.
> > > >> I am not able to see why this is not valid.
> > > >
> > > > I think there are two problems:
> > > >
> > > > 1) If the device responds to any address space, Linux needs to know
> > > > about it.  If we don't know about it, we might assign that space to
> > > > something else.
> > >
> > > If it is really needed I can mark the address space as reserved. And
> > > Linux generally should not use physical address space outside the
> > > areas specified by firmware of device tree.
> >
> > That's true, at least in theory.  Historically on x86, we only avoided
> > areas marked as reserved.  We *should* have only used areas marked as
> > available for MMIO, but Linux didn't pay much attention to the _CRS
> > methods that tell us that.  We just assumed anything that wasn't RAM
> > and wasn't marked as reserved in E820 was fair game for use by PCI.
> >
> > > > 2) The PCI core still reads and writes the BAR to size it, and we
> > > > don't know whether this will trigger the hardware issue.
> > >
> > > The sizing read/write is done after disabling the BAR by writing
> > > to CMD register - I don't see why you think this will be a problem.
> >
> > You're right; I forgot about that CMD enable bit.  There are a few
> > cases where we don't disable decoding (see mmio_always_on), the most
> > important being for PCI_CLASS_BRIDGE_HOST devices.  But I think your
> > device is a PCI_CLASS_BRIDGE_PCI device, not a BRIDGE_HOST device, so
> > we should disable decoding on your device.
> >
> > > > *You* might know that neither of these is an issue today, but special
> > > > incidental knowledge like that is a maintenance problem because I
> > > > can't tell what PCI core changes might make them an issue in the
> > > > future.
> > >
> > > The worst case maintenance problem is that a future PCI change
> > > might break our chip. This is not unexpected in Linux, and such
> > > breakages can be easily fixed.
> >
> > It's certainly unexpected by *me* :)  And the burden of fixing such
> > breakages doesn't scale well -- it usually involves triage by somebody
> > other than the chip folks, e.g., me.
> >
> > Andi is doing something similar for some Broadwell-EP devices that
> > have BARs that aren't really BARs.  I suggested custom config
> > accessors, which is fairly clean from the core point of view, but it's
> > definitely clunky on the device side.
> >
> > Andi's first proposals were to use a quirk to set a flag bit that told
> > us not to touch the BAR.  I'm starting to think that's a cleaner
> > approach.  I think it would be better to put the bit(s) in the
> > pci_dev, so generic struct resource users wouldn't see them, set them
> > via early quirks, then have __pci_read_base() check them and just
> > leave the struct resource zeroed out.
> 
> I spent some more time working with our hardware team on the nature
> of this issue. It looks like this is not a problem if we setup the PCI
> configuration in firmware better (this has to do with the sequence for
> configuring the PCI controller in firmware).
> 
> So I will drop the part in my patch that hides the BARs from linux, since
> the standard Linux resource allocation code will work Sorry for taking up
> your time on this.

No problem, this is the best possible outcome: the hardware isn't
broken, Linux isn't broken (at least in this tiny regard), we avoided
putting an unnecessary workaround into Linux, and we retained the
ability to use those debug registers if that's ever necessary.

Thanks a lot for chasing this down!

Bjorn

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