Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

Bjorn Helgaas
2017-07-03T13:27:09Z

[+cc Jingoo, Joao]

On Mon, Jul 03, 2017 at 10:35:50AM +0100, Ard Biesheuvel wrote:
> On 3 July 2017 at 00:18, Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Tue, Jun 20, 2017 at 10:17:40AM +0200, Marc Gonzalez wrote:
> >> This driver is required to work around several hardware bugs
> >> in the PCIe controller.
> >>
> >> NB: Revision 1 does not support legacy interrupts, or IO space.
> >
> > I had to apply these manually because of conflicts in Kconfig and
> > Makefile. What are these based on? Easiest for me is if you base
> > them on the current -rc1 tag.
> >
> >> Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
> >> ---
> >> drivers/pci/host/Kconfig | 8 +++
> >> drivers/pci/host/Makefile | 1 +
> >> drivers/pci/host/pcie-tango.c | 164 ++++++++++++++++++++++++++++++++++++++++++
> >> include/linux/pci_ids.h | 2 +
> >> 4 files changed, 175 insertions(+)
> >> create mode 100644 drivers/pci/host/pcie-tango.c
> >>
> [..]
> >> + /*
> >> + * QUIRK #2
> >> + * Unfortunately, config and mem spaces are muxed.
> >> + * Linux does not support such a setting, since drivers are free
> >> + * to access mem space directly, at any time.
> >> + * Therefore, we can only PRAY that config and mem space accesses
> >> + * NEVER occur concurrently.
> >> + */
> >> + writel_relaxed(1, pcie->mux);
> >> + ret = pci_generic_config_read(bus, devfn, where, size, val);
> >> + writel_relaxed(0, pcie->mux);
> >
> > I'm very hesitant about this. When people stress this, we're going to
> > get reports of data corruption. Even with the disclaimer below, I
> > don't feel good about this. Adding the driver is an implicit claim
> > that we support the device, but we know it can't be made reliable.
>
> I noticed that the Synopsys driver suffers from a similar issue: in
> dw_pcie_rd_other_conf(), it happily reprograms the outbound I/O window
> to perform a config space access, and switches it back to I/O space
> afterwards (unless it has more than 2 viewports, in which case it uses
> dedicated windows for I/O space and config space)

That doesn't sound good. Jingoo, Joao? I remember some discussion
about this, but not the details.

I/O accesses use wrappers (inb(), etc), so there's at least the
possibility of a mutex to serialize them with respect to config
accesses.

Bjorn