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

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Message-ID: <20170703132704.GF13824@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 3 Jul 2017 08:27:04 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Mason <slash.tmp@...e.fr>, Marc Zyngier <marc.zyngier@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>
Subject: Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

[+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@...nel.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@...madesigns.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