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

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Date:   Mon, 3 Jul 2017 10:35:50 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Bjorn Helgaas <helgaas@...nel.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>
Subject: Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

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)