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

Ard Biesheuvel
2017-07-04T09:38:34Z

On 4 July 2017 at 09:19, Jisheng Zhang <jszhang@marvell.com> wrote:
> On Tue, 4 Jul 2017 09:02:06 +0100 Ard Biesheuvel wrote:
>
>> On 4 July 2017 at 07:58, Jisheng Zhang <jszhang@marvell.com> wrote:
>> > On Mon, 3 Jul 2017 08:27:04 -0500 wrote:
>> >
>> >> [+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.
>> >>
>> >
>> > IIRC, for 2 viewports, we don't need to worry about the config space
>> > access, because config space access is serialized by pci_lock; We
>> > do have race between config space and io space. But the accessing config
>> > space and io space at the same time is rare.
>>
>> Being 'rare' is not sufficient, unfortunately. In the current
>> situation, I/O space accesses may occur when the outbound window is
>> directed to the config space of a potentially completely unrelated
>> device. This is bad.
>
> Yep, I agree with you.
>
>>
>> > And the PCIe EPs which
>> > has io space are rare too, supporting these EPs are not the potential
>> > target of those platforms with 2 viewports.
>> >
>>
>> I am not sure EP mode is relevant here. What I do know is that boards
>
> I mean those PCIe EP cards which have IO space, but that doesn't matter.
>

Ah, ok. But if such EP cards are so rare, why expose I/O space at all
if we cannot do it safely?

>> like the Marvell 8040 based MacchiatoBin uses this IP in RC mode, and
>> exposes config, MMIO and IO space windows using only 2 viewports. Note
>> that this is essentially a bug in the DT description, given that its
>> version of the IP supports 8 viewports. But the driver needs to be
>> fixed as well.
>
> To fix for 2 viewports situation, we need to serialize access of the io
> and config space. In internal repo, we can achieve it by modifying the
> io access helper functions such as inl/outl, but this won't be accepted
> by the mainline IMHO. Except fixing the HW, any elegant solution?
>
> Suggestions are appreciated.
>

I think the safe and upstreamable approach is to disable the I/O
window completely if num-viewports <= 2.