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

[Jisheng Zhang <jszhang@marvell.com>]

On Tue, 4 Jul 2017 14:58:40 +0800 Jisheng Zhang 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. 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.
> 

PS: I think most platforms choose 2 pcie designware viewports just because
it's the default setting. And I have send a feature request to ASIC people
to increase the viewports to 3 for future marvell berlin SoCs.