RE: [PATCH v7 01/10] PCI: host: rcar: Add Renesas R-Car PCIe driver

[Phil Edworthy <phil.edworthy@renesas.com>]

Hi Jason,

On 30 April 2014 16:44, Jason wrote:
> On Wed, Apr 30, 2014 at 10:33:08AM +0000, Phil Edworthy wrote:
> 
> > > You set the value of the integrated bus based on the bus number in the
> > > TYPE0 accesses the driver performs and it shows up in the PCI config
> > > space of the root port bridge:
> > >
> > > root@koelsch:~# lspci -vv
> > > 00:00.0 PCI bridge: Renesas Technology Corp. Device 001f (prog-if 00
> > > [Normal decode])
> > >         Bus: primary\0, secondary\x01, subordinate\x01, sec-latency=0
> > >
> > > The value here will be also be exported on-the-wire in certain PCI-E
> > > TLPs.
> > >
> > > So it is important, and must be set properly.
> > I am not so sure about this...
> >
> > There are three things we need to consider:
> > 1) talking to the host bridge itself
> > 2) talking to immediate device (connected directly to host bridge)
> > 3) talking to other devices
> >
> > The PCI standard tells us we use type 1's for 3) and type 0's for
> > 2). The driver uses root_bus_nr to make this decision.  For 1),
> > there is a note in the driver that the hardware cannot target
> > itself. The driver has code in rcar_pcie_config_access() to check if
> > it's an access to the host bridge, and if so, use I/O (readl/writel)
> > to perform the config access.
> 
> The logic should be:
>  if (bus = primary)
>     do io access to host bridge
>  else if (bus = secondary)
>     issue type 0 TLP on the wire
>  else if (bus > secondary && bus <= subordinate)
>     issue type 1 TLP on the wire
>  else
>     fail, invalid bus number
> Where the three values come from the register in the PCI host bridge's
> configuration space, and are kept in sync with the programming from
> the Linux PCI core.
> 
> It is just a happy hapenstance that root_bus_nr equals the value the
> PCI core programmed into secondary - that is not guarenteed, you must
> use the primary value directly.
For type0 TLPs, we are not checking that root_bus_nr equals the value the PCI core programmed into secondary, we are checking that the (root_bus_nr = bus->parent->number). The only way this wouldn't work is if root_bus_nr was not the root bus number.

Since the Synopsys DW driver also saves off sys->busnr and later uses this to determine if accesses are for the host bridge, I guess that means it won't always work either, right? Or is it ok because the DW driver saves sys->busnr in its .scan function?

When would the PCI core change the root bus number to something other than set in sys->busnr?

> I recommend capturing it on write to the host bridge config
> space.

Thanks
Phil