Re: PCI-ISA Bridge not operating

[David Brigada]

Jordan Crouse wrote:
> On 11/07/08 14:58 -0400, David Brigada wrote:
> > David Brigada wrote:
> > > Jordan Crouse wrote:
> > > > On 11/07/08 10:58 -0400, David Brigada wrote:
> > > > > Hi,
> > > > >
> > > > > I'm working with the MSM800XEV board from Digital-Logic.  This board 
> > > > > uses a Geode LX800 for a CPU and has the CS5536 companion board also 
> > > > > installed.  The board works with an IT8888G IC that provides a PCI/ISA 
> > > > > bridge to a PC/104 bus that is externally provided.
> > > > >
> > > > > If I boot with FreeDOS, I can twiddle I/O ports, and the proper ISA 
> > > > > signaling comes over the PC/104 bus.  In Linux, the /IOW or /IOR line 
> > > > > goes low as expected, but the address doesn't come over the bus.  The 
> > > > > DOS that I'm running doesn't seem to have any specific drivers for the 
> > > > > chip, I'm guessing that the hardware should "just work" --- the IT8888G 
> > > > > is designed to grab I/O requests in the ISA range off the PCI bus after 
> > > > > a short delay if nothing else grabs them first.
> > > > >
> > > > > I have a feeling that it has something to do with the CS5536 companion 
> > > > > chip, as it seems as though there is a driver for a PCI/ISA bridge on 
> > > > > that chip, though I can't get much detail from AMD's datasheet on that 
> > > > > functionality.  I do know that on the MSM800XEV, any such functionality 
> > > > > is wired to the IT8888G, not the CS5536.
> > > > >
> > > > > There are two kernel config options related to the PCI IDs of the parts 
> > > > > of the device that handle the ISA bus, CONFIG_SCx200_ACB and 
> > > > > CONFIG_CS5535_GPIO.  I've tried disabling both, but it doesn't seem to help.
> > > > >
> > > > > In lspci, the CS5536 PCI/ISA bridge is shown, but not the IT8888G.
> > > > >
> > > > > Any ideas?
> > > > ISA should indeed "just work".  The only thing I'm wondering is if
> > > > the kernel is interfering (it shouldn't).  I assume that since it works
> > > > in FreeDOS that there is no possibility that something on the PCI bus
> > > > is grabbing the cycles instead.
> > > That's what I'm thinking --- that the CS5536 PCI/ISA bridge is claiming 
> > > the cycles.
> > >
> > > > How are you trying to access the device in Linux?  Through a kernel module
> > > > or a user application running as root?
> > > I've tried both.  I have a kernel module that I wrote for the hardware. 
> > >   When I couldn't get that working, I tried looping some code that keeps 
> > > touching the same I/O port that I'm using.
> > >
> > > > Jordan
> > > Dave
> > Looking through the documentation for the CS5536, in the "CS56536 
> > Companion Device Data Book," section 5.2.8, it says the following:
> >
> >  > If the SDOFF (Subtractive Decode Off) bit in the GLPCI_MSR_CTRL (MSR
> >  > 51000010h[10]) is cleared (reset value), any PCI transaction, other
> >  > than Configuration Read/Write, Interrupt Acknowledge, and Special
> >  > Cycle transactions, not claimed by any device (i.3., not asserting
> >  > DEVSEL#) within the default active decode cycles (three cycles
> >  > immediately after FRAME# being asserted) will be accepted by GLPCI_SB
> >  > at the fourth clock edge.
> >
> > This is the same behavior that the IT8888G chip uses --- it waits three 
> > cycles for another device to claim it and then passes the transaction 
> > along.  I'm guessing that the CS5536 might be grabbing it (maybe it's 
> > electrically closer, or the logic is more optimized) before the IT8888G 
> > can handle it.
> >
> > Does this seem feasible as to what could be happening?
>
> Sure, but then why does FreeDOS work?  It shouldn't be any different
> when the bits hit the line.
>
> Jordan

That *is* puzzling.  When I do lspci, the entry for the IT8888G does not 
appear.  I don't have much experience with PCI internals.  Would that be 
because there is no driver for it in the kernel, or is there something 
more insidious afoot?

Dave
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