Message-ID: <4877AD63.3000405@rpi.edu>
Date:	Fri, 11 Jul 2008 14:58:43 -0400
From:	David Brigada <brigad@....edu>
To:	Jordan Crouse <jordan.crouse@....com>
CC:	jim.cromie@...il.com, LKML <linux-kernel@...r.kernel.org>,
	linux-geode@...ts.infradead.org
Subject: Re: PCI-ISA Bridge not operating

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?

Dave
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