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30 Dec 01 09:23
Jake Oshins
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Posts To This List: 1047
RE: ACPI Machines and IRQ 9 [was: Communicating with the NT developers]

This is a stupid story. And I'm embarrassed to tell you the truth. But here it is. The early ACPI machines were mostly laptops. And the laptops of that generation had most of their devices either embedded in the chipset or on the ISA bus. The PCI or AGP buses were used only for video, and to connect the north bridge with the south bridge. (In Intel's chipset terms, the North bridge has all the fast gates of the chipset, including the memory controller, AGP and in that generation, the PCI bus generation logic. The south bridge contains all the slow gates, including the IDE controller, the ISA bridge, all the PC legacy stuff and probably a USB controller. Today, the south bridge probably also has audio and a few other random odds and ends.) Because the laptops of that era had all of their devices on the ISA bus, interrupt sharing worked poorly. If you bought a mid-'90s laptop from IBM or Toshiba, the serial port and possibly IR would be disabled. There would be a utility packaged with the machine that allowed you to turn on your serial or IR, but at the cost of the bi-directional parallel port, or one of the PCMCIA slots, since there just weren't enough IRQs in the machine to guarantee that all of the peripherals worked, especially if you filled both PCMCIA slots with combo cards. I once debugged a Toshiba 750CDT in a docking station that had two PCMCIA cards plugged into the machine, two PCMCIA cards plugged into the slots in dock, two ISA cards in the dock and an extra IDE device in the dock, too. This meant that the total demand on the machine was 20 IRQs, when only 16 were actually available. (As an aside, I've been trying to convince Intel to put APIC interrupt controllers, which would allow many more IRQs, in their laptop chipsets since 1997. My predecessor had been trying since '94. They may actually manage it soon.) Along comes ACPI. When you turn on ACPI in a machine, it suddenly switches all the power management logic in the machine from delivering its interrupts as BIOS-visible, non-vectored System Management Interrupts over to OS-visible, vectored interrupts. And that interrupt is delivered level-triggered, active-low, which means that it can be shared with a PCI interrupt. Now consider that these early ACPI machines were already over-committed in terms of interrupts. There was no way to make them work with PCI devices spread out on lots of IRQs. So I just made the code collapse all the sharable devices onto the ACPI interrupt, which was fixed in the chipset by Intel at IRQ 9. By doing it this way, I could hide the fact that ACPI had just created a demand for one more IRQ. (If you use a non-Intel chipset that has ACPI coming in on some other IRQ, you'll see all the PCI devices in Win2K go to that IRQ, not 9.) Further complicating this story was that I was trying to get ACPI machines to work back in 1997, when the people working on Plug and Play in Win2K hadn't yet gotten their stuff going yet. At time, it wasn't possible to move a device from one set of resources to another after it had been started. This meant that any IRQ solution that I came up with had to work from the first try, so it had to be conservative. The everything-on-IRQ-9 solution worked. It got the machines to run, as long as none of the device drivers mis-handled their ISRs. (Later, this turned out to be a huge debugging problem, since when you chain eight or nine devices, you'll get somebody who fails.) The solution wasn't optimal, but it did work. I meant to go back and change it later, before we shipped Windows 2000. A couple of years passed. I had been working on multi-processor problems and on other aspects of ACPI. It got close to the time to ship Windows 2000 and somebody brought up the old question of IRQ stacking. I worked up a more-elegant solution, one that spread out interrupts on most machines. By that time, Plug and Play had been mostly completed, and that wasn't a bottleneck any more. But the test team told me that they wouldn't let me put it into the product, since they didn't have time to re-test the thousands of machines that had already been tested with the old algorithm. At the time, I thought that this was somewhat ridiculous. I thought that my code would work just fine. I thought that their fears were un-justified. But I was overruled, and I just put the code into what became Windows XP, letting Windows 2000 ship with the simple, safe, yet frustrating stacking. This is a good point in the story to explain that, in ACPI machines, the IRQ steering is accomplished by interpreting BIOS-supplied P-code called ASL. The IRQ routers are completely abstracted by the BIOS. The OS doesn't need to know about the actual hardware. The old IRQ steering code in Win9x, which was dropped into the non-ACPI HAL in Win2K, had to have code specific to each chipset, which meant that it didn't work when new chipsets were shipped. It was also written in a way that it assumed that there were exactly four IRQs coming from PCI. ACPI machines sometimes have many more. (This is the reason that you don't see the IRQ steering tab in ACPI machines. It just wasn't flexible enough and we didn't have time to re-do it.) What we discovered with Windows XP was that all of those ACPI machines that had been tested with their IRQs stacked on IRQ 9 tended to fail when you spread the IRQs out. A typical example of a failure would work like this: WinXP doesn't need the IRQ for the parallel port unless you're using one of the extended modes. So the parallel driver releases its IRQ until it's needed. The IRQ choosing logic (called an IRQ "arbiter") would move a PCI device onto the parallel IRQ. This action depends on re-programming the chipset so that the parallel port isn't actually triggering the IRQ. This is supposed to happen by interpreting even more BIOS P-code that manipulates the chipset, since there is no standard for parallel port configuration. If your chipset comes from Intel, this probably works, since the mere act of setting a PCI device to an IRQ also disconnects that IRQ from the ISA bus. But if your chipset comes from VIA or ALi, there is another step involved. The problem is that nearly all of the BIOS P-code out there is copied from old Intel example code. So they are almost all missing the extra step necessary in VIA and ALi machines. If the BIOS fails to stop the IRQ coming from the parallel port, the machine hangs, since the parallel port, which sends its IRQs active-high, edge-triggered, will ground the interrupt signal in the passive state. And grounding an interrupt which is enabled active-low, level-triggered will cause an endless stream of interrupts. The parallel port is just an example. Pick any device that is in the legacy SuperIO chip and the story repeats itself. In Windows XP, I made a bunch of changes. In machines without cardbus controllers, (which don't have the IRQ problems created by PCMCIA,) it will try to keep the PCI devices on the IRQs that the BIOS used during boot. If the BIOS didn't set the device up, then any IRQ may be chosen. But if your machine has a VIA chipset, or if it has a BIOS that we know to be broken, then we fall back to the Win2K-style stacking behavior. The unfortunate truth is that you guys on this list mostly build your own machines, rather than buying them from reputable manufacturers, which means that you guys own the machines with broken BIOSes and VIA chipsets. So even with WindowsXP, you'll see the same old stacking behavior. One notable addendum is that any machine with an APIC interrupt controller, and thus more than 16 IRQs, will spread interrupts out, even in Win2K. In the past, this was mostly limited to SMP machines. But any desktop machine shipping today that gets the Windows logo has to have an APIC. (This was another reason that I hadn't gone back to re-write this code earlier. Intel had promised that all machines would have APICs by 1998. If this had materialized, then none of you would have had any complaints by now.) I'm actually currently working on software for some future NT that will let an administrator configure the machine in any way he or she desires. - Jake -----Original Message----- Subject: Re: Communicating with the NT developers From: "Maxim S. Shatskih" <> Date: Thu, 27 Dec 2001 18:22:20 +0300 X-Message-Number: 19 Hi Jake, >expertise. (For those who are curious, I have spent the last five years >working on the NT HAL, with lots of forays into power management and >ACPI. Then maybe you're exactly the person who will be able to answer to one of the popular questions here: - why ACPI HAL puts all PCI cards to IRQ9? What were the technical backgrounds of such a decision? Why PCI IRQ steering is allowed for non-ACPI HAL (and even has a UI property sheet) and is disabled on ACPI? thanks, Max --- You are currently subscribed to ntdev as: $subst('Recip.EmailAddr') To unsubscribe send a blank email to leave-ntdev-$subst('Recip.MemberIDChar')
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