From - Wed Sep 24 08:05:39 1997
Path: news.mitre.org!blanket.mitre.org!agate!newsgate.duke.edu!nntprelay.mathworks.com!newsfeed.direct.ca!news.he.net!newshub.cts.com!newsfeed.cts.com!cmkrnl!jeh
From: jeh@cmkrnl.com (Jamie Hanrahan)
Newsgroups: comp.os.ms-windows.programmer.nt.kernel-mode
Subject: Re: CurrentIrp == NULL in my ISR
Message-ID: <1997Sep23.183233.8218@cmkrnl>
Date: 23 Sep 97 18:32:33 PST
References: <34283cbd.349053059@news.connectnet.com>
Organization: Kernel Mode Systems, San Diego, CA
Lines: 58

In article <34283cbd.349053059@news.connectnet.com>, jvasquez@getntds.com writes:
> Why is my deviceObject->currentIrp == NULL in my ISR?
> Should I be setting that field to my IRP just before I kick off the
> interrupt?

(I could swear I answered this recently, but here goes again:)

The only things in the system that change this field are IoStartPacket 
and IoStartNextPacket.  

Put it this way - these routines do approximately this (ignoring 
details of cancels, spinlocks, the Key argument, and so on):

IoStartPacket(dvcObj, Irp)
{
	if (dvcObj->DeviceQueue.Busy) {
		insert IRP on device queue
	} else {
		dvcObj->DeviceQueue.Busy = TRUE;
		dvcObj->CurrentIrp = Irp;
		call StartIo routine (dvcObj, newIrp);
	}
}

IoStartNextPacket(dvcObj)
{
	dvcObj->CurrentIrp = NULL;
	remove IRP from head of queue, store addr in newIrp
	if (queue was empty) {
		dvcObj->DeviceQueue.Busy = TRUE;
	} else {
		dvcObj->CurrentIrp = newIrp;
		call StartIo routine (dvcObj, newIrp);
	}

}

If you don't use IoStart[Next]Packet this field will always be NULL
unless you change it yourself -- and there is probably no real reason
to do so; since if you aren't using IoStart[Next]Packet it's probably
because you support more than one "current" IRP, so having a single
pointer to "the current IRP" would make no sense anyway. 

If you ARE using IoStart[Next]Packet and you find that this field is
NULL in your ISR, it means that you've already called
IoStartNextPacket before the ISR was invoked, and IoStartNextPacket
found no more IRPs on the the DeviceQueue.  Along with clearing the
DeviceQueue->Busy bit, IoStartNextPacket NULLs the CurrentIrp pointer
when the device queue is empty. 

And these are the ONLY two things in the system that manipulate the 
dvcObj->DeviceQueue.Busy or dvcObj->CurrentIrp .  

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
        Internet: jeh@cmkrnl.com (JH645)  CompuServe: 74140,2055  
drivers, internals, networks, applications, and training for VMS and Windows NT
NT driver FAQ, links, and other information:  http://www.cmkrnl.com/

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