Messages in this thread | | | Date | Tue, 30 Oct 2007 18:23:14 -0400 | From | Jeff Garzik <> | Subject | Re: Multiple MSI messages support |
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Shawn Jin wrote: > Hi, > > If this is really off-topic here, I apologize first. But I cannot > think a better place to ask this particular question. > > I understand that the current PCI subsystem or linux kernel (x86) > supports only one message when MSI is enabled even for devices having > multiple MSI messages. But why? Is this a limitation solely due to the > OS or due to the x86 APIC? > > I know the current linux kernel (2.6.23) changed MSI message data > format a little bit to support other architectures. But some older > version (e.g. 2.6.18) defined a specific format for the MSI msg data > in a way that 8 bits contain the irq number and the other 8 bits have > the interrupt attributes, which is x86 specific. Why does the msg data > need to contain the irq number? Here is my hypothetic explanation. The > device writes the MSI msg data to the specified MSI msg address. And > APIC uses the irq number in the msg data to generate appropriate > interrupt, which of course results in an appropriate ISR invoked. A > device having multiple MSI messages typically appends some information > of which MSI message to the msg data field. For example, if the system > (or OS) configures the MSI msg data as 0x5000, a device having 4 MSI > messages could write 0x5000, 0x5001, 0x5002, 0x5003 to differentiate > the MSI messages. However this cannot work with the APIC due to the > way how APIC asserts interrupts as I described above (if my > understanding is correct). > > Hence my answer to the question is this is due to the x86 APIC. For > other architectures such as powerpc this is probably not a problem > since the interrupt controller is different. Am I correct?
IMO it's more like there has never been enough need for anybody to look into it, I bet...
The way drivers are written, you are typically must touch a few key hardware registers _anyway_, so the multiple messages in practice are not much more useful than the simple fact that your MSI irq handler function was called (with all that indicates and implies).
Jeff
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