Re: PCIE

[Manu Abraham <abraham.manu@gmail.com>]

Roland Dreier wrote:
>  > > At least on my device (PCI ID 1131:7162) there is no MSI-X capability,
>  > > so that's not an option for you.  The current Linux implementation
>  > > does not support more than one MSI interrupt, so you just get one
>  > > interrupt with pci_enable_msi().
>  > 
>  > This would mean MSI or MSI-X ?  A bit confused now.
> 
> As I said, the device I have in my system:
> 
>     02:00.0 Multimedia controller: Philips Semiconductors Unknown device 7162
>             Subsystem: Animation Technologies Inc. Unknown device 0820
>             Flags: bus master, fast devsel, latency 0, IRQ 11
>             Memory at 90200000 (64-bit, non-prefetchable) [size=1M]
>             Capabilities: [40] Message Signalled Interrupts: Mask- 64bit+ Queue=0/5 Enable-
>             Capabilities: [50] Express Endpoint IRQ 0
>             Capabilities: [74] Power Management version 2
>             Capabilities: [80] Vendor Specific Information
> 
> ...has only an MSI capability (the "[40] Message Signalled Interrupts"
> line).  So MSI-X is not possible, since the device cannot do it.  And
> that means you can at most do pci_enable_msi().  The current Linux MSI
> support only handles a single interrupt, just like you get normally
> (no matter how many MSI interrupts a device can handle).  To get
> multiple interrupts from a single device under Linux, you must use
> MSI-X and pci_enable_msix -- but for this to work, your device must
> support MSI-X of course.
> 
> A device that supports both MSI and MSI-X would look like:
> 
>     0b:00.0 InfiniBand: Mellanox Technologies MT25204 [InfiniHost III Lx HCA] (rev 20)
>             Subsystem: Mellanox Technologies MT25204 [InfiniHost III Lx HCA]
>             Flags: bus master, fast devsel, latency 0, IRQ 16
>             Memory at fc600000 (64-bit, non-prefetchable) [size=1M]
>             Memory at d8800000 (64-bit, prefetchable) [size=8M]
>             Capabilities: [40] Power Management version 2
>             Capabilities: [48] Vital Product Data
>             Capabilities: [90] Message Signalled Interrupts: Mask- 64bit+ Queue=0/5 Enable-
>             Capabilities: [84] MSI-X: Enable- Mask- TabSize=32
>             Capabilities: [60] Express Endpoint IRQ 0
> 
> with both "Message Signalled Interrupts" and "MSI-X" capabilities.
> 

Thanks for the explanation.

> However, as I said before I think you shouldn't worry about MSI right
> now.  Since there are many systems where MSI doesn't work, you'll need
> to get the driver working with legacy (INTx) interrupts anyway.  And
> you seem to be in a bit over your head just doing that without adding
> the complexity of MSI on top, hence my recommendation to just focus on
> the basic driver.

True, compatibility would be important.

Thanks,
Manu

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