RE: [PATCH]2.6.7 MSI-X Update

[Nguyen, Tom L]

ON Thursday, June 24, 2004 Roland Dreier wrote:
>    Roland> I could imagine hardware where the driver does not know
>    Roland> exactly how many vectors it will use until it starts up.
>    Roland> As a hypothetical example, imagine some storage networking
>    Roland> host adapter that supports an interrupt vector per storage
>    Roland> target.  The driver does not know how many vectors it will
>    Roland> actually use until it has logged into the storage fabric;
>    Roland> in fact, the driver may want to keep some vectors "in
>    Roland> reserve" in case a new target is added to the fabric
>    Roland> later.
>
>    Roland> I think it would be better to preserve maximum flexibility
>    Roland> for devices and drivers, and not mandate that every
>    Roland> allocated MSI-X vector is always used.
>
>    Zwane> The MSI subsystem should at most reserve and the driver
>    Zwane> make a request.  There may be a limit per PCI device as
>    Zwane> specified by the MSI subsystem for some reason or
>    Zwane> other. Isn't this what we're all saying?
>
>No, Long is actually saying that a driver must actually call
>request_irq() on all the vectors that it is allocated.  I am saying
>that this requirement is too stringent, since there may be devices and
>drivers that cannot predict exactly how many MSI-X vectors they will
>use during driver initialization.

That is what we're all saying.

>    Roland> It seems in the code right now you are able to tell if any
>    Roland> MSI-X vectors are hooked, since you wait for the last
>    Roland> vector to be unhooked to disable MSI-X.  I would just have
>    Roland> it be a WARN_ON() (or maybe BUG_ON()) if a driver calls
>    Roland> pci_disable_msix() without calling free_irq for all its
>    Roland> MSI-X vectors.
>
>    Roland> Right now there is an issue if a driver is unloaded
>    Roland> without freeing all its IRQs -- the device will be left in
>    Roland> MSI-X mode and can not be recovered without rebooting.
>
>    Zwane> This sounds like a case of bad driver bug generally the
>    Zwane> kernel would oops when the ISR text gets unloaded. What
>    Zwane> kind of behaviour do you expect here?
>
>Yes, I agree, it is a bad driver bug if the driver is unloaded without
>doing free_irq() on all the vectors it has done request_irq() on.
>However, with Long's API, there is a problem if for example a device
>driver does pci_enable_msix() and is allocated 2 vectors, then
>correctly does request_irq()/free_irq() on one vector and doesn't
>touch the second vector, and then is unloaded.  The device will be
>left with MSI-X enabled and leak its vectors.

It's very convincing. The addition of the pci_disable_msi() and 
pci_disable_msix() functions are what is needed to handle this issue.

Thanks,
Long
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