[PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface

[Michael Ellerman]

On Fri, Sep 20, 2013 at 07:26:03AM -0500, Tejun Heo wrote:
> Hello,
> 
> On Wed, Sep 18, 2013 at 06:50:45PM +0200, Alexander Gordeev wrote:
> > Actually, I do not see much contradiction with what I proposed. The
> > key words here "determine the number of MSIs the controller wants".
> > 
> > In general case it is not what pci_msix_table_size() returns (or at
> > least we should not limit ourselves to it) - there could be non-
> > standard means to report number of MSIs: hardcoded, version-dependant,
> > device-specific registers etc.
> > 
> > Next, if we opt to determine the number of MSIs by non-MSI standard
> > means then there is no reason not to call pci_get_msix_limit() (or
> > whatever) at this step.
> 
> Yeah, that's all fine.  My point is that we shouldn't try to use
> "degraded" multiple MSI mode where the number of MSIs allocated is
> smaller than performing full multiple MSI operation.  How that number
> is determined doesn't really matter but that number is a property
> which is solely decided by the device driver, right?  If a device
> needs full multiple MSI mode, given specific configuration, it needs
> >= X number of MSIs and that's the number it should request.

Sure, the driver is in full control. If it can ONLY work with N MSIs
then it should try for N, else fallback to 1.

But some drivers are able to work with a range of values for N, and
performance is improved vs using a single MSI.

> > Being Captain Obvious here, but it is up to the device driver to handle
> > a failure. There could be no such option as single MSI mode after all :)
> 
> I don't think there actually is a mainstream device which can't
> fallback to single interrupt.  Anyways, the point is the same, let's
> please not try to create an interface which encourages complex retry
> logic in its users which are likely to involve less traveled and
> tested paths in both the driver and firmware.

Why support > 1 MSI at all? It just adds complex logic and less travelled
paths in the driver and firmware.

cheers