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


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