On Mon, Sep 02, 2013 at 04:43:01PM +0300, Xenia Ragiadakou wrote: > Hi Sarah and Alan, > > I am writing a patch for allocating as many msi-x vectors as the > number of event rings because now the msi-x vectors allocated are > more than are actually used, since currently xhci-hcd implements > only one event ring. > > xhci_setup_msix() limits the number of supported msi-x vectors based > on the number of cpus in the host system, and that confuses me: > > xhci->msix_count = min(num_online_cpus() + 1, > HCS_MAX_INTRS(xhci->hcs_params1)); > > Can you explain me why it does so?
I'm not the original author of that patch, so I'm not sure why they did that. I asked them to explain why in a comment above the code: http://marc.info/?l=linux-usb&m=127351043723713&w=2 The comment that they placed above the line didn't explain *why* they needed they wanted to allocate an MSI-X vector per CPU, just what they were doing: http://marc.info/?l=linux-usb&m=127365906500373&w=2 (And now you understand why kernel developers ask for a lot of documentation and comments. People disappear after submitting their code all the time.) I suspect the original author assumed that only one MSI-X vector could be tied to a CPU? If that's the case, I don't know why they allocated an extra vector. If you're going to allocate more MSI-X vectors than you have CPUs available, it probably makes sense to allocate as many vectors as you have event rings. > Also, another thing that bothers me is that, in case > pci_enable_msix() returns a positive number which > indicates the number of msi-x vectors supported based on the MSI-X > Capability structure of the PCI Configuration Address Space (this > value is returned when the requested number of msi-x vectors are > greater than the supported number), shall i free the msix_entries > and reallocate them for the new number > or reimplement this check before pci_enable_msix() is called and > take it into account when allocate msix_entries for first time? I think you should re-implement the check, so that you allocate the minimum of: - the number of MSI-X vectors the PCI device will support, - the number of interrupters the xHCI host will support, and - the number of event rings as the driver has allocated. Sarah Sharp -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
