On Tue, 24 Aug 2021 20:34:38 +0100, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > On Tue, Aug 24, 2021 at 10:46:59AM +1200, Barry Song wrote: > > On Mon, Aug 23, 2021 at 11:28 PM Marc Zyngier <maz@xxxxxxxxxx> wrote: > > > > > > On Mon, 23 Aug 2021 12:03:08 +0100, > > > Barry Song <21cnbao@xxxxxxxxx> wrote: > > > +static ssize_t irq_show(struct device *dev, > > + struct device_attribute *attr, > > + char *buf) > > +{ > > + struct pci_dev *pdev = to_pci_dev(dev); > > +#ifdef CONFIG_PCI_MSI > > + struct msi_desc *desc = first_pci_msi_entry(pdev); > > + > > + /* for MSI, return the 1st IRQ in IRQ vector */ > > + if (desc && !desc->msi_attrib.is_msix) > > + return sysfs_emit(buf, "%u\n", desc->irq); > > +#endif > > + > > + return sysfs_emit(buf, "%u\n", pdev->irq); > > +} > > +static DEVICE_ATTR_RO(irq); > > Makes sense to me. And with Marc's patch maybe we could get rid of > default_irq, which also seems nice. > > > > > if we don't want to change the behaviour of any existing ABI, it > > > > seems the only thing we can do here to document it well in ABI > > > > doc. i actually doubt anyone has really understood what the irq > > > > entry is really showing. > > > > > > Given that we can't prove that it is actually the case, I believe this > > > is the only option. > > > > we have to document the ABI like below though it seems quite annoying. > > > > 1. for devices which don't support MSI and MSI-X, show legacy INTx > > 2. for devices which support MSI > > a. if CONFIG_PCI_MSI is not enabled, show legacy INTx > > b. if CONFIG_PCI_MSI is enabled and devices are using MSI at this > > moment, show 1st IRQ in the vector > > c. if CONFIG_PCI_MSI is enabled, but we shutdown its MSI before > > the users call sysfs entry, > > so at this moment, devices are not using MSI, show legacy INTx > > 3. for devices which support MSI-X, no matter if it is using MSI-X, > > show legacy INTx > > 4. In Addition, INTx might be broken due to incomplete firmware or > > hardware design for MSI and MSI-X cases > > > > To be honest, it sounds like a disaster :-) but if this is what we > > have to do, I'd like to try it in v3. > > It doesn't seem necessary to me to get into the gory details of > CONFIG_PCI_MSI -- if that's not enabled, drivers can't use MSI anyway. > > I don't understand 3. If a device supports both MSI and MSI-X and a > driver enables MSI, msi_capability_init() writes dev->irq, so it looks > like "irq" should contain the first MSI vector. > > I don't understand 4, either. Is the possibility of broken hardware > or firmware something we need to document? > > What about something like this? > > If a driver has enabled MSI (not MSI-X), "irq" contains the IRQ of > the first MSI vector. Otherwise "irq" contains the IRQ of the > legacy INTx interrupt. > I think that pretty much nails it. CONFIG_MSI is not something that userspace can (nor should) discover anyway. For (4), you may want to add that "irq" being set to 0 indicates that the device isn't capable of generating legacy INTx interrupts. Thanks, M. -- Without deviation from the norm, progress is not possible.