On Thu, May 07, 2009 at 06:19:53PM +0800, Sheng Yang wrote: > On Thursday 07 May 2009 17:53:02 Matthew Wilcox wrote: > > On Thu, May 07, 2009 at 05:40:15PM +0800, Sheng Yang wrote: > > > It's indeed weird. Why the semantic of pci_enable_msix can be changed to > > > "enable msix, or tell me how many vector do you have"? You can simply > > > call pci_msix_table_size() to get what you want, also without any more > > > work, no? I can't understand... > > > > Here's a good example. Let's suppose you have a driver which supports > > two different models of cards, one has 16 MSI-X interrupts, the other > > has 10. You can call pci_enable_msix() asking for 16 vectors. If your > > card is model A, you get 16 interrupts. If your card is model B, it says > > "you can have 10". > > > > This is less work in the driver (since it must implement falling back to > > a smaller number of interrupts *anyway*) than interrogating the card to > > find out how many interrupts there are, then requesting the right number, > > and still having the fallback path which is going to be less tested. > > Yeah, partly understand now. > > But the confusing of return value is not that pleasure compared to this > benefit. And even you have to fall back if return > 0 anyway, but in the past, > you just need fall back once at most; but now you may fall back twice. I don't think that's right - you might not be able to get the number of interrupts that pci_enable_msix reported. > This > make thing more complex - you need either two ifs or a simple loop. And just > one "if" can deal with it before. All that required is one call for > pci_msix_table_size(), and I believe most driver would like to know how much > vector it have before it fill the vectors, so mostly no extra cost. But for > this ambiguous return meaning, you have to add more code for fall back - yes, > the driver may can assert that the positive return value always would be irq > numbers if it call pci_msix_table_size() before, but is it safe in logic? If you know how many vectors the card has, then the only failure mode is when you are out of irqs. No change there. -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
