On Tue, Mar 29, 2011 at 10:58:54AM +1100, Benjamin Herrenschmidt wrote: > On Mon, 2011-03-28 at 15:34 -0700, Sarah Sharp wrote: > > > > What I'd like to do is take out the read of the status register out of > > the interrupt handler (which is killing performance), and make it only > > check the status register when xhci_handle_event() returns a negative > > error status. If the status register shows the host controller has a > > critical error, the driver should call usb_hcd_died(). > > Be careful with removing that read... > > Without MSIs, that read is what guarantees that all pending DMA writes > by the xHCI have been "flushed" before you start poking at memory. > > IE. If the chip writes an event and sends an LSI, without that read, you > might get the interrupt before the writes to memory have completed and > your driver will "miss" the event. > > With MSIs (provided they are not broken on your PCI host bridge of > course, this is typically the #1 cause of MSI breakage), you don't need > that as the MSI itself is a DMA store by the device which is ordered > after the stores done to update the event. So by the time you get the > MSI interrupt, you -should- have all the updates visible in memory. > > But that means that your PCI host bridge is doing the right thing, by > ensuring whatever queues to the coherency domain it has have been > properly flushed before it signals the interrupts caused by the MSI to > the processors. Hopefully most systems get that right nowadays. > > Point is: you need to keep that read if MSIs aren't enabled. Sorry for the sloppy language, yes, I understand I still need the status register read if only legacy IRQs are enabled. Sarah Sharp -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
