On Friday 12 October 2007 00:23:50 Alan Stern wrote: > On Fri, 12 Oct 2007, Rafael J. Wysocki wrote: > > > On Thursday, 11 October 2007 05:13, Maxim Levitsky wrote: > > > Hi, > > > > > > I have few questions about .suspend()/.resume() driver functions and how best to write them. > > > > > > I have written a support for suspend/resume for saa7134 v4l driver. > > > Now looking at code again and again, I found few problems, and I am seeking your advice how to fix them. > > > > > > First of all the .suspend() function: > > > > > > Looking at various drivers (including v4l ones) it seems that in general the function: > > > > > > 1) tells upper layers that it is suspended > > > 2) saves the state of device > > > (generally there is nothing to save, since the driver maintains a copy of device state in memory) > > > > > > 3) disables the device (including DMA) > > > 4) does usual pci_save_state+pci_set_power_state(pci_dev, pci_choose_state(pci_dev, state)) > > > (I am talking about pci devices of course) > > > > > > But there is one problem that my .suspend() function have together with quite a lot of drivers: > > > It can race with IRQ handler. Suppose the handler is called just before .suspend(), and thus .suspend() literally > > > pulls the hadware from that handler. > > > > > > I was told that I should use synchronize_irq(), and it looks exactly like the solution. > > > > Yes, that seems to be the right solution. > > > > > But I was surprised to see that very few drivers use it in their .suspend() routines. > > > > Frankly, I'm not sure why that is so. > > USB calls it. > > > > Another issue, even bigger happens during the resume: > > > Suppose the IRQ line is shared with some other device and it gets resumed first. > > > And my IRQ handler is called because of the other device. > > > Now my IRQ handler can't determine whenever the IRQ for the device, since the hardware is still powered off. > > > > > > Probably I can fix the following issues doing this: > > > > > > 1) do free_irq() in .suspend(), and request_irq() in .resume > > > this solves both problems since free_irq() calls synchronize_irq() > > > Few drivers do that this way. I would like to know if this is the right way. > > > > AFAICS, it is not and these drivers should probably be modified not to do so. > > > > The problem, as I see it, is that we don't know what state the device will be > > in during the resume (this may be a resume from disk and the device may have > > been initialized by the BIOS and we get it actually generating interrupts). > > > > > 2) Disable the card's IRQ register , then call synchronize_irq(), at that > > > point I can be sure that no IRQ handler is running and will run then set some > > > per device flag say dev->insuspend > > > > > > let IRQ handler check this flag, and bail out if set, so false interrupts are caught on resume > > > Probably not a good idea, since I didn't find such implementation in kernel > > > > I'm not sure of that. Sounds better than freeing the IRQs in .suspend() to me. > > USB uses 2). Actually we set the "insuspend" flag after disabling IRQ > generation but before calling synchronize_irq() -- presumably this > doesn't lead to any problems. > > The problem of devices initialized by the BIOS and generating unwanted > interrupts is handled by a PCI quirk routine. The device's IRQ-enable > flag is turned off early on in the boot kernel. > > Alan Stern > > Thank you very much. I will now implement (2) knowing that this is the right way. Thanks again, Best regards, Maxim Levitsky _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
