On Wed, 16 Jan 2013, Stephen Warren wrote: > On 01/16/2013 08:08 AM, Alan Stern wrote: > > On Wed, 16 Jan 2013, Venu Byravarasu wrote: > > > >> As Tegra PHY driver needs to access one of the Host registers, > >> added few APIs. > > >> --- a/drivers/usb/host/ehci-tegra.c > >> +++ b/drivers/usb/host/ehci-tegra.c > > >> +void tegra_ehci_set_wakeon_events(struct usb_phy *x, bool enable) > >> +{ > >> + unsigned long val; > >> + struct usb_hcd *hcd = bus_to_hcd(x->otg->host); > >> + void __iomem *base = hcd->regs; > >> + u32 wake = USB_PORTSC1_WKOC | USB_PORTSC1_WKDS | USB_PORTSC1_WKCN; > >> + > >> + val = readl(base + USB_PORTSC1); > >> + if (enable) > >> + val |= wake; > >> + else > >> + val &= ~wake; > >> + writel(val, base + USB_PORTSC1); > >> +} > > > > Here and below, this sort of code is highly questionable. You > > evidently don't realize that some of the bits in the PORTSC registers > > are R/WC. This means writing a 1 to these bits will clear them. > > > > Consequently it is almost always wrong to read a PORTSC register and > > then write back the same (or a slightly modified) value. > > Sorry I'm not familiar with USB... Are the bits being manipulated here > (i.e. USB_PORTSC1_WKOC | USB_PORTSC1_WKDS | USB_PORTSC1_WKCN) > standardized USBisms, or some custom Tegra stuff? The _symbols_ are not standard (i.e., not used in the regular ehci-hcd driver) but the _values_ are: compare with PORT_WKOC_E etc. in include/linux/usb/ehci_defs.h. I'm not sure about the register addresses, however. > Anyway, is the solution here to do: > > val = readl(addr) > > // i.e. add the following line: > val &= ~(all write-to-clear bits); > > if (enable) val |= wake; else val &= ~wake; > > writel(val, addr) > > ... or is there more broken than just that? Nope, that's the right way to do it. There are loads of examples in drivers/usb/host/ehci-hub.c; look at usages of PORT_RWC_BITS. > Also note that the driver is already doing exactly what is in these new > functions; the code is just being split out so that only the EHCI driver > touches EHCI registers, and the PHY driver only touches PHY registers. > Still, I'll admit it's a good time to fix any mistakes in this part of > the code. Agreed. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html