On Tue, Sep 11, 2012 at 9:06 PM, Anton Vorontsov <anton.vorontsov@xxxxxxxxxx> wrote: > On Tue, Sep 11, 2012 at 08:42:46PM -0700, Colin Cross wrote: > [...] >> > The "problem" is in the last step. If we exit NMI without making UART >> > know that we're done with the interrupt, we will reenter the NMI >> > immediately, even without any new characters from the UART. >> >> The UART irq line should go low when you read the character out of the > > Probably some controllers may lower the line by themselves, but not > all, and probably most of them need an explicit clear. Anything 8250-based will clear the interrupt automatically, assuming you read the status registers as well as the character register. >> receive buffer, or the polling rx function should clear the interrupt >> for you. > > Yes, that's an option. But that way we add a new semantic for the > polling routines, and effecitvely we just merge the two callbacks. > > Of course, if Alan is OK with this, I'm more than OK too. :-) > > (But the polling routines would need to clear all interrupts, not > just rx/tx. For example, if the controller indicated some error, and > nobody clears it, then we'll start reentering infinitely.) For exynos5, the only non-8250 based serial port I've come across, we clear all interrupts in the rx poll function (see https://android.googlesource.com/kernel/exynos/+/ef427aafffb7153dde59745e440fd7ec41ea969d/arch/arm/mach-exynos/exynos_fiq_debugger.c). >> If you use a clear_irqs callback, you can drop characters if >> one arrives between the last character buffer read and calling >> clear_irqs. > > Only if we call clear_irqs() after reading the characters, but we do > it before. So if new characters are available, we will reenter NMI, > which is OK. > > But if used incorrectly, it truly can cause dropping (or staling) of > characters, so I'd better add some comments about this. What does clear_irqs() mean for a status or tx interrupt? The tx interrupt will generally re-assert as long as the tx fifo is empty, which would require disabling it. On 8250 ports, status interrupts will re-assert until the corresponding status register is read. -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
