[RFC] tty/serial/kgdboc: Add and wire up clear_irqs callback

[Colin Cross]

On Tue, Sep 11, 2012 at 9:06 PM, Anton Vorontsov
<anton.vorontsov at linaro.org> 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.