linux-kernel - Re: [RFC] tty/serial/kgdboc: Add and wire up clear

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Date:	Wed, 12 Sep 2012 03:32:05 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	Colin Cross <ccross@...roid.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Jason Wessel <jason.wessel@...driver.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Brian Swetland <swetland@...gle.com>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	kernel-team@...roid.com, kgdb-bugreport@...ts.sourceforge.net,
	linux-serial@...r.kernel.org
Subject: Re: [RFC] tty/serial/kgdboc: Add and wire up clear_irqs callback

On Wed, Sep 12, 2012 at 10:44:20AM +0100, Alan Cox wrote:
> > Of course, if Alan is OK with this, I'm more than OK too. :-)
> 
> It may well be better.
> 
> > (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 a lot of devices and platforms you'd probably mask them instead ?

If there is no way to clear them, yes, we obviously would want to
mask them before using the port for NMI debugger. Then we'd need
three callbacks:

- mask_all_irqs_but_rx() -- used before we want to start using the port
  for the NMI debugger;
- clear_rx_irq() -- (optional) clears rx IRQ for controllers that need
  it;
- restore_irqs() -- unmasks interrupts that were previously masked.

If we ever encounter a case when just clearing interrupts doesn't work,
we can surely implement the above scheme... It's just so far I don't
see any need to over-design this, but again, it's your call, I told my
opinion on this, but I'll do whatever you guys like more. :-)

> > > 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.
> 
> Recursively or not... again you get platform specific magic in places
> we don't want.

I really really don't see how this is platform-specific. All we ask the
serial driver is to quiesce its interrupt. Whether we can handle
NMIs/IRQs recursively or not is not serial driver's worry, since its
IRQ handler is not going to fire anyway. The polling routines already
gave us the power to steal/inject the data, so now we're stealing the
interrupt too.

How we use the callback is indeed platform-specific, but so is the
whole KGDB, and that knowledge is hidden there.

For serial driver it's all pretty much clear: lower the interrupt, but
don't turn off rx detection.

Thanks!

Anton.
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