linux-kernel - Re: [PATCH 3/3] IRQ: Maintain regs pointer globally rather than passing to IRQ handlers

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Date:	Fri, 6 Oct 2006 13:42:11 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Brownell <david-b@...bell.net>
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] IRQ: Maintain regs pointer globally rather than
 passing to IRQ handlers

On Thu, 5 Oct 2006, David Brownell wrote:

> On Tuesday 03 October 2006 11:03 am, Alan Stern wrote:
> 
> > > > Notice another questionable use of hcd->state. 
> > > 
> > > Questionable in what way?  When that code is called to clean up
> > > after driver death, that loop must be ignored ... every pending I/O
> > > can safely be scrubbed.  That's the main point of that particular
> > > HC_IS_RUNNING() test.  In other cases, it's essential not to touch
> > > DMA queue entries that the host controller is still using.
> > 
> > Questionable because changes to hcd->state aren't synchronized with the
> > driver.  In this case it probably doesn't end up making any difference.
> 
> The driver changes hcd->state with its spinlock held ... or it did,
> last time I audited that code.

Yeah, but usbcore changes hcd->state without holding the spinlock.

> > Removing "regs &&" might change other aspects too.  For instance, does 
> > this routine ever get called from a timer routine, where regs would 
> > normally be NULL?  In such situations removing "regs &&" would reverse 
> > the sense of the test.
> 
> As I said in my previous comments:  should not be an issue.  OHCI doesn't
> have timers.  That routine is normally called in_irq(), with the other
> two call sites being cases where the controller is stopped.

(Or suspended?)

Well, I'm not familiar with the code so I accept your statement.

However, consider that even though ohci-hcd may not have timers, usbcore
does have the root-hub status timer, and it calls into ohci-hcd whenever
that timer expires.  Even though root-hub interrupts are now used, there
still are occasions when the driver asks for polling.  More than there
used to be, thanks to my auto-stop changes.

Alan Stern

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