Re: Use of in_atomic in i2c_transfer (Was: use of preempt_count instead of in_atomic() at leds-gpio.c)

[Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>]

[thanks for not copying the ARM lists - this mail would've triggered
the nasty mailman CC/To header mangling bug, resulting in folk being
bounced off the list.]

On Fri, Mar 21, 2008 at 04:42:35PM +0100, Jean Delvare wrote:
> On Thu, 20 Mar 2008 18:08:02 -0700, Andrew Morton wrote:
> > On Thu, 20 Mar 2008 21:36:04 -0300 Henrique de Moraes Holschuh <hmh-N3TV7GIv+o9fyO9Q7EP/yw@public.gmane.org> wrote:
> > 
> > > Well, so far so good for LEDs, but what about the other users of in_atomic
> > > that apparently should not be doing it either?
> > 
> > Ho hum. (...)
> >
> > ./drivers/i2c/i2c-core.c
> > 
> >   Possibly buggy: deadlockable
> > 
> > 
> > The usual pattern for most of the above is
> > 
> > 	if (!in_atomic())
> > 		do_something_which_might_sleep();
> > 
> > problem is, in_atomic() returns false inside spinlock on non-preptible
> > kernels.  So if anyone calls those functions inside spinlock they will
> > incorrectly schedule and another task can then come in and try take the
> > already-held lock.
> > 
> > Now, it happens that in_atomic() returns true on non-preemtible kernels
> > when running in interrupt or softirq context.  But if the above code really
> > is using in_atomic() to detect am-i-called-from-interrupt and NOT
> > am-i-called-from-inside-spinlock, they should be using in_irq(),
> > in_softirq() or in_interrupt().
> 
> Mike, Russell: Andrew says that we shouldn't use in_atmomic() in
> i2c_transfer(). This means that the approach I suggested to implement
> polled-mode to i2c bus drivers, and on which i2c-pxa relies, is not
> correct, sorry about that. We will have to come up with something else,
> but I am not sure what.
> 
> Andrew says that the caller should know in advance whether sleeping is
> allowed or not, so we should presumably have a different i2c_transfer
> function for cases where the caller can't sleep. In our case my feeling
> is that the bus driver knows whether it can sleep or not (at least
> that's how it is implemented in i2c-pxa right now) but the caller does
> not, so I don't know on what basis a different i2c_transfer would be
> called.
> 
> Maybe we can move the use_pio flag from pxa-private structure to the
> i2c_adapter structure, so that i2c_transfer can access it? Originally I
> wanted to avoid making this structure bigger but now it seems we don't
> really have the choice.

My original suggestion was that transfers should either be always PIO
or never PIO per I2C bus.

Yes, it means a performance hit on those systems which place a PMIC on
the same bus as other peripherals, but that's a design mistake from the
start.

What I'm in favour of obtaining is _something_ that can work to make
what is not currently possible possible, rather than something that's
100% perfect for everyone.  If we need to later go back and change it
in some way, then so be it, but that's no bad thing.  What is bad is
preventing folk from doing something because a perfect solution hasn't
been found.

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