On Wed, 14 Nov 2007, David Brownell wrote:
> > The protection of the chip list can be converted to a mutex and
> > does not need to be a spinlock at all.
>
> No, we still need to use a spinlock to protect table changes.
> The reason for that is briefly:
>
> - gpio_request()/gpio_free() have so far been optional. Most
> platforms implement them as NOPs, not all drivers use them.
> (Having gpiolib in place should help change that ...)
By magically doing the request of the pin ? See below.
> - gpio_direction_input()/gpio_direction_output() implicitly
> request the pins, if they weren't already requested.
Eek, that's completely wrong. Allowing to access a resource _before_
it is assigned and then doing the assignment implicit is a really bad
idea.
> - Those input/output direction-setting calls may be called
> in IRQ contexts, which means (on non-RT kernels) no mutex.
There is no reason to do that if you actually have a useful reference
to the chip _before_ accessing the pin.
> So we're actually in good shape; just take out a bit of code
> (or turn it into debugging instrumentation) and I don't think
> anyone will complain about the locking any more.
This still does not solve the lookup, which is done for each operation
on a pin (direction setting, read, write).
tglx
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