Messages in this thread | | | From | David Brownell <> | Subject | Re: [PATCH] Bitbanging i2c bus driver using the GPIO API | Date | Fri, 9 Mar 2007 13:17:56 -0800 |
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On Friday 09 March 2007 12:08 pm, Russell King wrote: > On Fri, Mar 09, 2007 at 11:30:12AM -0800, David Brownell wrote: > > On Friday 09 March 2007 10:48 am, Haavard Skinnemoen wrote: > > > This is a very simple bitbanging i2c bus driver utilizing the new > > > arch-neutral GPIO API. Useful for chips that don't have a built-in > > > i2c controller, additional i2c busses, or testing purposes. > > > > That's the right idea! But remember that not all GPIOs support > > reading back the actual value on SCL (it's an OUT pin, so lacking > > multidrive capability the values "should" be what you wrote), so > > getscl() support should depend on a flag in platform data. In > > the same vein, if SCL is an output-only pin, you won't be able > > to change its direction ... but then, I'm not sure why you were > > changing its direction in setscl() rather than just its value. > > That's a more correct I2C implementation. If you read the specs, the > SDA and SCL signals are supposed to be driven by open-collector or > open-drain drivers, such that devices only pull the bus low. Pull-up > resistors pull the signals high when undriven.
Exactly as I had mentioned:
> > I2C has another interesting special case. at91_set_multi_drive() > > would be appropriate (yes?) for ARCH_AT91 to use on SCL, to best > > support both clock stretching and multi-master configurations.
Where "multi-drive" == open-drain. You're saying it should also be used on SDA too ... OK, I was focussing on clock stretching but it applies there too.
> This avoids the possibility of damage caused when one device drives > a signal low and another device tries to drive it high. > > Therefore, the correct I2C GPIO implementation is one where you drive > both SDA and SCL low by using a combination of the data direction > register and the output level register, but avoid driving the output > high.
I see your point -- it does answer my question -- but you seem to have overlooked a highly relevant one of mine. :)
Regardless, those nuances should be captured in comments in that driver: that for GPIOs that don't support open drain output (only push/pull drivers), that it's faked by otherwise-unnecessary tweaking of GPIO direction.
- Dave
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