Hi David, On Tue, Oct 07, 2014 at 12:14:20PM -0700, David E. Box wrote: > Hi Maxime, > > On Thu, Sep 25, 2014 at 11:47:52AM +0200, Maxime Ripard wrote: > > Hi David, > > > > On Tue, Sep 23, 2014 at 12:58:54PM -0700, David E. Box wrote: > > > Hi Maxime, > > > > > > On Tue, Sep 23, 2014 at 09:00:57PM +0200, Maxime Ripard wrote: > > > > Hi David, > > > > > > > > On Tue, Sep 23, 2014 at 11:40:26AM -0700, David E. Box wrote: > > > > > This patch implements an I2C bus sharing mechanism between the host and platform > > > > > hardware on select Intel BayTrail SoC platforms using the X-Powers AXP288 PMIC. > > > > > > > > > > On these platforms access to the PMIC must be shared with platform hardware. The > > > > > hardware unit assumes full control of the I2C bus and the host must request > > > > > access through a special semaphore. Hardware control of the bus also makes it > > > > > necessary to disable runtime pm to avoid interfering with hardware transactions. > > > > > > > > > > Signed-off-by: David E. Box <david.e.box@xxxxxxxxxxxxxxx> > > > > > > > > Sorry for stepping in like this without really knowing your platform, > > > > but wouldn't using the hwspinlock framework make more sense than > > > > hardcoding your own internal functions here? > > > > > > I looked into this but didn't see a clear way on our platform to identify the > > > semaphore seperately from doing it in the designware platform driver. The way > > > we can find it now is through evaluating an ACPI _SEM object on every i2c device > > > that gets probed by the dw driver since at probe time we can get the acpi handle. > > > > And you have no way to turn it around and identify which semaphore is > > associated to which i2c bus? > > > > If so, there is probably some way to associate a given instance of the > > i2c driver to one semaphore. > > > > > Without this handle however there isn't a clear way of evaluating the _SEM > > > object which would be needed to register a hwspinlock in separate code. > > > > > > Plus it would still require changes to the designware i2c core, though admittedly > > > having a generic hwspinlock pointer added to the struct is cleaner. > > > > Not only cleaner, but that could also be used by other platforms that > > are using this I2C driver (and since it's a designware IP, there must > > be quite a lot) together with hardware locking. > > > > After again considering a way to make this work I don't think this api can fit > well with our platform. Acquisition of this semaphore is through a mailbox > sequence where we set one register and then poll another for a value that > confirms we have the lock. For best performance we need to be able to > periodically sleep while waiting for that confirmation. This time can vary > widely as it's dependent on the component we are requesting the semaphore from > which is itself a user of that bus. > > While we could simply fail after a short time, reattempts would still need > to happen in the i2c-designware driver and the timing would be completely > dependent on our hardware, making it less clean for reuse. In addition, > if we timed out, we would have to immediately call unlock to cancel the > mailbox transaction. This may not fit well with reuse either. Ok, if Wolfram is ok with it, and if it makes your life much easier, I'm ok :) Thanks! Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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