On Fri, Aug 15, 2014 at 08:07:01PM +0200, Stephen Warren wrote: > >> However, the new code sets the clock rate after the clock is prepared. I > >> think the rate should be set first, then the clock prepared. While this > >> likely doesn't apply to the Tegra clock controller, prepare() is allowed > >> to enable the clock if enable() can't be implemented in an atomic > >> fashion (in which case enable/disable would be no-ops), and we should > >> make sure that the driver correctly configures the clock before > >> potentially enabling it. > >> > >> I'm not sure if a similar change to our SPI drivers is possible; after > >> all, the SPI transfer rate can vary per message, so if clk_set_rate() > >> acquires a lock, it seems there's no way to avoid the issue there. > > > > Even for i2c this could be the case I think if you use the highspeed > > (3.4Mhz) > > mode? From what I remember, a highspeed i2c transaction starts with a lower > > speed preamble to make sure non highspeed slaves don't get confused? Which > > means you could change the bus speed depending on the slave you're > > addressing. > > Since there's no separate chip-select for I2C, I believe all I2C devices > need to be able to understand the entire transaction, so the I2C bus > speed is fixed. >
Does it? I would assume the slave only needs to check if the address matches its own address after a START condition and if not can just wait until the STOP condition appears on the bus? > At least, that's my understanding between 100KHz and 400KHz I2C. I don't > know if 3.4MHz I2C introduced something new, although considering that > slower I2C never had anything about being compatible with fast stuff in > the spec AFAIK, and such speed-switching would only be useful for > backwards-compatibility, I don't see how that would work. > Looking at http://www.i2c-bus.org/highspeed/ they at least claim some form of backwards compatibility ('High-speed IC devices are downward compatible allowing for mixed bus systems. ') > >> Luckily, we don't have any SPI-based chips that do anything related to > >> clocks on any of our current boards... > > > > And we don't use SPI to talk to the PMIC, which is the usecase were actually > > run into problems with the locking. > > IIRC, the I2C-based clock provider (or consumer?) issue was something > mentioned (later on?) in the email thread linked by the patch description. Yes, that's another usecase, but we don't have that on Tegra. I was refering to Tegra usecases here. Cheers, Peter. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
