On Fri, Aug 15, 2014 at 11:46:49PM +0200, Stephen Warren wrote: > On 08/15/2014 03:34 PM, Peter De Schrijver wrote: > > On Fri, Aug 15, 2014 at 09:45:46PM +0200, Peter De Schrijver wrote: > >> 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? > >> > > > > http://www.nxp.com/documents/user_manual/UM10204.pdf says you can mix them by > > using an interconnect bridge between the highspeed and the non-highspeed > > capable slaves. The bridge uses the special preamble to disconnect the non- > > highspeed part of the bus when a highspeed transaction is ongoing. It's afaics > > transparent to the master. > > I expect that works by echoing the slow-speed pre-amble to the > slow-speed bus segment, then emitting a stop and turning off the echo. > For actual slow-speed transactions, the whole thing would be echo'd. > That way the slow-speed devices don't ever see any high-speed pulses. > Indeed. > That all said, that does indeed imply that a master supporting the > high-speed transactions would need to emit a varying-speed signal. My > assumption would be that this happens inside the I2C HW, rather than > under SW control though, since the transition would need to happen > mid-protocol. Still, perhaps the selection between low-speed and > high-speed-with-a-slow-preamble might need SW clock programming > depending on the HW though... Who knows. > That's true if the master wants to do a high-speed transaction. If the master wants to do a normal-speed transaction to a slave on the same bus, the master will need to select a lower speed clock under software control I think. Cheers, Peter. -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
