Hello Eddie, On Wed, May 20, 2015 at 03:59:35PM +0800, Eddie Huang wrote: > On Wed, 2015-05-20 at 09:11 +0200, Uwe Kleine-König wrote: > > On Wed, May 20, 2015 at 10:40:11AM +0800, Eddie Huang wrote: > > > On Mon, 2015-05-18 at 20:43 +0200, Uwe Kleine-König wrote: > > > > On Tue, May 19, 2015 at 12:40:08AM +0800, Eddie Huang wrote: > > > > > +/* calculate i2c port speed */ > > > > It would be nice to summarize the clock frequency settings here. > > > > Something like: > > > > > > > > /* > > > > * The input clock is divided by the value specified in the > > > > * device tree as clock-div. The actual bus speed is then > > > > * derived from this frequency by the following formula: > > > > * .... > > > > > > > > This would make it possible to verify your calculations below. > > > > > > The comment will be: > > > /* > > > * khz: I2C bus clock > > > * hclk: The input clock is divided by the value specified in the > > > * device tree as clock-div > > and which one of the two clocks you're writing about is hclk now? I > > assume the divided one. > > > * div = (sample_cnt + 1) * (step_cnt + 1) > > > * khz = (hclk / 2) / div > > khz for the 2nd time. > > > > > * > > > * The calculation is to get div value that let result of > > > * ((hclk / 2) / div) most approach and less than khz > > > */ > > I imagined something more hardware related. A list of register (or > > register bit fields) that influence the frequency and a formula > > > i2c_freq = parent_clk / clock-div * (...) > > > > (It seems to be a bit more complicated here as there are two registers > > involved that are set differently depending on the target frequency.) > > Yes, hardware is a little complicated. I rewrite the comment: > > /* > * Calculate i2c port speed > * > * Hardware design: > * i2c_bus_freq = parent_clk / (clock-div * 2 * (sample_cnt) * > (step_cnt)) > * clock-div: fixed in hardware, but may be various in different SoCs ... fixed in hardware, but different on different SoCs. > * > * The calculation want to pick the highest bus frequency that is still > * less than or equal to i2c->speed_hz. and the calculation try to get > * sample_cnt and step_cnt to fill in hardware register. "The calculation picks the highest bus frequency that is still less than or equal to i2c->speed_hz." and I'd drop the last sentence. With that in mind I'll reply once more to the original patch. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- 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
