Hello Linus, On Wed Oct 9, 2024 at 1:34 PM CEST, Linus Walleij wrote: > On Wed, Oct 9, 2024 at 12:23 PM Théo Lebrun <theo.lebrun@xxxxxxxxxxx> wrote: > > --- a/drivers/i2c/busses/i2c-nomadik.c > > +++ b/drivers/i2c/busses/i2c-nomadik.c > > @@ -454,9 +454,12 @@ static void setup_i2c_controller(struct nmk_i2c_dev *priv) > > * operation, and the other is for std, fast mode, fast mode > > * plus operation. Currently we do not supprt high speed mode > > * so set brcr1 to 0. > > + * > > + * BRCR is a clock divider amount. Pick highest value that > > + * leads to rate strictly below target. > > */ > > You could push in some more details from the commit message here so it's not > so terse. Most of the details from the commit message come from behavior changes: what was done previously versus what is the new behavior we implement. Having a clock divider picking the bus rate that is below the target speed rather than above sounds rather intuitive. Eg when you ask for 400kHz you want <=400kHz, not >=400kHz. I'll add that last sentence "Eg when you ask for 400kHz you want a bus rate <=400kHz (and not >=400kHz)". It is straight forward and easy to understand. > > brcr1 = FIELD_PREP(I2C_BRCR_BRCNT1, 0); > > - brcr2 = FIELD_PREP(I2C_BRCR_BRCNT2, i2c_clk / (priv->clk_freq * div)); > > + brcr2 = FIELD_PREP(I2C_BRCR_BRCNT2, i2c_clk / (priv->clk_freq * div) + 1); > > Doesn't the last part correspond to something like > #include <linux/math.h> > u64 scaler = DIV_ROUND_DOWN_ULL(i2c_clk, (priv->clk_freq * div)); > brcr2 = FIELD_PREP(I2C_BRCR_BRCNT2, (u32)scaler); > > Certianly one of the in-kernel division helpers like DIV_ROUND_DOWN > round_up() etc are better to use IMO, but I might not be understanding the > fine details of the math here. Indeed what we want is: DIV_ROUND_DOWN(i2c_clk, priv->clk_freq * div) I see no reason to use DIV_ROUND_DOWN_ULL(). It would be useful if i2c_clk + (priv->clk_freq * div) had a chance to overflow. Worst case is: 3_400_000 + (48_000_000 * 3) = 147_400_000 Will send v3 straight away as this is a significant change, thanks Linus! -- Théo Lebrun, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
