Messages in this thread | | | Date | Fri, 15 Apr 2022 10:52:13 +0200 | Subject | Re: [PATCH v2] clk: bcm2835: Round UART input clock up | From | Stefan Wahren <> |
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Hi Ivan,
Am 14.04.22 um 12:56 schrieb Ivan T. Ivanov: > Hi Stefan, > > Please, could you take a look into following patch? yes, but i cannot give a technical review. But from my gut feeling this doesn't look really elegant to me. > > Thanks! > Ivan > > On 04-04 15:51, Ivan T. Ivanov wrote: >> Subject: [PATCH v2] clk: bcm2835: Round UART input clock up >> Message-Id: <20220404125113.80239-1-iivanov@suse.de> >> >> The UART clock is initialised to be as close to the requested >> frequency as possible without exceeding it. Now that there is a >> clock manager that returns the actual frequencies, an expected >> 48MHz clock is reported as 47999625. If the requested baudrate >> == requested clock/16, there is no headroom and the slight >> reduction in actual clock rate results in failure. >> >> If increasing a clock by less than 0.1% changes it from ..999.. >> to ..000.., round it up.
Based on this commit message this looks like a fix / workaround for an issue. It would be very helpful to know:
What issue should be fixed?
Why is it fixed here and not in the UART driver for instance?
In case it fixes a regression, a Fixes tag should be necessary.
In best case this is explained in the commit message.
Best regards
>> >> This is reworked version of a downstream fix: >> https://github.com/raspberrypi/linux/commit/ab3f1b39537f6d3825b8873006fbe2fc5ff057b7 >> >> Cc: Phil Elwell <phil@raspberrypi.org> >> Signed-off-by: Ivan T. Ivanov <iivanov@suse.de> >> --- >> Changes since v1 >> Make bcm2835_clock_round() static to fix following warning >> when compiling for riscv: >> drivers/clk/bcm/clk-bcm2835.c:997:15: warning: no previous prototype for 'bcm2835_clock_round' [-Wmissing-prototypes] >> Reported-by: kernel test robot <lkp@intel.com> >> >> drivers/clk/bcm/clk-bcm2835.c | 30 ++++++++++++++++++++++++++++-- >> 1 file changed, 28 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c >> index 3ad20e75fd23..c29b643d1bf5 100644 >> --- a/drivers/clk/bcm/clk-bcm2835.c >> +++ b/drivers/clk/bcm/clk-bcm2835.c >> @@ -502,6 +502,8 @@ struct bcm2835_clock_data { >> bool low_jitter; >> >> u32 tcnt_mux; >> + >> + bool round_up; >> }; >> >> struct bcm2835_gate_data { >> @@ -992,12 +994,30 @@ static long bcm2835_clock_rate_from_divisor(struct bcm2835_clock *clock, >> return temp; >> } >> >> +static unsigned long bcm2835_clock_round(unsigned long clk) >> +{ >> + unsigned long scaler; >> + >> + /* >> + * If increasing a clock by less than 0.1% changes it >> + * from ..999.. to ..000.., round up. >> + */ >> + scaler = 1; >> + while (scaler * 100000 < clk) >> + scaler *= 10; >> + if ((clk + scaler - 1) / scaler % 1000 == 0) >> + clk = (clk / scaler + 1) * scaler; >> + >> + return clk; >> +} >> + >> static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw, >> unsigned long parent_rate) >> { >> struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw); >> struct bcm2835_cprman *cprman = clock->cprman; >> const struct bcm2835_clock_data *data = clock->data; >> + unsigned long rate; >> u32 div; >> >> if (data->int_bits == 0 && data->frac_bits == 0) >> @@ -1005,7 +1025,12 @@ static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw, >> >> div = cprman_read(cprman, data->div_reg); >> >> - return bcm2835_clock_rate_from_divisor(clock, parent_rate, div); >> + rate = bcm2835_clock_rate_from_divisor(clock, parent_rate, div); >> + >> + if (data->round_up) >> + rate = bcm2835_clock_round(rate); >> + >> + return rate; >> } >> >> static void bcm2835_clock_wait_busy(struct bcm2835_clock *clock) >> @@ -2142,7 +2167,8 @@ static const struct bcm2835_clk_desc clk_desc_array[] = { >> .div_reg = CM_UARTDIV, >> .int_bits = 10, >> .frac_bits = 12, >> - .tcnt_mux = 28), >> + .tcnt_mux = 28, >> + .round_up = true), >> >> /* TV encoder clock. Only operating frequency is 108Mhz. */ >> [BCM2835_CLOCK_VEC] = REGISTER_PER_CLK( >> -- >> 2.26.2
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