On Thu, 26 Sep 2024 13:33:20 +0200, Sascha Hauer wrote:
> Adoption of Linux commit:
>
> | commit c5a8045a553e32529ffb6bfb33fcad4d38aec2c7
> | Author: Emil Lundmark <emil@xxxxxxxxxxxxxx>
> | Date: Wed Oct 12 12:31:41 2016 +0200
> |
> | clk: imx: improve precision of AV PLL to 1 Hz
> |
> | The audio and video PLLs are designed to have a precision of 1 Hz if some
> | conditions are met. The current implementation only allows a precision that
> | depends on the rate of the parent clock. E.g., if the parent clock is 24
> | MHz, the precision will be 24 Hz; or more generally the precision will be
> |
> | p / 10^6 Hz
> |
> | where p is the parent clock rate. This comes down to how the register
> | values for the PLL's fractional loop divider are chosen.
> |
> | The clock rate calculation for the PLL is
> |
> | PLL output frequency = Fref * (DIV_SELECT + NUM / DENOM)
> |
> | or with a shorter notation
> |
> | r = p * (d + a / b)
> |
> | In addition to all variables being integers, we also have the following
> | conditions:
> |
> | 27 <= d <= 54
> |
> | -2^29 <= a <= 2^29-1
> | 0 < b <= 2^30-1
> | |a| < b
> |
> | Here, d, a and b are register values for the fractional loop divider. We
> | want to chose d, a and b such that f(p, r) = p, i.e. f is our round_rate
> | function. Currently, d and b are chosen as
> |
> | d = r / p
> | b = 10^6
> |
> | hence we get the poor precision. And a is defined in terms of r, d, p and
> | b:
> |
> | a = (r - d * p) * b / p
> |
> | I propose that if p <= 2^30-1 (i.e., the max value for b), we chose b as
> |
> | b = p
> |
> | We can do this since
> |
> | |a| < b
> |
> | |(r - d * p) * b / p| < b
> |
> | |r - d * p| < p
> |
> | Which have two solutions, one of them is when p < 0, so we can skip that
> | one. The other is when p > 0 and
> |
> | p * (d - 1) < r < p * (d + 1)
> |
> | Substitute d = r / p:
> |
> | (r - p) < r < (r + p) <=> p > 0
> |
> | So, as long as p > 0, we can chose b = p. This is a good choise for b since
> |
> | a = (r - d * p) * b / p
> | = (r - d * p) * p / p
> | = r - d * p
> |
> | r = p * (d + a / b)
> | = p * d + p * a / b
> | = p * d + p * a / p
> | = p * d + a
> |
> | and if d = r / p:
> |
> | a = r - d * p
> | = r - r / p * p
> | = 0
> |
> | r = p * d + a
> | = p * d + 0
> | = p * r / p
> | = r
> |
> | I reckon this is the intention by the design of the clock rate formula.
> |
> | Signed-off-by: Emil Lundmark <emil@xxxxxxxxxxxxxx>
> | Reviewed-by: Fabio Estevam <fabio.estevam@xxxxxxx>
> | Acked-by: Shawn Guo <shawnguo@xxxxxxxxxx>
> | Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>
> [...]
Applied, thanks!
[1/1] clk: imx: improve precision of AV PLL to 1 Hz
https://git.pengutronix.de/cgit/barebox/commit/?id=af3d99396a8b (link may not be stable)
Best regards,
--
Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
