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authorEmil Lundmark <emil@limesaudio.com>2016-10-12 12:31:41 +0200
committerStephen Boyd <sboyd@codeaurora.org>2016-11-01 17:12:50 -0700
commitc5a8045a553e32529ffb6bfb33fcad4d38aec2c7 (patch)
treeb031b6a3b282dbbd930da586a059804b751aab00 /scripts/checksyscalls.sh
parent0f1e2f891a8eb25513add7411be4f0960846146f (diff)
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@limesaudio.com> Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com> Acked-by: Shawn Guo <shawnguo@kernel.org> Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
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