[alsa-devel] [PATCH] ASoC: tas2552: Fix fraction overflow in PLL calculation
Setting the PLL involves the calculation of a fixed point ratio with 4 decimal digits fraction, referred to as "J.D". The fraction "D" is stored separately from the integer part "J" and is limited to 0..9999.
The current algorithm uses integer registers to calculate the fraction part, but failed to compensate for rounding errors, resulting in values larger than 9999 for the fraction part occasionally, e.g. for 44.1kHz audio rate and pll_clkin = 3763400 it would set J to 11 and D to 10002, which will at best result in wrong pitch.
The critical part is the "pll_clkin / 10000", which would be ok with real numbers, but using integer arithmetic the rounding decreases the divisor, thus increasing the final quotient.
The issue is solved by linear interpolation over the reciprocal function between the two adjacent points with integer divisor, i.e. pll_clkin / 10000 and pll_clkin / 10000 + 1, and doing all rounding to the lower result.
As a side effect to the bug fix, the approximation to the desired frequency is much better, for the above mentioned example we get 11.9993, while the true ratio is 11.9993623.
Signed-off-by: Oskar Schirmer oskar@scara.com --- sound/soc/codecs/tas2552.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/sound/soc/codecs/tas2552.c b/sound/soc/codecs/tas2552.c index 8840f72..1011616 100644 --- a/sound/soc/codecs/tas2552.c +++ b/sound/soc/codecs/tas2552.c @@ -192,7 +192,7 @@ static int tas2552_setup_pll(struct snd_soc_codec *codec, * pll_clk = (.5 * pll_clkin * J.D) / 2^p * Need to fill in J and D here based on incoming freq */ - unsigned int d; + unsigned int d, q, t; u8 j; u8 pll_sel = (tas2552->pll_clk_id << 3) & TAS2552_PLL_SRC_MASK; u8 p = snd_soc_read(codec, TAS2552_PLL_CTRL_1); @@ -200,9 +200,12 @@ static int tas2552_setup_pll(struct snd_soc_codec *codec, p = (p >> 7);
recalc: - j = (pll_clk * 2 * (1 << p)) / pll_clkin; - d = (pll_clk * 2 * (1 << p)) % pll_clkin; - d /= (pll_clkin / 10000); + t = (pll_clk * 2) << p; + j = t / pll_clkin; + d = t % pll_clkin; + t = pll_clkin / 10000; + q = d / (t + 1); + d = q + ((9999 - pll_clkin % 10000) * (d / t - q)) / 10000;
if (d && (pll_clkin < 512000 || pll_clkin > 9200000)) { if (tas2552->pll_clk_id == TAS2552_PLL_CLKIN_BCLK) {
The patch
ASoC: tas2552: Fix fraction overflow in PLL calculation
has been applied to the asoc tree at
git://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
All being well this means that it will be integrated into the linux-next tree (usually sometime in the next 24 hours) and sent to Linus during the next merge window (or sooner if it is a bug fix), however if problems are discovered then the patch may be dropped or reverted.
You may get further e-mails resulting from automated or manual testing and review of the tree, please engage with people reporting problems and send followup patches addressing any issues that are reported if needed.
If any updates are required or you are submitting further changes they should be sent as incremental updates against current git, existing patches will not be replaced.
Please add any relevant lists and maintainers to the CCs when replying to this mail.
Thanks, Mark
From 1bb7cb68feb73a8106546cbd3379590297b0a950 Mon Sep 17 00:00:00 2001
From: Oskar Schirmer oskar@scara.com Date: Fri, 25 Aug 2017 21:36:56 +0000 Subject: [PATCH] ASoC: tas2552: Fix fraction overflow in PLL calculation
Setting the PLL involves the calculation of a fixed point ratio with 4 decimal digits fraction, referred to as "J.D". The fraction "D" is stored separately from the integer part "J" and is limited to 0..9999.
The current algorithm uses integer registers to calculate the fraction part, but failed to compensate for rounding errors, resulting in values larger than 9999 for the fraction part occasionally, e.g. for 44.1kHz audio rate and pll_clkin = 3763400 it would set J to 11 and D to 10002, which will at best result in wrong pitch.
The critical part is the "pll_clkin / 10000", which would be ok with real numbers, but using integer arithmetic the rounding decreases the divisor, thus increasing the final quotient.
The issue is solved by linear interpolation over the reciprocal function between the two adjacent points with integer divisor, i.e. pll_clkin / 10000 and pll_clkin / 10000 + 1, and doing all rounding to the lower result.
As a side effect to the bug fix, the approximation to the desired frequency is much better, for the above mentioned example we get 11.9993, while the true ratio is 11.9993623.
Signed-off-by: Oskar Schirmer oskar@scara.com Signed-off-by: Mark Brown broonie@kernel.org --- sound/soc/codecs/tas2552.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/sound/soc/codecs/tas2552.c b/sound/soc/codecs/tas2552.c index 8840f72f3c4a..10116169a239 100644 --- a/sound/soc/codecs/tas2552.c +++ b/sound/soc/codecs/tas2552.c @@ -192,7 +192,7 @@ static int tas2552_setup_pll(struct snd_soc_codec *codec, * pll_clk = (.5 * pll_clkin * J.D) / 2^p * Need to fill in J and D here based on incoming freq */ - unsigned int d; + unsigned int d, q, t; u8 j; u8 pll_sel = (tas2552->pll_clk_id << 3) & TAS2552_PLL_SRC_MASK; u8 p = snd_soc_read(codec, TAS2552_PLL_CTRL_1); @@ -200,9 +200,12 @@ static int tas2552_setup_pll(struct snd_soc_codec *codec, p = (p >> 7);
recalc: - j = (pll_clk * 2 * (1 << p)) / pll_clkin; - d = (pll_clk * 2 * (1 << p)) % pll_clkin; - d /= (pll_clkin / 10000); + t = (pll_clk * 2) << p; + j = t / pll_clkin; + d = t % pll_clkin; + t = pll_clkin / 10000; + q = d / (t + 1); + d = q + ((9999 - pll_clkin % 10000) * (d / t - q)) / 10000;
if (d && (pll_clkin < 512000 || pll_clkin > 9200000)) { if (tas2552->pll_clk_id == TAS2552_PLL_CLKIN_BCLK) {
participants (2)
-
Mark Brown -
Oskar Schirmer