aptina-pll.c

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/*
 * Aptina Sensor PLL Configuration
 *
 * Copyright (C) 2012 Laurent Pinchart <[email protected]>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA
 */

#include <linux/device.h>
#include <linux/gcd.h>
#include <linux/kernel.h>
#include <linux/lcm.h>
#include <linux/module.h>

#include "aptina-pll.h"

int aptina_pll_calculate(struct device *dev,
             const struct aptina_pll_limits *limits,
             struct aptina_pll *pll)
{
    unsigned int mf_min;
    unsigned int mf_max;
    unsigned int p1_min;
    unsigned int p1_max;
    unsigned int p1;
    unsigned int div;

    dev_dbg(dev, "PLL: ext clock %u pix clock %u\n",
        pll->ext_clock, pll->pix_clock);

    if (pll->ext_clock < limits->ext_clock_min ||
        pll->ext_clock > limits->ext_clock_max) {
        dev_err(dev, "pll: invalid external clock frequency.\n");
        return -EINVAL;
    }

    if (pll->pix_clock == 0 || pll->pix_clock > limits->pix_clock_max) {
        dev_err(dev, "pll: invalid pixel clock frequency.\n");
        return -EINVAL;
    }

    /* Compute the multiplier M and combined N*P1 divisor. */
    div = gcd(pll->pix_clock, pll->ext_clock);
    pll->m = pll->pix_clock / div;
    div = pll->ext_clock / div;

    /* We now have the smallest M and N*P1 values that will result in the
     * desired pixel clock frequency, but they might be out of the valid
     * range. Compute the factor by which we should multiply them given the
     * following constraints:
     *
     * - minimum/maximum multiplier
     * - minimum/maximum multiplier output clock frequency assuming the
     *   minimum/maximum N value
     * - minimum/maximum combined N*P1 divisor
     */
    mf_min = DIV_ROUND_UP(limits->m_min, pll->m);
    mf_min = max(mf_min, limits->out_clock_min /
             (pll->ext_clock / limits->n_min * pll->m));
    mf_min = max(mf_min, limits->n_min * limits->p1_min / div);
    mf_max = limits->m_max / pll->m;
    mf_max = min(mf_max, limits->out_clock_max /
            (pll->ext_clock / limits->n_max * pll->m));
    mf_max = min(mf_max, DIV_ROUND_UP(limits->n_max * limits->p1_max, div));

    dev_dbg(dev, "pll: mf min %u max %u\n", mf_min, mf_max);
    if (mf_min > mf_max) {
        dev_err(dev, "pll: no valid combined N*P1 divisor.\n");
        return -EINVAL;
    }

    /*
     * We're looking for the highest acceptable P1 value for which a
     * multiplier factor MF exists that fulfills the following conditions:
     *
     * 1. p1 is in the [p1_min, p1_max] range given by the limits and is
     *    even
     * 2. mf is in the [mf_min, mf_max] range computed above
     * 3. div * mf is a multiple of p1, in order to compute
     *  n = div * mf / p1
     *  m = pll->m * mf
     * 4. the internal clock frequency, given by ext_clock / n, is in the
     *    [int_clock_min, int_clock_max] range given by the limits
     * 5. the output clock frequency, given by ext_clock / n * m, is in the
     *    [out_clock_min, out_clock_max] range given by the limits
     *
     * The first naive approach is to iterate over all p1 values acceptable
     * according to (1) and all mf values acceptable according to (2), and
     * stop at the first combination that fulfills (3), (4) and (5). This
     * has a O(n^2) complexity.
     *
     * Instead of iterating over all mf values in the [mf_min, mf_max] range
     * we can compute the mf increment between two acceptable values
     * according to (3) with
     *
     *  mf_inc = p1 / gcd(div, p1)          (6)
     *
     * and round the minimum up to the nearest multiple of mf_inc. This will
     * restrict the number of mf values to be checked.
     *
     * Furthermore, conditions (4) and (5) only restrict the range of
     * acceptable p1 and mf values by modifying the minimum and maximum
     * limits. (5) can be expressed as
     *
     *  ext_clock / (div * mf / p1) * m * mf >= out_clock_min
     *  ext_clock / (div * mf / p1) * m * mf <= out_clock_max
     *
     * or
     *
     *  p1 >= out_clock_min * div / (ext_clock * m) (7)
     *  p1 <= out_clock_max * div / (ext_clock * m)
     *
     * Similarly, (4) can be expressed as
     *
     *  mf >= ext_clock * p1 / (int_clock_max * div)    (8)
     *  mf <= ext_clock * p1 / (int_clock_min * div)
     *
     * We can thus iterate over the restricted p1 range defined by the
     * combination of (1) and (7), and then compute the restricted mf range
     * defined by the combination of (2), (6) and (8). If the resulting mf
     * range is not empty, any value in the mf range is acceptable. We thus
     * select the mf lwoer bound and the corresponding p1 value.
     */
    if (limits->p1_min == 0) {
        dev_err(dev, "pll: P1 minimum value must be >0.\n");
        return -EINVAL;
    }

    p1_min = max(limits->p1_min, DIV_ROUND_UP(limits->out_clock_min * div,
             pll->ext_clock * pll->m));
    p1_max = min(limits->p1_max, limits->out_clock_max * div /
             (pll->ext_clock * pll->m));

    for (p1 = p1_max & ~1; p1 >= p1_min; p1 -= 2) {
        unsigned int mf_inc = p1 / gcd(div, p1);
        unsigned int mf_high;
        unsigned int mf_low;

        mf_low = roundup(max(mf_min, DIV_ROUND_UP(pll->ext_clock * p1,
                    limits->int_clock_max * div)), mf_inc);
        mf_high = min(mf_max, pll->ext_clock * p1 /
                  (limits->int_clock_min * div));

        if (mf_low > mf_high)
            continue;

        pll->n = div * mf_low / p1;
        pll->m *= mf_low;
        pll->p1 = p1;
        dev_dbg(dev, "PLL: N %u M %u P1 %u\n", pll->n, pll->m, pll->p1);
        return 0;
    }

    dev_err(dev, "pll: no valid N and P1 divisors found.\n");
    return -EINVAL;
}
EXPORT_SYMBOL_GPL(aptina_pll_calculate);

MODULE_DESCRIPTION("Aptina PLL Helpers");
MODULE_AUTHOR("Laurent Pinchart <[email protected]>");
MODULE_LICENSE("GPL v2");