drm/omap: work-around for errata i886

DRA7 errata i886 (FPDLink PLL Unlocks With Certain SoC PLL M/N Values)
says that FPDLink is sensitive to jitter on the vout clock, and that low
PLL M and N values result in more jitter than high M and N values.

This patch implements a workaround for the problem by changing the PLL
setup to search for clocks starting from high M and N values, instead of
low values. This should not cause any functional change, and only
reduces the jitter.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

diff --git a/drivers/gpu/drm/omapdrm/dss/pll.c b/drivers/gpu/drm/omapdrm/dss/pll.c
index 0a76c89..5e22130 100644 (file)
--- a/drivers/gpu/drm/omapdrm/dss/pll.c
+++ b/drivers/gpu/drm/omapdrm/dss/pll.c
@@ -215,8 +215,8 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
                dss_pll_calc_func func, void *data)
 {
        const struct dss_pll_hw *hw = pll->hw;
-       int n, n_start, n_stop;
-       int m, m_start, m_stop;
+       int n, n_min, n_max;
+       int m, m_min, m_max;
        unsigned long fint, clkdco;
        unsigned long pll_hw_max;
        unsigned long fint_hw_min, fint_hw_max;
@@ -226,21 +226,22 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
        fint_hw_min = hw->fint_min;
        fint_hw_max = hw->fint_max;
 
-       n_start = max(DIV_ROUND_UP(clkin, fint_hw_max), 1ul);
-       n_stop = min((unsigned)(clkin / fint_hw_min), hw->n_max);
+       n_min = max(DIV_ROUND_UP(clkin, fint_hw_max), 1ul);
+       n_max = min((unsigned)(clkin / fint_hw_min), hw->n_max);
 
        pll_max = pll_max ? pll_max : ULONG_MAX;
 
-       for (n = n_start; n <= n_stop; ++n) {
+       /* Try to find high N & M to avoid jitter (DRA7 errata i886) */
+       for (n = n_max; n >= n_min; --n) {
                fint = clkin / n;
 
-               m_start = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
+               m_min = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
                                1ul);
-               m_stop = min3((unsigned)(pll_max / fint / 2),
+               m_max = min3((unsigned)(pll_max / fint / 2),
                                (unsigned)(pll_hw_max / fint / 2),
                                hw->m_max);
 
-               for (m = m_start; m <= m_stop; ++m) {
+               for (m = m_max; m >= m_min; --m) {
                        clkdco = 2 * m * fint;
 
                        if (func(n, m, fint, clkdco, data))