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 --- drivers/gpu/drm/omapdrm/dss/pll.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/drivers/gpu/drm/omapdrm/dss/pll.c b/drivers/gpu/drm/omapdrm/dss/pll.c index 0a76c89cdc2e..5e221302768b 100644 --- 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))