[01/36] drm/omap: work-around for errata i886

Commit Message

Tomi Valkeinen Nov. 30, 2016, 11:17 a.m. UTC

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 <[e-mail]>
---
 drivers/gpu/drm/omapdrm/dss/pll.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Patch

diff --git a/drivers/gpu/drm/omapdrm/dss/pll.c b/drivers/gpu/drm/omapdrm/dss/pll.c
index 0a76c89cdc2e..65c478d85caa 100644
--- a/drivers/gpu/drm/omapdrm/dss/pll.c
+++ b/drivers/gpu/drm/omapdrm/dss/pll.c
@@ -231,7 +231,7 @@  bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
 
 	pll_max = pll_max ? pll_max : ULONG_MAX;
 
-	for (n = n_start; n <= n_stop; ++n) {
+	for (n = n_stop; n >= n_start; --n) {
 		fint = clkin / n;
 
 		m_start = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
@@ -240,7 +240,7 @@  bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
 				(unsigned)(pll_hw_max / fint / 2),
 				hw->m_max);
 
-		for (m = m_start; m <= m_stop; ++m) {
+		for (m = m_stop; m >= m_start; --m) {
 			clkdco = 2 * m * fint;
 
 			if (func(n, m, fint, clkdco, data))