drm/omap: fix i886 work-around

7d267f06 ("drm/omap: work-around for
errata i886") changed how the PLL dividers and multipliers are
calculated. While the new way should work fine for all the PLLs, it
breaks omap5 PLLs. The issues seen are rather odd: seemed that the
output clock rate is half of what we asked. It is unclear what's causing
there issues.

As a work-around this patch adds a "errata_i886" flag, which is set only
for DRA7's PLLs, and the PLL setup is done according to that flag.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>

drivers/gpu/drm/omapdrm/dss/dss.h

@@ -185,6 +185,9 @@ struct dss_pll_hw {
 	bool has_freqsel;
 	bool has_selfreqdco;
 	bool has_refsel;
+
+	/* DRA7 errata i886: use high N & M to avoid jitter */
+	bool errata_i886;
 };
 
 struct dss_pll {

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_min, n_max;
-	int m, m_min, m_max;
+	int n, n_start, n_stop, n_inc;
+	int m, m_start, m_stop, m_inc;
 	unsigned long fint, clkdco;
 	unsigned long pll_hw_max;
 	unsigned long fint_hw_min, fint_hw_max;
@@ -226,22 +226,33 @@ 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_min = max(DIV_ROUND_UP(clkin, fint_hw_max), 1ul);
-	n_max = min((unsigned)(clkin / fint_hw_min), hw->n_max);
+	n_start = max(DIV_ROUND_UP(clkin, fint_hw_max), 1ul);
+	n_stop = min((unsigned)(clkin / fint_hw_min), hw->n_max);
+	n_inc = 1;
+
+	if (hw->errata_i886) {
+		swap(n_start, n_stop);
+		n_inc = -1;
+	}
 
 	pll_max = pll_max ? pll_max : ULONG_MAX;
 
-	/* Try to find high N & M to avoid jitter (DRA7 errata i886) */
-	for (n = n_max; n >= n_min; --n) {
+	for (n = n_start; n != n_stop; n += n_inc) {
 		fint = clkin / n;
 
-		m_min = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
+		m_start = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
 				1ul);
-		m_max = min3((unsigned)(pll_max / fint / 2),
+		m_stop = min3((unsigned)(pll_max / fint / 2),
 				(unsigned)(pll_hw_max / fint / 2),
 				hw->m_max);
+		m_inc = 1;
+
+		if (hw->errata_i886) {
+			swap(m_start, m_stop);
+			m_inc = -1;
+		}
 
-		for (m = m_max; m >= m_min; --m) {
+		for (m = m_start; m != m_stop; m += m_inc) {
 			clkdco = 2 * m * fint;
 
 			if (func(n, m, fint, clkdco, data))

drivers/gpu/drm/omapdrm/dss/video-pll.c

@@ -130,6 +130,8 @@ static const struct dss_pll_hw dss_dra7_video_pll_hw = {
 	.mX_lsb[3] = 5,
 
 	.has_refsel = true,
+
+	.errata_i886 = true,
 };
 
 struct dss_pll *dss_video_pll_init(struct platform_device *pdev, int id,