drm/omap: pll: fix iteration loop check

If the PLL calc function is given bad parameters, n_start/m_start may be
higher than n_stop/m_stop, which leads to the loops iterating through
the whole u32 number space.

Fix this by failing early on such cases.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-59-tomi.valkeinen@ti.com

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

@@ -222,6 +222,9 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
 	n_stop = min((unsigned)(clkin / fint_hw_min), hw->n_max);
 	n_inc = 1;
 
+	if (n_start > n_stop)
+		return false;
+
 	if (hw->errata_i886) {
 		swap(n_start, n_stop);
 		n_inc = -1;
@@ -239,6 +242,9 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
 				hw->m_max);
 		m_inc = 1;
 
+		if (m_start > m_stop)
+			continue;
+
 		if (hw->errata_i886) {
 			swap(m_start, m_stop);
 			m_inc = -1;