Allwinner clocks fixes for 5.1

Some fixes for odd cases of the NKMP clocks.
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Merge tag 'clk-fixes-for-5.1' of https://git.kernel.org/pub/scm/linux/kernel/git/sunxi/linux into clk-fixes

Pull Allwinner clk fixes from Maxime Ripard:

 - Some fixes for odd cases of the NKMP clocks

* tag 'clk-fixes-for-5.1' of https://git.kernel.org/pub/scm/linux/kernel/git/sunxi/linux:
  clk: sunxi-ng: nkmp: Explain why zero width check is needed
  clk: sunxi-ng: nkmp: Avoid GENMASK(-1, 0)

drivers/clk/sunxi-ng/ccu_nkmp.c

@@ -167,7 +167,7 @@ static int ccu_nkmp_set_rate(struct clk_hw *hw, unsigned long rate,
 			   unsigned long parent_rate)
 {
 	struct ccu_nkmp *nkmp = hw_to_ccu_nkmp(hw);
-	u32 n_mask, k_mask, m_mask, p_mask;
+	u32 n_mask = 0, k_mask = 0, m_mask = 0, p_mask = 0;
 	struct _ccu_nkmp _nkmp;
 	unsigned long flags;
 	u32 reg;
@@ -186,10 +186,24 @@ static int ccu_nkmp_set_rate(struct clk_hw *hw, unsigned long rate,
 
 	ccu_nkmp_find_best(parent_rate, rate, &_nkmp);
 
-	n_mask = GENMASK(nkmp->n.width + nkmp->n.shift - 1, nkmp->n.shift);
-	k_mask = GENMASK(nkmp->k.width + nkmp->k.shift - 1, nkmp->k.shift);
-	m_mask = GENMASK(nkmp->m.width + nkmp->m.shift - 1, nkmp->m.shift);
-	p_mask = GENMASK(nkmp->p.width + nkmp->p.shift - 1, nkmp->p.shift);
+	/*
+	 * If width is 0, GENMASK() macro may not generate expected mask (0)
+	 * as it falls under undefined behaviour by C standard due to shifts
+	 * which are equal or greater than width of left operand. This can
+	 * be easily avoided by explicitly checking if width is 0.
+	 */
+	if (nkmp->n.width)
+		n_mask = GENMASK(nkmp->n.width + nkmp->n.shift - 1,
+				 nkmp->n.shift);
+	if (nkmp->k.width)
+		k_mask = GENMASK(nkmp->k.width + nkmp->k.shift - 1,
+				 nkmp->k.shift);
+	if (nkmp->m.width)
+		m_mask = GENMASK(nkmp->m.width + nkmp->m.shift - 1,
+				 nkmp->m.shift);
+	if (nkmp->p.width)
+		p_mask = GENMASK(nkmp->p.width + nkmp->p.shift - 1,
+				 nkmp->p.shift);
 
 	spin_lock_irqsave(nkmp->common.lock, flags);