[PATCH v2] clk: meson: pll: Fix by 0 division in __pll_params_to_rate()

[Remi Pommarel <repk@triplefau.lt>]

Some meson pll registers can be initialized with 0 as N value, introducing
the following division by 0 when computing rate :

  UBSAN: Undefined behaviour in drivers/clk/meson/clk-pll.c:75:9
  division by zero
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc3-608075-g86c9af8630e1-dirty #400
  Call trace:
   dump_backtrace+0x0/0x1c0
   show_stack+0x14/0x20
   dump_stack+0xc4/0x100
   ubsan_epilogue+0x14/0x68
   __ubsan_handle_divrem_overflow+0x98/0xb8
   __pll_params_to_rate+0xdc/0x140
   meson_clk_pll_recalc_rate+0x278/0x3a0
   __clk_register+0x7c8/0xbb0
   devm_clk_hw_register+0x54/0xc0
   meson_eeclkc_probe+0xf4/0x1a0
   platform_drv_probe+0x54/0xd8
   really_probe+0x16c/0x438
   driver_probe_device+0xb0/0xf0
   device_driver_attach+0x94/0xa0
   __driver_attach+0x70/0x108
   bus_for_each_dev+0xd8/0x128
   driver_attach+0x30/0x40
   bus_add_driver+0x1b0/0x2d8
   driver_register+0xbc/0x1d0
   __platform_driver_register+0x78/0x88
   axg_driver_init+0x18/0x20
   do_one_initcall+0xc8/0x24c
   kernel_init_freeable+0x2b0/0x344
   kernel_init+0x10/0x128
   ret_from_fork+0x10/0x18

This checks if N is null before doing the division.

Fixes: 7a29a869434e ("clk: meson: Add support for Meson clock controller")
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
---
Changes since v1:
  - Change Fix tag
  - Move null test to .recalc_rate()
---
 drivers/clk/meson/clk-pll.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/clk/meson/clk-pll.c b/drivers/clk/meson/clk-pll.c
index ddb1e5634739..4d3a8003ca20 100644
--- a/drivers/clk/meson/clk-pll.c
+++ b/drivers/clk/meson/clk-pll.c
@@ -77,6 +77,10 @@ static unsigned long meson_clk_pll_recalc_rate(struct clk_hw *hw,
 	unsigned int m, n, frac;
 
 	n = meson_parm_read(clk->map, &pll->n);
+	/* Some hw may have n set to 0 at init, avoid div by 0 in that case */
+	if (n == 0)
+		return 0;
+
 	m = meson_parm_read(clk->map, &pll->m);
 
 	frac = MESON_PARM_APPLICABLE(&pll->frac) ?
-- 
2.24.0