[PATCH net v5 3/5] dpll: zl3073x: reject output frequencies with too small divisor

[Ivan Vecera <ivecera@redhat.com>]

The output divisor has to be at least 2: a divisor of 1 bypasses the
divider and the hardware then ignores the pulse width and eSync
registers. For the N-pin of an N-divided output the N divider (>= 2)
applies on top, so its effective divisor has to be at least 4.

zl3073x_pin_check_freq() only checked that the frequency divides the
synth frequency. Require also the minimum divisor. For N-divided
outputs this is necessary but not sufficient, the rest is checked by
the frequency set callback.

Hit during development: an N-pin frequency that needs an N divider of 1
was accepted, and with esync_n_period = 1 the N-pin produced no output
on an oscilloscope.

Fixes: a99a9f0ebdaa ("dpll: zl3073x: Read DPLL types and pin properties from system firmware")
Reviewed-by: Petr Oros <poros@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
---
 drivers/dpll/zl3073x/prop.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/drivers/dpll/zl3073x/prop.c b/drivers/dpll/zl3073x/prop.c
index 768e5424de2d..4e006ed5950a 100644
--- a/drivers/dpll/zl3073x/prop.c
+++ b/drivers/dpll/zl3073x/prop.c
@@ -22,7 +22,8 @@
  * The function checks the given frequency is valid for the device. For input
  * pins it checks that the frequency can be factorized using supported base
  * frequencies. For output pins it checks that the frequency divides connected
- * synth frequency without remainder.
+ * synth frequency without remainder and that the resulting divisor is not
+ * below the minimum the driver supports.
  *
  * Return: true if the frequency is valid, false if not.
  */
@@ -41,18 +42,38 @@ zl3073x_pin_check_freq(struct zl3073x_dev *zldev, enum dpll_pin_direction dir,
 		if (rc)
 			goto err_inv_freq;
 	} else {
-		u32 synth_freq;
-		u8 out, synth;
+		u32 synth_freq, div, min_div, rem;
+		const struct zl3073x_out *out;
+		u8 out_id, synth;
 
 		/* Get output pin synthesizer */
-		out = zl3073x_output_pin_out_get(id);
-		synth = zl3073x_dev_out_synth_get(zldev, out);
+		out_id = zl3073x_output_pin_out_get(id);
+		synth = zl3073x_dev_out_synth_get(zldev, out_id);
 
 		/* Get synth frequency */
 		synth_freq = zl3073x_dev_synth_freq_get(zldev, synth);
 
 		/* Check the frequency divides synth frequency */
-		if (synth_freq % (u32)freq)
+		div = div_u64_rem(synth_freq, freq, &rem);
+		if (rem)
+			goto err_inv_freq;
+
+		/* The driver requires an output divisor of at least 2 -
+		 * below that the hardware bypasses the divider and ignores
+		 * the pulse width and eSync registers the driver programs.
+		 * For the N-pin of an N-divided output the effective divisor
+		 * also includes the N divider (>= 2), so the minimum is 4.
+		 * For N-divided outputs this is only a necessary condition -
+		 * whether the frequency can be set also depends on the output
+		 * divisor shared by both pins, which is checked when the
+		 * frequency is set.
+		 */
+		out = zl3073x_out_state_get(zldev, out_id);
+		if (zl3073x_out_is_ndiv(out) && zl3073x_is_n_pin(id))
+			min_div = 4;
+		else
+			min_div = 2;
+		if (div < min_div)
 			goto err_inv_freq;
 	}
 
-- 
2.56.0