[PATCH] clk: at91: clk-generated: Limit the requested rate to our range

On clk_generated_determine_rate(), the requested rate could be outside
of clk's range. Limit the rate to the clock's range to not return an
error.

Fixes: df70aeef6083 ("clk: at91: add generated clock driver")
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
---
 drivers/clk/at91/clk-generated.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/clk/at91/clk-generated.c b/drivers/clk/at91/clk-generated.c
index b4fc8d71daf2..b656d25a9767 100644
--- a/drivers/clk/at91/clk-generated.c
+++ b/drivers/clk/at91/clk-generated.c

@@ -128,6 +128,12 @@ static int clk_generated_determine_rate(struct clk_hw *hw,
 	int i;
 	u32 div;
 
+	/* do not look for a rate that is outside of our range */
+	if (gck->range.max && req->rate > gck->range.max)
+		req->rate = gck->range.max;
+	if (gck->range.min && req->rate < gck->range.min)
+		req->rate = gck->range.min;
+
 	for (i = 0; i < clk_hw_get_num_parents(hw); i++) {
 		if (gck->chg_pid == i)
 			continue;

-- 
2.30.2


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On 07/07/2021 at 15:12, Codrin Ciubotariu wrote:

On clk_generated_determine_rate(), the requested rate could be outside
of clk's range. Limit the rate to the clock's range to not return an
error.

Isn't it saner for the user to return an error code instead of 
automatically restrain the dynamics requested without notice?

Can you elaborate the use case where returning an error is not convenient?

Regards,
   Nicolas

-- 
Nicolas Ferre

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On 09.07.2021 12:21, Nicolas Ferre wrote:

On 07/07/2021 at 15:12, Codrin Ciubotariu wrote:

quoted

On clk_generated_determine_rate(), the requested rate could be outside
of clk's range. Limit the rate to the clock's range to not return an
error.

Isn't it saner for the user to return an error code instead of 
automatically restrain the dynamics requested without notice?

Can you elaborate the use case where returning an error is not convenient?

The way I see it, if the user requests a rate that is out of clock's 
range, the driver's determine_rate() should return min/max, not an 
error. That is actually the closest rate supported by the clock, which 
is what determine_rate() should accomplish. The user has no clk API to 
get clock's range, so there is no way to call clk_round_rate() only for 
values within our range.

The use cause is with sam9x60's I2S driver, which has to try different 
rates to get the closest one to what it needs. There is no 'perfect' 
rate, because there is no AUDIO PLL and we have to try different values 
for our internal dividers to find the closest one.

https://elixir.bootlin.com/linux/latest/source/sound/soc/atmel/mchp-i2s-mcc.c#L416

Best regards,
Codrin

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On 09/07/2021 at 11:59, Codrin Ciubotariu - M19940 wrote:

On 09.07.2021 12:21, Nicolas Ferre wrote:

quoted

On 07/07/2021 at 15:12, Codrin Ciubotariu wrote:

quoted

On clk_generated_determine_rate(), the requested rate could be outside
of clk's range. Limit the rate to the clock's range to not return an
error.

Isn't it saner for the user to return an error code instead of
automatically restrain the dynamics requested without notice?

Can you elaborate the use case where returning an error is not convenient?

The way I see it, if the user requests a rate that is out of clock's
range, the driver's determine_rate() should return min/max, not an
error. That is actually the closest rate supported by the clock, which
is what determine_rate() should accomplish. The user has no clk API to
get clock's range, so there is no way to call clk_round_rate() only for
values within our range.

The use cause is with sam9x60's I2S driver, which has to try different
rates to get the closest one to what it needs. There is no 'perfect'
rate, because there is no AUDIO PLL and we have to try different values
for our internal dividers to find the closest one.

https://elixir.bootlin.com/linux/latest/source/sound/soc/atmel/mchp-i2s-mcc.c#L416

Sure, makes sense:

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>

Regards,
   Nicolas

-- 
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Quoting Codrin Ciubotariu (2021-07-07 06:12:13)

On clk_generated_determine_rate(), the requested rate could be outside
of clk's range. Limit the rate to the clock's range to not return an
error.

Fixes: df70aeef6083 ("clk: at91: add generated clock driver")
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
---

Applied to clk-next

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