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Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection

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  Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection Eric Anholt <eric@anholt.net> - 2015-11-18 19:40 +0100
    Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection Remi Pommarel <repk@triplefau.lt> - 2015-11-18 20:30 +0100

#1272464 — Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection

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Remi Pommarel <repk@triplefau.lt> writes:

> Some bcm2835 clocks used by hardware (like "PWM" or "H264") can have multiple
> parent clocks. These clocks divide the rate of a parent which can be selected by
> setting the proper bits in the clock control register.
>
> Previously all these parents where handled by a mux clock. But a mux clock
> cannot be used because updating clock control register to select parent needs a
> password to be xor'd with the parent index.
>
> This patch get rid of mux clock and make these clocks handle their own parent,
> allowing them to select the one to use.
>
> Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> ---
>  drivers/clk/bcm/clk-bcm2835.c | 122 ++++++++++++++++++++++++++----------------
>  1 file changed, 77 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index 1237716..2b01a53 100644
> --- a/drivers/clk/bcm/clk-bcm2835.c
> +++ b/drivers/clk/bcm/clk-bcm2835.c
> @@ -1198,16 +1198,6 @@ static long bcm2835_clock_rate_from_divisor(struct bcm2835_clock *clock,
>  	return temp;
>  }
>  
> -static long bcm2835_clock_round_rate(struct clk_hw *hw,
> -				     unsigned long rate,
> -				     unsigned long *parent_rate)
> -{
> -	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> -	u32 div = bcm2835_clock_choose_div(hw, rate, *parent_rate);
> -
> -	return bcm2835_clock_rate_from_divisor(clock, *parent_rate, div);
> -}
> -
>  static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw,
>  					    unsigned long parent_rate)
>  {
> @@ -1279,13 +1269,75 @@ static int bcm2835_clock_set_rate(struct clk_hw *hw,
>  	return 0;
>  }
>  
> +static int bcm2835_clock_determine_rate(struct clk_hw *hw,
> +		struct clk_rate_request *req)
> +{
> +	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> +	struct clk_hw *parent, *best_parent = NULL;
> +	struct clk_rate_request parent_req;
> +	unsigned long rate, best_rate = 0;
> +	unsigned long prate, best_prate = 0;
> +	size_t i;
> +	u32 div;
> +
> +	/*
> +	 * Select parent clock that results in the closest but lower rate
> +	 */
> +	for (i = 0; i < clk_hw_get_num_parents(hw); ++i) {
> +		parent = clk_hw_get_parent_by_index(hw, i);
> +		if (!parent)
> +			continue;
> +		parent_req = *req;

parent_req appears dead, so it should be removed.

> +		prate = clk_hw_get_rate(parent);
> +		div = bcm2835_clock_choose_div(hw, req->rate, prate);
> +		rate = bcm2835_clock_rate_from_divisor(clock, prate, div);
> +		if (rate > best_rate && rate <= req->rate) {
> +			best_parent = parent;
> +			best_prate = prate;
> +			best_rate = rate;
> +		}
> +	}
> +
> +	if (!best_parent)
> +		return -EINVAL;
> +
> +	req->best_parent_hw = best_parent;
> +	req->best_parent_rate = best_prate;

I think you're supposed to req->rate = best_rate, here, too.  With these
two fixes,

Reviewed-by: Eric Anholt <eric@anholt.net>

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#1272493

On Wed, Nov 18, 2015 at 10:30:17AM -0800, Eric Anholt wrote:

[...]

> > +static int bcm2835_clock_determine_rate(struct clk_hw *hw,
> > +		struct clk_rate_request *req)
> > +{
> > +	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> > +	struct clk_hw *parent, *best_parent = NULL;
> > +	struct clk_rate_request parent_req;
> > +	unsigned long rate, best_rate = 0;
> > +	unsigned long prate, best_prate = 0;
> > +	size_t i;
> > +	u32 div;
> > +
> > +	/*
> > +	 * Select parent clock that results in the closest but lower rate
> > +	 */
> > +	for (i = 0; i < clk_hw_get_num_parents(hw); ++i) {
> > +		parent = clk_hw_get_parent_by_index(hw, i);
> > +		if (!parent)
> > +			continue;
> > +		parent_req = *req;
> 
> parent_req appears dead, so it should be removed.

Yes, will do thanks.

> > +		prate = clk_hw_get_rate(parent);
> > +		div = bcm2835_clock_choose_div(hw, req->rate, prate);
> > +		rate = bcm2835_clock_rate_from_divisor(clock, prate, div);
> > +		if (rate > best_rate && rate <= req->rate) {
> > +			best_parent = parent;
> > +			best_prate = prate;
> > +			best_rate = rate;
> > +		}
> > +	}
> > +
> > +	if (!best_parent)
> > +		return -EINVAL;
> > +
> > +	req->best_parent_hw = best_parent;
> > +	req->best_parent_rate = best_prate;
> 
> I think you're supposed to req->rate = best_rate, here, too.  With these
> two fixes,

I did not set req->rate to best_rate in order to avoid rounding down
twice the actual clock rate.

Indeed with patch 1 from this patchset bcm2835_clock_choose_div()
chooses a divisor that produces a rate lower or equal to the requested
one. As we call bcm2835_clock_choose_div() twice when using
clk_set_rate() (once with ->determine_rate() and once with ->set_rate()),
if I set req->rate in bcm2835_clock_determine_rate to the rounded down
one, the final rate will likely be again rounded down in
bcm2835_clock_set_rate().

Thanks,

-- 
Rémi Pommarel
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