[PATCH V4] clk: at91: PLL recalc_rate() now using cached MUL and DIV values

[Boris Brezillon]

On Sun, 29 Apr 2018 15:01:11 -0400
Marcin Ziemianowicz <marcin at ziemianowicz.com> wrote:

> When a USB device is connected to the USB host port on the SAM9N12 then
> you get "-62" error which seems to indicate USB replies from the device
> are timing out. Based on a logic sniffer, I saw the USB bus was running
> at half speed.
> 
> The PLL code uses cached MUL and DIV values which get set in set_rate()
> and applied in prepare(), but the recalc_rate() function instead
> queries the hardware instead of using these cached values. Therefore,
> if recalc_rate() is called between a set_rate() and prepare(), the
> wrong frequency is calculated and later the USB clock divider for the
> SAM9N12 SOC will be configured for an incorrect clock.
> 
> In my case, the PLL hardware was set to 96 Mhz before the OHCI
> driver loads, and therefore the usb clock divider was being set
> to /2 even though the OHCI driver set the PLL to 48 Mhz.
> 
> As an alternative explanation, I noticed this was fixed in the past by
> 87e2ed338f1b ("clk: at91: fix recalc_rate implementation of PLL
> driver") but the bug was later re-introduced by 1bdf02326b71 ("clk:
> at91: make use of syscon/regmap internally").
> 
> Fixes: 1bdf02326b71 ("clk: at91: make use of syscon/regmap internally)
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Marcin Ziemianowicz <marcin at ziemianowicz.com>

Acked-by: Boris Brezillon <boris.brezillon at bootlin.com>

> ---
> Thank you for bearing with me about this Boris.
> 
> Changes since V3:
>   Fix for double returns found by kbluild test robot
>   > Comments by Boris Brezillon about email formatting issues  
> Changes since V2:
>   Removed all logging/debug messages I added
>   > Comment by Boris Brezillon about my fix being wrong addressed  
> Changes since V1:
>   Added patch set cover letter
>   Shortened lines which were over >80 characters long
>   > Comment by Greg Kroah-Hartman about "from" field in email addressed
>   > Comment by Alan Stern about redundant debug lines addressed  
> 
>  drivers/clk/at91/clk-pll.c | 13 +------------
>  1 file changed, 1 insertion(+), 12 deletions(-)
> 
> diff --git a/drivers/clk/at91/clk-pll.c b/drivers/clk/at91/clk-pll.c
> index 7d3223fc..72b6091e 100644
> --- a/drivers/clk/at91/clk-pll.c
> +++ b/drivers/clk/at91/clk-pll.c
> @@ -132,19 +132,8 @@ static unsigned long clk_pll_recalc_rate(struct clk_hw *hw,
>  					 unsigned long parent_rate)
>  {
>  	struct clk_pll *pll = to_clk_pll(hw);
> -	unsigned int pllr;
> -	u16 mul;
> -	u8 div;
> -
> -	regmap_read(pll->regmap, PLL_REG(pll->id), &pllr);
> -
> -	div = PLL_DIV(pllr);
> -	mul = PLL_MUL(pllr, pll->layout);
> -
> -	if (!div || !mul)
> -		return 0;
>  
> -	return (parent_rate / div) * (mul + 1);
> +	return (parent_rate / pll->div) * (pll->mul + 1);
>  }
>  
>  static long clk_pll_get_best_div_mul(struct clk_pll *pll, unsigned long rate,