[PATCHv2 01/31] drm/omap: work-around for errata i886

[Jyri Sarha]

On 03/24/17 11:40, Tomi Valkeinen wrote:
> DRA7 errata i886 (FPDLink PLL Unlocks With Certain SoC PLL M/N Values)
> says that FPDLink is sensitive to jitter on the vout clock, and that low
> PLL M and N values result in more jitter than high M and N values.
> 
> This patch implements a workaround for the problem by changing the PLL
> setup to search for clocks starting from high M and N values, instead of
> low values. This should not cause any functional change, and only
> reduces the jitter.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ti.com>
> ---
>  drivers/gpu/drm/omapdrm/dss/pll.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/omapdrm/dss/pll.c b/drivers/gpu/drm/omapdrm/dss/pll.c
> index 0a76c89cdc2e..65c478d85caa 100644
> --- a/drivers/gpu/drm/omapdrm/dss/pll.c
> +++ b/drivers/gpu/drm/omapdrm/dss/pll.c
> @@ -231,7 +231,7 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
>  
>  	pll_max = pll_max ? pll_max : ULONG_MAX;
>  
> -	for (n = n_start; n <= n_stop; ++n) {
> +	for (n = n_stop; n >= n_start; --n) {

The patch looks simple like this, but just reading the code without a
hint what is going on may be confusing. Loops beginning from stop and
ending in start and all.

Could these variables be called [nm]_max and [nm]_min, or maybe one
simple comment to tell why we are going from max to min, or both?

>  		fint = clkin / n;
>  
>  		m_start = max(DIV_ROUND_UP(DIV_ROUND_UP(pll_min, fint), 2),
> @@ -240,7 +240,7 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
>  				(unsigned)(pll_hw_max / fint / 2),
>  				hw->m_max);
>  
> -		for (m = m_start; m <= m_stop; ++m) {
> +		for (m = m_stop; m >= m_start; --m) {
>  			clkdco = 2 * m * fint;
>  
>  			if (func(n, m, fint, clkdco, data))
>