linux-kernel - Re: [PATCH v2 19/26] drm/rockchip: dw-mipi-dsi: improve PLL configuration

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Message-ID: <58855EAE.2010704@rock-chips.com>
Date:   Mon, 23 Jan 2017 09:38:54 +0800
From:   Chris Zhong <zyw@...k-chips.com>
To:     John Keeping <john@...anate.com>,
        Mark Yao <mark.yao@...k-chips.com>
Cc:     dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 19/26] drm/rockchip: dw-mipi-dsi: improve PLL
 configuration

Hi John

On 01/22/2017 12:31 AM, John Keeping wrote:
> The multiplication ratio for the PLL is required to be even due to the
> use of a "by 2 pre-scaler".  Currently we are likely to end up with an
> odd multiplier even though there is an equivalent set of parameters with
> an even multiplier.
>
> For example, using the 324MHz bit rate with a reference clock of 24MHz
> we end up with M = 27, N = 2 whereas the example in the PHY databook
> gives M = 54, N = 4 for this bit rate and reference clock.
>
> By walking down through the available multiplier instead of up we are
> more likely to hit an even multiplier.  With the above example we do now
> get M = 54, N = 4 as given by the databook.
>
> While doing this, change the loop limits to encode the actual limits on
> the divisor, which are:
>
> 	40MHz >= (pllref / N) >= 5MHz

This formula is limit for N, but we still can not guarantee to get an 
even M.
Do you think we should do a check for M.
such as:
if (m % 2)
     continue;
...
     for (i = pllref / 5; i > (pllref / 40); i--) {
         pre = pllref / i;
         if ((tmp > (target_mbps % pre)) && (target_mbps / pre < 512)) {
             tmp = target_mbps % pre;
             n = i;
             m = target_mbps / pre;
             if (m % 2)
                 continue;
         }
         if (tmp == 0)
             break;
     }

if (m % 2)
     m++;

     dsi->lane_mbps = pllref / n * m;
     dsi->input_div = n;
     dsi->feedback_div = m;



>
> Signed-off-by: John Keeping <john@...anate.com>
> ---
> Unchanged in v2
> ---
>   drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> index 12432e41971b..f2320cf1366c 100644
> --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> @@ -519,7 +519,7 @@ static int dw_mipi_dsi_get_lane_bps(struct dw_mipi_dsi *dsi,
>   	pllref = DIV_ROUND_UP(clk_get_rate(dsi->pllref_clk), USEC_PER_SEC);
>   	tmp = pllref;
>   
> -	for (i = 1; i < 6; i++) {
> +	for (i = pllref / 5; i > (pllref / 40); i--) {
>   		pre = pllref / i;
>   		if ((tmp > (target_mbps % pre)) && (target_mbps / pre < 512)) {
>   			tmp = target_mbps % pre;