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

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Message-ID: <5886BF09.6090301@rock-chips.com>
Date:   Tue, 24 Jan 2017 10:42:17 +0800
From:   Chris Zhong <zyw@...k-chips.com>
To:     John Keeping <john@...anate.com>
Cc:     Mark Yao <mark.yao@...k-chips.com>,
        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



On 01/23/2017 08:49 PM, John Keeping wrote:
> Hi Chris,
>
> On Mon, 23 Jan 2017 09:38:54 +0800, Chris Zhong wrote:
>> 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;
> Yes, I agree that there should be a check for M, but I'm not sure if
> the version above is sufficient.  The "m % 2" check inside the loop
> means that we don't break immediately when tmp=0 but then we are
> guaranteed to break next time without having modified n, m because now
> tmp=0 so "tmp > (target_mbps % pre)" is always false and we just hit the
> "if (tmp == 0) break" case next time.
>
> Given that the descending loop already means that if we can hit "tmp"
> exactly we are more likely to do so with a bigger N and even M, I think
> it might be better to just fix M after the loop like:
>
> 	if (m % 2) {
> 		if (m < 256 && (n * 2) <= (pllref / 5)) {
> 			n *= 2;
> 			m *= 2;
> 		} else {
> 			m++;
> 		}
> 	}
>
> but I haven't thought about this too carefully.
>
> For this series, I'd rather either keep this patch as it is or drop it
> in favour of a more comprehensive solution.  I don't want to block the
> other fixes waiting for a perfect fix here and we can always improve
> this further with a follow-up patch.
Agree, We can improve the whole formula in the future.

>
>>> 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;
>>
>
>