Re: [PATCH v3 7/7] clk: rockchip: rk3399: Add support frac mode frequencies

[Xing Zheng <zhengxing@rock-chips.com>]

Hi Heiko,

On 2016年08月05日 16:48, Heiko Stübner wrote:
> Hi Xing,
>
> Am Freitag, 5. August 2016, 10:26:57 schrieb Xing Zheng:
>> On 2016年08月05日 03:19, Heiko Stübner wrote:
>>> Am Dienstag, 2. August 2016, 15:22:59 schrieb Xing Zheng:
>>>> We need to support various display resolutions for external
>>>> display devices like HDMI/DP, the frac mode can help us to
>>>> acquire almost any frequencies, and need higher VCOs to reduce
>>>> clock jitters.
>>>>
>>>> Signed-off-by: Xing Zheng<zhengxing@rock-chips.com>
>>> why does this need to be a separate rate array and cannot live in the
>>> general pll rate array?
>>>
>>> The plls are general purpose, so we shouldn't limit them arbitarily.
>> Yes, I understand your mean. :-)
>>
>>> I currently only see some frequencies (594MHz, 297MHz, 54MHz) that are
>>> present in both arrays but have different settings. As your patch
>>> description says that these settings reduce clock jitter, wouldn't the
>>> general frequencies also profit from merging these new values into the
>>> general rate array?
>> and here are some of our ideas:
>>
>> "WIth the frac mode and higher VCO to reduce clock jitters" that
>> suggestion is from IC designer.
>> There are many and various kinds resolution and needed frequencies for
>> external disaplay devices. For example, the DP needs:
>> 3840x2160 533250KHz
>> 3840x2160 297000KHz
>> 3840x2160 296703KHz
>> 2560x1440 241500KHz
>> 1920x1080 148500KHz
>> 1920x1080 148352KHz
>> 1680x1050 146250KHz
>> 1600x900 108000KHz
>> 1280x1024 135000KHz
>> 1280x1024 108000KHz
>> ... and so on
>>
>> There some frequencies must be allocated with frac mode. We separate
>> these frequencies that are only used for display (VPLL) from the general
>> rate table, and put them to be classified into a frac mode table, we can
>> reduce the frequency of the query time, the two rate tables will not
>> interfere with each other. Because other PLLs don't need to assgin these
>> various frequencies with frac mode.
> Hmm, you're adding 14 frequencies to that new table (4 or so of them
> duplicating existing frequencies). So even if the effective number of new
> frequencies goes from now 10 to 20, I don't think walking that table will take
> an excessive time longer than now.
>
> After the patch introducing the automatic rate calculation, the rate table we
> need to walk, will even get smaller.
>
> Other components might also profit from the updated standard frequencies with
> less jitter you're introducing here.
>
> And of course there is also the possibility somebody might want to build some
> rk3399 device without any graphics output at all [arm-server seem to be the
> new hype :-) ], so may want to use the vpll for something else completely.
>
> So I still don't see an argument why it needs to be a separate table, as I
> currently don't see a case were it will really hurt the other PLLs.
>
>
> Heiko
>
Yes, sorry to this idea is not comprehensive. I will try to find a 
better way.

Thanks for your comments. :-)

-- 
- Xing Zheng