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

[Heiko Stübner]

Am Freitag, 5. August 2016, 21:23:14 schrieb Xing Zheng:
> Hi Heiko,
> 
> On 2016å08æ05æ 16:48, Heiko StÃbner wrote:
> > Hi Xing,
> > 
> > Am Freitag, 5. August 2016, 10:26:57 schrieb Xing Zheng:
> >> On 2016å08æ05æ 03:19, Heiko StÃbner 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@xxxxxxxxxxxxxx>
> >>> 
> >>> 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. :-)

as I said, to me just merging the new clock rates into the existing pll rate 
array looks like it should work just fine :-)