[linux-sunxi] Re: [RFC PATCH 07/11] drm: sun4i: add support for the TV encoder in H3 SoC

[icenowy at aosc.io]

在 2017-05-23 20:53，Maxime Ripard 写道：
> On Mon, May 22, 2017 at 07:55:56PM +0200, Jernej Škrabec wrote:
>> Hi,
>> 
>> Dne sobota, 20. maj 2017 ob 03:37:53 CEST je Chen-Yu Tsai napisal(a):
>> > On Sat, May 20, 2017 at 2:23 AM, Jernej Škrabec <jernej.skrabec at siol.net>
>> wrote:
>> > > Hi,
>> > >
>> > > Dne petek, 19. maj 2017 ob 20:08:18 CEST je Icenowy Zheng napisal(a):
>> > >> 于 2017年5月20日 GMT+08:00 上午2:03:30, Maxime Ripard <maxime.ripard at free-
>> > >
>> > > electrons.com> 写到:
>> > >> >On Thu, May 18, 2017 at 12:43:50AM +0800, Icenowy Zheng wrote:
>> > >> >> Allwinner H3 features a TV encoder similar to the one in earlier
>> > >> >
>> > >> >SoCs,
>> > >> >
>> > >> >> but with some different points about clocks:
>> > >> >> - It has a mod clock and a bus clock.
>> > >> >> - The mod clock must be at a fixed rate to generate signal.
>> > >> >
>> > >> >Why?
>> > >>
>> > >> It's experiment result by Jernej.
>> > >>
>> > >> The clock rates in BSP kernel is also specially designed
>> > >> (PLL_DE at 432MHz) in order to be able to feed the TVE.
>> > >
>> > > My experiments and search through BSP code showed that TVE seems to have
>> > > additional fixed predivider 8. So if you want to generate 27 MHz clock,
>> > > unit has to be feed with 216 MHz.
>> > >
>> > > TVE has only one PLL source PLL_DE. And since 216 MHz is a bit low for
>> > > DE2,
>> > > BSP defaults to 432 MHz for PLL_DE and use divider 2 to generate 216 MHz.
>> > > This clock is then divided by 8 internaly to get final 27 MHz.
>> > >
>> > > Please note that I don't have any hard evidence to support that, only
>> > > experimental data. However, only that explanation make sense to me.
>> > >
>> > > BTW, BSP H3/H5 TV driver supports only PAL and NTSC which both use 27 MHz
>> > > base clock. Further experiments are needed to check if there is any
>> > > possibility to have other resolutions by manipulating clocks and give
>> > > other proper settings. I plan to do that, but not in very near future.
>> >
>> > You only have composite video output, and those are the only 2 standard
>> > resolutions that make any sense.
>> 
>> Right, other resolutions are for VGA.
>> 
>> Anyway, I did some more digging in A10 and R40 datasheets. I think 
>> that H3 TVE
>> unit is something in between. R40 TVE has a setting to select "up 
>> sample".
> 
> That might be just another translation of oversampling :)
> 
> I didn't know it could be applied to composite signals though, but I
> guess this is just another analog signal after all.
> 
>> Possible settings are 27 MHz, 54 MHz, 108 MHz and 216 MHz. BSP driver 
>> on R40
>> has this setting enabled only for PAL and NTSC and it is always 216 
>> MHz. I
>> think that H3 may have this hardwired to 216 MHz and this would be the 
>> reason
>> why 216 MHz is needed.
>> 
>> Has anyone else any better explanation?
> 
> That's already a pretty good one.
> 
> Either way, wether this is upsampling, oversampling or just a
> pre-divider, this can and should be dealt with in the mode_set
> callback, and not in the probe.

I got a better idea -- let TVE driver have the CLK_TVE as an
input and create a subclock output with divider 16, and feed this
subclock to TCON lcd-ch1.

This is a model of the real hardware -- the clock divider is in
TVE, not TCON.

> 
> Thanks!
> Maxime
> 
> --
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com