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

[Icenowy Zheng]

于 2017年5月24日 GMT+08:00 下午3:30:19, Maxime Ripard <maxime.ripard at free-electrons.com> 写到:
>On Tue, May 23, 2017 at 09:00:59PM +0800, icenowy at aosc.io wrote:
>> 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.
>
>That's definitely not a good representation of the hardware. There's
>one clock, it goes to the TCON, period.

No, I still think it goes to the TVE as:

1. it's named TVE in datasheet.
2. Generating signal with such a low resolution but such
a high dotclock is not a good situation.

>
>However, the TV encoder has a constraint on that clock rate. This can
>be easily implemented using a custom encoder state where you'd set the
>multiplier to set on that clock, and the TCON will use it.
>
>Maxime