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

[Maxime Ripard]

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.

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

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