[linux-sunxi] Re: [PATCH v2 11/27] drm/sun4i: tcon: Add support for tcon-top gate

[Jernej Škrabec]

Dne sobota, 16. junij 2018 ob 07:48:38 CEST je Chen-Yu Tsai napisal(a):
> On Sat, Jun 16, 2018 at 1:33 AM, Jernej Škrabec <jernej.skrabec at siol.net> 
wrote:
> > Dne petek, 15. junij 2018 ob 19:13:17 CEST je Chen-Yu Tsai napisal(a):
> >> On Sat, Jun 16, 2018 at 12:41 AM, Jernej Škrabec
> >> 
> >> <jernej.skrabec at siol.net> wrote:
> >> > Hi,
> >> > 
> >> > Dne petek, 15. junij 2018 ob 10:31:10 CEST je Maxime Ripard napisal(a):
> >> >> Hi,
> >> >> 
> >> >> On Tue, Jun 12, 2018 at 10:00:20PM +0200, Jernej Skrabec wrote:
> >> >> > TV TCONs connected to TCON TOP have to enable additional gate in
> >> >> > order
> >> >> > to work.
> >> >> > 
> >> >> > Add support for such TCONs.
> >> >> > 
> >> >> > Signed-off-by: Jernej Skrabec <jernej.skrabec at siol.net>
> >> >> > ---
> >> >> > 
> >> >> >  drivers/gpu/drm/sun4i/sun4i_tcon.c | 11 +++++++++++
> >> >> >  drivers/gpu/drm/sun4i/sun4i_tcon.h |  4 ++++
> >> >> >  2 files changed, 15 insertions(+)
> >> >> > 
> >> >> > diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> >> >> > b/drivers/gpu/drm/sun4i/sun4i_tcon.c index
> >> >> > 08747fc3ee71..0afb5a94a414
> >> >> > 100644
> >> >> > --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> >> >> > +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> >> >> > @@ -688,6 +688,16 @@ static int sun4i_tcon_init_clocks(struct device
> >> >> > *dev,
> >> >> > 
> >> >> >             dev_err(dev, "Couldn't get the TCON bus clock\n");
> >> >> >             return PTR_ERR(tcon->clk);
> >> >> >     
> >> >> >     }
> >> >> > 
> >> >> > +
> >> >> > +   if (tcon->quirks->has_tcon_top_gate) {
> >> >> > +           tcon->top_clk = devm_clk_get(dev, "tcon-top");
> >> >> > +           if (IS_ERR(tcon->top_clk)) {
> >> >> > +                   dev_err(dev, "Couldn't get the TCON TOP bus
> >> >> > clock\n");
> >> >> > +                   return PTR_ERR(tcon->top_clk);
> >> >> > +           }
> >> >> > +           clk_prepare_enable(tcon->top_clk);
> >> >> > +   }
> >> >> > +
> >> >> 
> >> >> Is it required for the TCON itself to operate, or does the TCON
> >> >> requires the TCON TOP, which in turn requires that clock to be
> >> >> functional?
> >> >> 
> >> >> I find it quite odd to have a clock that isn't meant for a particular
> >> >> device to actually be wired to another device. I'm not saying this
> >> >> isn't the case, but it would be a first.
> >> > 
> >> > Documentation doesn't say much about that gate. I did few tests and
> >> > TCON
> >> > registers can be read and written even if TCON TOP TV TCON gate is
> >> > disabled. However, there is no image, as expected.
> >> 
> >> The R40 manual does include it in the diagram, on page 504. There's also
> >> a
> >> mux to select whether the clock comes directly from the CCU or the TV
> >> encoder (a feedback mode?). I assume this is the gate you are referring
> >> to
> >> here, in which case it is not a bus clock, but rather the TCON module or
> >> channel clock, strangely routed.
> >> 
> >> > More interestingly, I enabled test pattern directly in TCON to
> >> > eliminate
> >> > influence of the mixer. As soon as I disabled that gate, test pattern
> >> > on
> >> > HDMI screen was gone, which suggest that this gate influences something
> >> > inside TCON.
> >> > 
> >> > Another test I did was that I moved enable/disable gate code to
> >> > sun4i_tcon_channel_set_status() and it worked just as well.
> >> > 
> >> > I'll ask AW engineer what that gate actually does, but from what I saw,
> >> > I
> >> > would say that most appropriate location to enable/disable TCON TOP TV
> >> > TCON
> >> > gate is TCON driver. Alternatively, TCON TOP driver could check if any
> >> > TV
> >> > TCON is in use and enable appropriate gate. However, that doesn't sound
> >> > right to me for some reason.
> >> 
> >> If what I said above it true, then yes, the appropriate location to
> >> enable
> >> it is the TCON driver, but moreover, the representation of the clock tree
> >> should be fixed such that the TCON takes the clock from the TCON TOP as
> >> its
> >> channel/ module clock instead. That way you don't need this patch, but
> >> you'd add another for all the clock routing.
> > 
> > Can you be more specific? I not sure what you mean here.
> 
> For clock related properties in the device tree:
> 
> &tcon_top {
>     clocks = <&ccu CLK_BUS_TCON_TOP>,
>              <&ccu CLK_TCON_TV0>,
>              <&tve0>,
>              <&ccu CLK_TCON_TV1>,
>              <&tve1>;
>     clock-names = "bus", "tcon-tv0", "tve0", "tcon-tv1", "tve1";
>     clock-output-names = "tcon-top-tv0", "tcon-top-tv1";
> };
> 
> &tcon_tv0 {
>     clocks = <&ccu CLK_BUS_TCON_TV0>, <&tcon_top 0>'
>     clock-names = "ahb", "tcon-ch1";
> };
> 
> A diagram would look like:
>                    | This part is TCON TOP |
> 
>                    v                       v
> CCU CLK_TCON_TV0 --|----\                  |
> 
>                    |     mux ---- gate ----|-- TCON_TV0
> 
> TVE0 --------------|----/                  |
> 
> And the same goes for TCON_TV1 and TVE1.
> 
> The user manual is a bit lacking on how TVE outputs a clock though.

I didn't yet received any response on HW details from AW till now, but I would 
like to post new version of patches soon.

While chaining like you described could be implemented easily, I don't think 
it really represents HW as it is. Tests showed that these two clocks are 
independent, otherwise register writes/reads wouldn't be possible with tcon-
top gate disabled. I chose tcon-top bus clock as a parent becase if it is not 
enabled, it simply won't work.

However, if everyone feels chaining is the best way to implement it, I'll do 
it.

Best regards,
Jernej