Re: [PATCH v8 09/24] drm/rockchip: dw_hdmi: Add support for niu clk

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Mar 16, 2022 at 04:01:49PM +0300, Dmitry Osipenko wrote:
> On 3/16/22 12:12, Sascha Hauer wrote:
> > On Mon, Mar 14, 2022 at 08:54:27PM +0300, Dmitry Osipenko wrote:
> >> On 3/14/22 11:18, Sascha Hauer wrote:
> >>> On Sun, Mar 13, 2022 at 12:07:56AM +0300, Dmitry Osipenko wrote:
> >>>> On 3/11/22 11:33, Sascha Hauer wrote:
> >>>>> The rk3568 HDMI has an additional clock that needs to be enabled for the
> >>>>> HDMI controller to work. This clock is not needed for the HDMI
> >>>>> controller itself, but to make the SoC internal bus logic work. From the
> >>>>> reference manual:
> >>>>>
> >>>>>> 2.8.6 NIU Clock gating reliance
> >>>>>>
> >>>>>> A part of niu clocks have a dependence on another niu clock in order to
> >>>>>> sharing the internal bus. When these clocks are in use, another niu
> >>>>>> clock must be opened, and cannot be gated.  These clocks and the special
> >>>>>> clock on which they are relied are as following:
> >>>>>>
> >>>>>> Clocks which have dependency     The clock which can not be gated
> >>>>>> -----------------------------------------------------------------
> >>>>>> ...
> >>>>>> pclk_vo_niu, hclk_vo_s_niu       hclk_vo_niu
> >>>>>> ...
> >>>>> The clock framework does not support turning on a clock whenever another
> >>>>> clock is turned on, so this patch adds support for the dependent clock
> >>>>> to the HDMI driver. We call it "NIU", which is for "Native Interface
> >>>>> Unit"
> >>>>
> >>>> This still doesn't make sense to me. You're saying that "pclk_vo_niu,
> >>>> hclk_vo_s_niu" depend on "hclk_vo_niu", but HDMI doesn't use pclk_vo, it
> >>>> uses pclk_hdmi.
> >>>
> >>> pclk_hdmi_host is a child clock of pclk_vo:
> >>>
> >>>      aclk_vo                  2        2        0   300000000          0     0  50000         Y
> >>>         aclk_hdcp             0        0        0   300000000          0     0  50000         N
> >>>         pclk_vo               2        3        0    75000000          0     0  50000         Y
> >>>            pclk_edp_ctrl      0        0        0    75000000          0     0  50000         N
> >>>            pclk_dsitx_1       0        0        0    75000000          0     0  50000         N
> >>>            pclk_dsitx_0       1        2        0    75000000          0     0  50000         Y
> >>>            pclk_hdmi_host     1        2        0    75000000          0     0  50000         Y
> >>>            pclk_hdcp          0        0        0    75000000          0     0  50000         N
> >>>         hclk_vo               2        5        0   150000000          0     0  50000         Y
> >>>            hclk_hdcp          0        0        0   150000000          0     0  50000         N
> >>>            hclk_vop           0        2        0   150000000          0     0  50000         N
> >>
> >> It was unclear that the pclk_hdmi is the child of pclk_vo by looking at
> >> the clk driver's code, thank you!
> >>
> >> Won't be better if the implicit clk dependency would be handled
> >> internally by the RK clk driver?
> > 
> > I have considered handling something like coupled clocks in the clock
> > framework, but I have not yet considered handling this internally in the
> > Rockchip clock driver.
> > 
> > I just had a quick look at the driver. While it sounds like an easy task
> > to enable gate A whenever gate B is enabled I haven't found a good way to
> > integrate that into the clk driver. It seems to me that it's not easier
> > to implement in the clk driver than it is to implement it in the clk
> > framework where it could be used by others as well.
> > 
> >> For example, by making the common gate
> >> shared/refcounted. Have you considered this variant? Then we won't need
> >> to change the DT bindings.
> > 
> > For the DT bindings it is just an additional clock. Should we have a
> > better way to handle that case in the future we could simply ignore the
> > additional clock. I wouldn't bother too much about this.
> 
> To me that NIU quirk should be internal to the clk h/w module, so it
> doesn't feel nice to mix the clk h/w description with the HDMI h/w
> description.
> 
> On the other hand, making clk driver to handle this case indeed will
> take some effort as I see now. For example, clk driver of NVIDIA Tegra
> has concept of shared gates, but bringing it to the RK clk driver will
> be quite messy.
> 
> Alright, let's work around the clk limitation like you're suggesting. I
> agree that it shouldn't really be a problem to deprecate the extra clock
> later on.
> 
> I have last questions..
> 
> 1. Previously you said that the PD driver takes care of enabling all the
> clocks it can find in the device by itself on RPM-resume, then why HDMI
> driver needs to enable the clock explicitly?

The PD driver only does so when the device has pm_runtime enabled.
Currently that's not the case for the hdmi driver. That could be changed
of course, but I am not convinced that it's a good idea that the PD
driver messes with the consumers clocks. It would also make the PD
driver mandatory which it currently not is.

> 
> 2. You added clk_prepare_enable(), but there is no corresponding
> clk_disable_unprepare(), AFAICS. Why?

Because I missed it ;)

Sascha

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