Question regarding clocks in the DW-HDMI DT bindings

[Laurent Pinchart]

Hello Fabio and Vladimir,

Thank you for your quick responses.

On Friday 25 Nov 2016 00:16:00 Vladimir Zapolskiy wrote:
> On 11/25/2016 12:07 AM, Fabio Estevam wrote:
> > On Thu, Nov 24, 2016 at 7:16 PM, Laurent Pinchart wrote:
> >> Hi Andy,
> >> 
> >> As the author of the DW-HDMI DT bindings this question is addressed to
> >> you, but information from anyone is more than welcome.
> >> 
> >> The DT bindings specify two clocks named "iahb" and "isfr" but don't
> >> describe them. While I assume that the "isfr" clock corresponds to the
> >> "isfrclk" input signal of the DW HDMI, there is no "iahb" clock
> >> described in the IP core datasheet.
> > 
> > i.MX6Q has a DW-HDMI IP block.
> > 
> > The names in the devicetree binding matches the ones listed at the
> > i.MX6Q Reference Manual - Table 33-1. HDMI Clocks
>
> correct, for your convenience the table is copied below:
> 
> Clock name |     Clock Root     | Description
> -----------+--------------------+---------------------------------------
>    iahbclk  | ahb_clk_root       | Bus clock
>    icecclk  | ckil_sync_clk_root | CEC low-frequency clock (32kHZ)
>    ihclk    | ahb_clk_root       | Module clock
>    isfrclk  | video_27m_clk_root | Internal SFR clock (video clock 27MHz)
> 
> Here AHB stands for ARM Advanced High-performance Bus.

That's what I suspected. I believe the "iahb" name is wrong, as the DW HDMI TX 
IP core clearly documents the bus clock as being called "iapbclk". We could 
rename that in the DT bindings (with compatibility code in the driver to keep 
supporting the old name) but it might not be worth it. The bindings should 
however document that the "iahb" clock is the IP core's "iapbclk" bus clock.

Another question I have about the bus clock (CC'ing the devicetree mailing 
list as well as the clock maintainers) is whether it should be made optional. 
The clock is obviously mandatory from a hardware point of view (given that APB 
is a synchronous bus and thus requires a clock), but in some SoCs 
(specifically for the Renesas SoCs) that clock is always on and can't be 
controlled. We already omit bus clocks in DT for most IP cores when the clock 
can never be controlled (and we also omit a bunch of other clocks that we 
don't even know exist), so it could make sense to make the clock optional. 
Otherwise there would be runtime overhead trying to handle a clock that can't 
be controlled.

> By the way while we're discussing DW HDMI bindings specific to iMX,
> I would recommend to remove utterly hackish and iMX only "gpr"
> property from the example in bindings/display/bridge/dw_hdmi.txt

-- 
Regards,

Laurent Pinchart