[PATCH v7 10/24] drm/rockchip: dw_hdmi: Add support for hclk

[Sascha Hauer]

Hi Elaine,

On Wed, Mar 09, 2022 at 09:41:39AM +0800, zhangqing at rock-chips.com wrote:
>    hi，all：
>    Let me explain the clock dependency：
>    From the clock tree, pclk_vo0 and hclk_vo0 are completely independent
>    clocks with different parent clocks and different clock frequencies。
>    But the niu path is :
>    pclk_vo is dependent on hclk_vo, and the pclk_vo niu goes through  hclk_vo
>    niu.

Thanks, this is the information we are looking for. What is "NIU" btw?
I think this is even documented in the Reference Manual. With the right
pointer I just found:

> 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 tree and NIU bus paths are designed independently
>    So there are three solutions to this problem:
>    1. DTS adds a reference to Hclk while referencing Pclk.
>    2, The dependent clock is always on, such as HCLK_VO0, but this is not
>    friendly for the system power.
>    3. Create a non-clock-tree reference. Clk-link, for example, we have an
>    implementation in our internal branch, but Upstream is not sure how to
>    push it.

I thought about something similar. That would help us here and on i.MX
we have a similar situation: We have one bit that switches multiple
clocks. That as well cannot be designed properly in the clock framework
currently, but could be modelled with a concept of linked clocks.

Doing this sounds like quite a bit of work and discussion though, I
don't really like having this as a dependency to mainline the VOP2
driver. I vote for 1. in that case, we could still ignore the hclk in
dts later when we have linked clocks.

Sascha

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