Re: [PATCH 0/5] clk: rockchip: add full support for HDMI clock on rk3288

[Doug Anderson]

Tomeu,

On Tue, Jan 19, 2016 at 4:02 AM, Tomeu Vizoso <tomeu@xxxxxxxxxxxxxxx> wrote:
> On 13 November 2014 at 23:59, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>> Hi,
>>
>> On Thu, Nov 13, 2014 at 12:52 AM, Kever Yang <kever.yang@xxxxxxxxxxxxxx> wrote:
>>> Hi Heiko,
>>>
>>> On 11/07/2014 05:06 AM, Heiko StÃbner wrote:
>>>>
>>>> Hi Kever,
>>>>
>>>> Am Dienstag, 4. November 2014, 15:52:34 schrieb Kever Yang:
>>>>>
>>>>> we are going to make a clock usage solution for rk3288:
>>>>> 1. CPLL and GPLL always not change after assign init;
>>>>> 2. NPLL default as 500MHz, may used for most scene;
>>>>> 3. NPLL may be changed by VOP(HDMI) clock for some special
>>>>>     frequency requirement.
>>>>>
>>>>>      I test it with rk3288 evb on top of Heiko's clk-for-next
>>>>
>>>> In general I'm not really sure if allowing one component to arbitarily
>>>> change
>>>> a shared clock wouldn't result in trouble.
>>>>
>>>> At the moment only dclk_vop0 is included in your series, while the hdmi
>>>> controller can connect to both vop0 and vop1.
>>>> And as Doug mentioned the gpu also has the npll as one possible source.
>>>
>>> I think the problem GPU HANGs with 480MHz clock from usbphy has
>>> been fixed with my patch to gerrit:
>>> https://chromium-review.googlesource.com/#/c/229554/
>>>>
>>>>
>>>> Looking through the clock-tree there are a lot more components possibly
>>>> using
>>>> (or wanting to use) the npll: of course the VOPs, the edp, hdmi, isp,
>>>> hevc,
>>>> gpu, tsp uart0 and gmac. So I'm slightly uncomfortable with somehow
>>>> reserving
>>>> the npll for VOP0 alone.
>>>
>>> It's true that I customized the usage of npll for VOP0 when we need some
>>> very special frequency, but it doesn't means other modules can't use the
>>> npll, it will always decided by clock core for module clocks that which
>>> parent
>>> is the best.
>>
>> We will just need to be very careful.  As I've mentioned in the past
>> we'll need to think about what happens to other clocks that happen to
>> be parented by NPLL whenever we change it.
>>
>> So if we do this:
>>
>> 1. NPLL happens to be 500MHz.
>> 2. We set GPU to be 500MHz and it picks NPLL.
>> 3. We change NPLL to a different speed (like 600MHz).
>>
>> ...I believe in this scenario the GPU would start running at 600MHz
>> immediately.  We'd need to add special code to watch out for this and
>> pick an alternate clock for the GPU (like the USB 480) before the NPLL
>> change.  If NPLL later changes back to 500MHz and the GPU still wanted
>> 500MHz, we'd have to decide what to do.
>>
>> The summary is: it's pretty complicated
>
> Hello,
>
> I have heard that this is still an issue in RK3288 support in mainline
> so have given a look and wonder if you have considered having the
> consumers of a shared clock (which could be a child clock) use
> PRE_RATE_CHANGE notifications to adapt to changes in a parent's rate,
> or to abort them if that's not possible.
>
> Alternatively, a consumer can set min and max constraints if it
> doesn't want for others to mess with its provider, and let the other
> consumer deal with failed rate changes (probably by choosing another
> provider).
>
> Have I missed any problems with that?

I don't think it's really that simple, but I certainly could be wrong.

Summary of the system:

* There are two VOPs (video processors) on rk3288: the big one and the
little one.

* The big VOP can handle higher resolutions than the little one but
uses more power.

* Both VOPs two can be sourced from CPLL, GPLL, or NPLL.

* Both VOPs can be setup to process video for either HDMI, eDP, LVDS, ...

* NPLL is the "new" PLL and is kinda like an extra PLL that's not used
for any of the core peripherals.  It's intended to be one that's
changed dynamically based on system needs.

* GPLL appears intended (in current designs) to run at 594 MHz on
rk3288 using a special "low jitter" setting.  This is intended to be
the source clock for HDMI when running the very common 148.5 and 74.25
rates.  It also it intended to be the source clock for various other
peripherals like i2c, eMMC, SD, etc.  This also runs the GPU when the
GPU is running at ~600MHz or ~300MHz.

* CPLL appears intended (in current designs) to run at 400MHz.  There
are some peripherals that run off this plus the GPU at 400MHz, 200MHz,
or 100MHz.


Let's imagine that we're booting up on a laptop where the builtin
panel is hooked up via eDP.  Let's say that the panel really wants
76.42 MHz but that will work just fine with 74.25 MHz too.  If this is
a system that has no other graphics needs, we'd want eDP to take NPLL
and grab 76.42 MHz.  Make the best rate, right?

...but what about if it's a system with an HDMI port?  The eDP panel
is built in and we know it works with 74.25 MHz, but the user could
plug into all sorts of HDMI devices and they may require some very
special PLL rates to get there.  So if we happen to take NPLL for eDP
we'll need to know to switch away from it once something is plugged
into HDMI.  We don't want to block the HDMI request since we know we
can run eDP at 74.25 off GPLL.  Presumably we could see the HDMI
request and then try to remux the eDP on the fly.  ...but what decides
the HDMI is more important to eDP?  You might want different rules if
the eDP port is somehow exposed and the user might plug different
panels into it...


-Doug