Re: [PATCH 1/3] dt-bindings: clock: ti,sci-clk: Add support for parsing clock info from DT

[Tero Kristo <t-kristo@ti.com>]

On 22/01/2019 09:33, Tero Kristo wrote:
> On 21/01/2019 23:04, Rob Herring wrote:
>> On Tue, Jan 08, 2019 at 03:30:21PM +0200, Tero Kristo wrote:
>>> By default, the available clock info is queried from firmware, which can
>>> be quite a lengthy operation if there is a very large amount of clocks
>>> available. Add option for parsing the used clocks from DT instead, and
>>> only register these which can improve the boot time of the device quite
>>> a lot.
>>>
>>> Signed-off-by: Tero Kristo <t-kristo@ti.com>
>>> ---
>>>   Documentation/devicetree/bindings/clock/ti,sci-clk.txt | 7 +++++++
>>>   1 file changed, 7 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/clock/ti,sci-clk.txt 
>>> b/Documentation/devicetree/bindings/clock/ti,sci-clk.txt
>>> index 4e59dc6..c757ae1 100644
>>> --- a/Documentation/devicetree/bindings/clock/ti,sci-clk.txt
>>> +++ b/Documentation/devicetree/bindings/clock/ti,sci-clk.txt
>>> @@ -18,6 +18,13 @@ Required properties:
>>>     and clocks IDs for 66AK2G SoC are documented at
>>>     http://processors.wiki.ti.com/index.php/TISCI#66AK2G02_Data
>>> +Optional properties:
>>> +-------------------
>>> +- ti,scan-clocks-from-dt: Scan clock tree info from DT. By default,
>>> +  clocks are queried from firmware, which can be rather slow operation,
>>> +  especially if there is a really large number of clocks available out
>>> +  of which only a handful are ever used by kernel.
>>
>> At first, I thought this was an either/or thing. Use firmware or use DT,
>> but it is really only get the clocks used in the DT from firmware.
>>
>> Why wouldn't you just always do that? I can think of 3 cases:
>> reparenting, debug and overlays. This breaks reparenting and overlays,
>> right? Debug could be handled with some userspace trigger to get all the
>> clocks.
> 
> Re-parenting this does not break, as the scan still checks every 
> possible parent of a clock scanned. Overlays are broken for sure, as we 
> don't know which overlays we would be applying, and what clocks would be 
> in them. Debug is kind of broken as we only scan a small portion of the 
> clocks.
> 
>>
>> Why scan any of the clocks up front? Why not just create the clocks on
>> demand? If an unknown clock id is requested, then create the clock and
>> query the firmware at that point. That would avoid the DT scan too.
>> Maybe there's some issues in the clk framework preventing that, but
>> that's not really a DT problem.
> 
> The very initial version I did a couple of years back, did scan the 
> clocks based on need, and registered them dynamically. Stephen shot down 
> this based on the assessment that there might be locking issues with the 
> common clock framework with this approach leading into potential 
> deadlock situations.

So Rob, what is the final call on this binding? Ack/NAK? If NAK, shall I 
implement a kernel cmdline param to select the parsing method or what is 
preferred? Doing it build time with a simple Kconfig seems too limiting.

-Tero
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