Re: [PATCH RFC v6 1/6] dt-bindings: exynos-bus: Add documentation for interconnect properties

[Sylwester Nawrocki <s.nawrocki@samsung.com>]

On 09.07.2020 23:04, Rob Herring wrote:
> On Thu, Jul 02, 2020 at 06:37:19PM +0200, Sylwester Nawrocki wrote:
>> Add documentation for new optional properties in the exynos bus nodes:
>> samsung,interconnect-parent, #interconnect-cells, bus-width.
>> These properties allow to specify the SoC interconnect structure which
>> then allows the interconnect consumer devices to request specific
>> bandwidth requirements.
>>
>> Signed-off-by: Artur Świgoń <a.swigon@samsung.com>
>> Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>

>> ---
>>  .../devicetree/bindings/devfreq/exynos-bus.txt     | 68 +++++++++++++++++++++-
>>  1 file changed, 66 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/devfreq/exynos-bus.txt b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> index e71f752..4035e3e 100644
>> --- a/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> +++ b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt
>> @@ -51,6 +51,13 @@ Optional properties only for parent bus device:
>>  - exynos,saturation-ratio: the percentage value which is used to calibrate
>>  			the performance count against total cycle count.
>>  
>> +Optional properties for interconnect functionality (QoS frequency constraints):
>> +- samsung,interconnect-parent: phandle to the parent interconnect node; for
>> +  passive devices should point to same node as the exynos,parent-bus property.
> 
> Adding vendor specific properties for a common binding defeats the 
> point.

Should we make it then a common interconnect-parent property? Perhaps allowing
also a second cell after the phandle to indicate the target interconnect id?

Currently the links are only being defined in drivers, I'm not sure if we want 
to go that direction and extend the interconnect binding so it is possible
to define any link between the nodes.

With the samsung,interconnect-parent property there was an assumption that
each DT node ("samsung,exynos-bus" compatible) corresponds to an interconnect 
provider with single interconnect node. The source and destination node id 
for the link were unspecified and dynamically allocated by the driver.

I guess we don't want a property that would contain pairs of the interconnect 
node specifiers (phandle + interconnect id) to define all links, since usually 
additional data is required per each link.

Then perhaps we could make the new interconnect-parent property applicable
only to DT nodes with #interconnect-cells == 0, i.e. valid only in such DT 
nodes?

>> +- #interconnect-cells: should be 0.
>> +- bus-width: the interconnect bus width in bits, default value is 8 when this
>> +  property is missing.
> 
> Your bus is 8-bits or 4-bits as the example?
Bus width might not be a good term for the intended purpose of that property.
It has been added to specify minimum bus clock rate required for given data
throughput. After checking the documentation again the AXI bus width is
actually 128 bits everywhere for instance. The example defines data path 
leftbus <- dmc <- (memory) and for leftbus we have bus-width=<8> and for dmc 
bus-width=<4>. 

Perhaps it's better to use a vendor specific property instead, e.g.
samsung, data-clock-ratio?

-- 
Thanks,
Sylwester