[Linux-stm32] [PATCH 3/4] media: dt-bindings: media: Document pclk-max-frequency property

[Hugues FRUCHET]

Hi Sakari,

On 09/28/2018 09:03 AM, Sakari Ailus wrote:
> Hi Hugues,
> 
> On Thu, Sep 27, 2018 at 04:46:06PM +0200, Hugues Fruchet wrote:
>> This optional property aims to inform parallel video devices
>> of the maximum pixel clock frequency admissible by host video
>> interface. If bandwidth of data to be transferred requires a
>> pixel clock which is higher than this value, parallel video
>> device could then typically adapt framerate to reach
>> this constraint.
>>
>> Signed-off-by: Hugues Fruchet <hugues.fruchet at st.com>
>> ---
>>   Documentation/devicetree/bindings/media/video-interfaces.txt | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/media/video-interfaces.txt b/Documentation/devicetree/bindings/media/video-interfaces.txt
>> index baf9d97..fa4c112 100644
>> --- a/Documentation/devicetree/bindings/media/video-interfaces.txt
>> +++ b/Documentation/devicetree/bindings/media/video-interfaces.txt
>> @@ -147,6 +147,8 @@ Optional endpoint properties
>>     as 0 (normal). This property is valid for serial busses only.
>>   - strobe: Whether the clock signal is used as clock (0) or strobe (1). Used
>>     with CCP2, for instance.
>> +- pclk-max-frequency: maximum pixel clock frequency admissible by video
>> +  host interface.
> 
> Is there a limit on the pixel clock or the link frequency?

The constraint is the frequency of the clock in input of the SoC (pixel 
clock line).

> 
> We do have a property for the link frequency and a control for the pixel
> lock as well as for the link frequency. Could these be used for the
> purpose?

As this was documented mainly for MIPI-CSI2 I was not clear if this 
could be used or not, but video-interface.txt binding let open the door
to parallel port usage...
I had also some hesitations to use this property because what I was 
searching for here was a maximum limit to not exceed while 
"link-frequencies" is described as frequencies to use: "Allowed data bus 
frequencies".
The fact that there was several entries for this property was also quite 
confusing.
What I can do is to use this property and add a comment explaining that 
this can also be used for parallel port as the frequency to not exceed 
on pixel clock signal, what do you think about it ?

Checking drivers which are implementing "link-frequencies", I've found 
OV2659 sensor which is doing almost what I want to, ie compute the clock 
rate depending on link-frequency, see ov2659_pll_calc_params().

> 
> The link frequency in general should be specified for the board, and that
> limits the pixel clock as well in the case the bus transfers a given number
> of pixels per clock.
> 
> The OMAP3ISP driver also address this by reading back the pixel clock from
> the sensor before starting streaming.
> 

BR,
Hugues.