Re: [PATCH 16/19] media: i2c: ds90ub960: Enable SSCG for UB9702

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

On 16/01/2025 11:58, Sakari Ailus wrote:
> Moi,
> 
> On Wed, Jan 15, 2025 at 06:04:40PM +0200, Tomi Valkeinen wrote:
>> On 15/01/2025 16:26, Sakari Ailus wrote:
>>> Moi,
>>>
>>> On Fri, Jan 10, 2025 at 11:14:16AM +0200, Tomi Valkeinen wrote:
>>>> From: Jai Luthra <jai.luthra@ideasonboard.com>
>>>>
>>>> UB9702 supports spread-spectrum clock generation for the back-channel
>>>> clock, which is futher used by serializers in synchronous mode to
>>>> generate the forward-channel clock, which can help reduce peak EMI
>>>> energy. The SSCG is common to all RX ports, so it can only be used if
>>>> all the ports are in the same mode.
>>>>
>>>> Add basic support for SSCG by adding a module parameter to enable the
>>>> SSCG. The SSCG uses hardcoded configurationg, with 0.5% center-spread at
>>>> 33kHz modulation rate. See datasheet if different configuration is
>>>> required.
>>>>
>>>> Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>> ---
>>>>    drivers/media/i2c/ds90ub960.c | 102 ++++++++++++++++++++++++++++++++++++++++++
>>>>    1 file changed, 102 insertions(+)
>>>>
>>>> diff --git a/drivers/media/i2c/ds90ub960.c b/drivers/media/i2c/ds90ub960.c
>>>> index f6d6c2fe12cd..a534d077f045 100644
>>>> --- a/drivers/media/i2c/ds90ub960.c
>>>> +++ b/drivers/media/i2c/ds90ub960.c
>>>> @@ -52,6 +52,10 @@
>>>>    #include <media/v4l2-fwnode.h>
>>>>    #include <media/v4l2-subdev.h>
>>>> +static bool ub960_enable_sscg;
>>>> +module_param_named(enable_sscg, ub960_enable_sscg, bool, 0644);
>>>> +MODULE_PARM_DESC(enable_sscg, "Enable SSCG (if available)");
>>>
>>> Shouldn't this come from DT instead?
>>
>> SSCG is an option to use or not to use, based on what the user wants. DT
>> should describe the hardware.
>>
>> A module parameter is bad for this, though, as it's then used for all ub960
>> devices... But I'm not sure what other options we have. We need to have it
>> at probe time.
>>
>> Probably the whole driver could be changed to not connect the serializers at
>> probe, but instead would offer a set of userspace APIs to enable/disable
>> SSCG, and to enable the links. But that brings in its own set of problems,
>> as the links are used for i2c communication to the ser and sensor, not to
>> mention new userspace APIs which always complicates things.
> 
> Typically determining whether you need spread spectrum signalling is highly
> specific to the board. I wonder if this topic has been discussed (and
> possibly somehow resolved) for other subsystems. CSI-2 appears to allow for
> spread spectrum clocks but I don't think I've seen that implemeted
> anywhere, scrambling may be more popular (but also that is right now
> unsupported but could be supported, probably this would be best to consider
> in conjunction with other CSI-2 parameters).

Well, I personally would rather have this option in the DT, so I'm all 
for it =). We can change this to a DT parameter, and see how the DT 
maintainers react.

  Tomi