Re: [RFC 13/17] omap3isp: Configure CSI-2 phy based on platform data

[Sylwester Nawrocki <snjw23@gmail.com>]

Hi,

On 01/08/2012 11:26 AM, Sakari Ailus wrote:
>>>> Shouldn't lane configuration be retrieved from the sensor instead ?
>>>> Sensors could use different lane configuration depending on the mode.
>>>> This could also be implemented later when needed, but I don't think it
>>>> would be too difficult to get it right now.
>>>
>>> I think we'd first need to standardise the CSI-2 bus configuration. I
>>> don't see a practical need to make the lane configuration dynamic. You
>>> could just use a lower frequency to achieve the same if you really need to.
>>>
>>> Ideally it might be nice to do but there's really nothing I know that
>>> required or even benefited from it --- at least for now.
>>
>> Does this mean that lane configuration needs to be duplicated in board code, 
>> on for the SMIA++ platform data and one of the OMAP3 ISP platform data ?
> 
> It's mostly the number of lanes, and the polarity --- in theory it is
> possible to invert the signals on the bus, albeit I'm not sure if anyone
> does that; I can't see a reason for that, but hey, I don't know why it's
> possible to specify polarity either. :-)

I've never seen polarity configuration option in any datasheet, neither
MIPI CSI-2 or D-PHY mentions that. Does OMAP3 ISP really allow MIPI-CSI
lane signal polarity configuration ? MIPI-CSI2 uses differential signals
after all. What would be a point of changing polarity ?

> If both sides support mapping of the lanes, a mapping that matches on
> both sides has to be provided.

In Samsung SoC (both sensor and host interface) I've seen only possibility
to configure the number of data lanes, FWIW I think it is assumed that
when you use e.g. 2 data lanes always lane1 and lane2 are utilised for
transmission, for 3 lanes - lane 1,2,3, etc. Also I've never seen on
schematics that someone wires data lane3 and lane4 when only 2 lanes
are utilised, so this makes me wonder if the lane mapping is ever needed.

Has anyone different experience with that ?

Also the standard seem to specify that Data1+ lane at a transmitter(Tx) is
connected to Data1+ lane at a receiver(Rx), Data1-(Tx) to Data1-(Rx),
Data2+(Tx) to Data2+(Rx), etc. I think this is needed due to explicitly
defined data distribution and merging scheme among the lanes, i.e. to allow
interworking of various receivers and transmitters.

Thus it seems all we need need is just a number of data lanes used.

> I agree we should standardise the configuration of CSI-2 as well as the
> configuration of other busses. However, I would prefer to do that later
> on since I'm already depending on a number of other patches on the
> SMIA++ driver.

--

Regards,
Sylwester