Re: Suggestion regarding x8 gang mode device tree changes

[Sowjanya Komatineni <skomatineni@nvidia.com>]

On 10/30/20 3:06 AM, Hans Verkuil wrote:
> On 30/10/2020 10:56, Sakari Ailus wrote:
>> Hi Hans,
>>
>> On Fri, Oct 30, 2020 at 10:31:06AM +0100, Hans Verkuil wrote:
>>> On 29/10/2020 18:07, Sowjanya Komatineni wrote:
>>>> On 10/29/20 9:52 AM, Sakari Ailus wrote:
>>>>> On Thu, Oct 29, 2020 at 09:49:57AM -0700, Sowjanya Komatineni wrote:
>>>>>> On 10/29/20 8:36 AM, Sowjanya Komatineni wrote:
>>>>>>> On 10/29/20 7:50 AM, Sakari Ailus wrote:
>>>>>>>> Hi Sowjanya,
>>>>>>>>
>>>>>>>> On Wed, Oct 28, 2020 at 06:48:59PM -0700, Sowjanya Komatineni wrote:
>>>>>>>>> Hi Sakari,
>>>>>>>>>
>>>>>>>>> Missed to add you to below patch series for HDMI2CSI bridge support
>>>>>>>>>
>>>>>>>>> https://patchwork.kernel.org/project/linux-media/cover/1603768763-25590-1-git-send-email-skomatineni@nvidia.com/
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Patch-10 of this series is for x8 capture from HDMI2CSI bridge.
>>>>>>>>>
>>>>>>>>> Would like to get your suggestion on x8 gang/combined ports capture
>>>>>>>>> implementation.
>>>>>>>> The majority of CSI-2 receiver devices support partitioning the
>>>>>>>> lanes among
>>>>>>>> different PHYs in various ways. They do support usually up to four
>>>>>>>> lanes,
>>>>>>>> but adding four more lanes is not a reason for making the API different.
>>>>>>>>
>>>>>>>> So instead, you should implement this as a single port that simply has 8
>>>>>>>> lanes.
>>>>>>>>
>>>>>>> Thanks Sakari for your reply.
>>>>>>>
>>>>>>> current v2 series treats as 8 lanes. You mean to not expose 2nd port in
>>>>>>> device tree as VI/CSI side takes care of 2nd port as combined to treat
>>>>>>> as 8 lane?
>>>>> Correct.
>>>>>
>>>>> Although you can have the second port connected if fewer lanes are assigned
>>>>> to the first one.
>>>>>
>>>>> How does it work for this device, are the lanes statically allocated to
>>>>> ports, apart from the special 8 lane mode?
>>>> Tegra CSI each port supports max 4 lanes. For x8, 2 x4 ports together
>>>> are programmed for simultaneous streaming during the same video/subdev
>>>> stream ops.
>>>>
>>>> Physically, CSI RX side 4 lanes goes to x4 port and other 4 lanes goes
>>>> to another x4 port.
>>>>
>>>> HDMI Bridge TX0 -> CSI RX0 (x4 port)
>>>>
>>>> HDMI Bridge TX1 -> CSI RX1 (x4 port)
>>>>
>>>> HDMI bridge side single image is split into 2 x4 ports and on RX side
>>>> image from both ports are captured simultaneously with buffer offsets
>>>> adjusted side-by-side to get combined image for same video buf of video
>>>> device.
>>>>
>>>> Both these 2 x4 ports together are used for streaming by Tegra VI and
>>>> buffer offsets are adjusted side by side for these ports and for video
>>>> device node stream, its single buffer which contains combined image from
>>>> capture.
>>>>
>>>>>> AS csi2 bus type supports max 4 data lanes with endpoint parse API.
>>>>>>
>>>>>> Currently with x8 as single port, I am using bus-width with bus type as
>>>>>> parallel in device tree and when using x4 using data-lanes with csi2 bus
>>>>>> type and driver gets lanes based on either of this from DT.
>>>>>>
>>>>>> Instead should we update endpoint parse API for max up to 8 lanes for
>>>>>> data-lanes?
>>>>> Yes, please. Could you send a patch?
>>>>>
>>>>> The standard AFAIK supports up to four lanes but as we know, hardware
>>>>> sometimes has more than that.
>>>> Sure once Hans also agrees with this to have it as single x8 port (just
>>>> like I have now in v2), will send v3 to update endpoint parse to allow
>>>> upto max 8 data-lanes and will also update Tegra CSI driver accordingly
>>>> to retrieve lanes using csi2 bus type.
>>>>
>>>> Hans, Please confirm if you agree with this.
>>>>
>>> I'm not sure if I agree with this. Shouldn't a device tree reflect the
>>> hardware? And how would you represent the use case where the ganging
>>> mode stitches together two synced sensors (left and right) into a single
>>> 3D side-by-side image? Then you would have:
>>>
>>>   Left Sensor TX -> CSI RX0 (x4 port)
>>> Right Sensor TX -> CSI RX1 (x4 port)
>>>
>>> And for the tc358840 something similar might be true: in the case of the
>>> Tegra you have this nice ganging mode available, but for other SoCs each
>>> half would have to go to a separate CSI port and captured via a separate
>>> video DMA channel, and software or a GPU is needed to combine the two
>>> halves.
>>>
>>> In both these examples it is my understanding that you have to model this
>>> in the DT as separate x4 ports.
>> Do note that a "port" as such is a logical concept. On modern hardware, a
>> port consists of two or more lanes --- one clock, plus at least one data
>> lane. Perhaps an example could be useful. For instance, if you have ten
>> lanes on a device, this could be split into following configurations, based
>> on the board design:
>>
>> configuration \ data lanes	port 0	port 1	port 2	port 3
>>
>> 1:				4	4
>> 2:				4	2	1
>> 3:				2	2	2
>> 4:				2	2	1	1
>>
>> So if you add one more, say:
>>
>> 5:				8
>>
>> So what we're discussing is just how the lanes are distributed across the
>> ports.
>>
>> There are usually hardware specific limitations how the lanes can be
>> distributed. The interface we have in DT (data-lanes + clock-lanes
>> properties) allows describing the hardware in general case, so what the
>> interface allows may not be possible in hardware, but what hardware
>> implements is supported by the interface.
>>
> So for this particular instance using a single logical 8-lane port would
> make sense, but in the two other scenarios (left/right sensor or supporting
> tc358840 in a SoC that doesn't support ganging) I described you would still
> have to model it as two 4-lane ports.
>
> Is that correct?
>
> Regards,
>
> 	Hans

Hi Hans,

As its a single image split here, even it comes from 2 TX ports it goes 
through same video channel where we adjust video buffer offset to align 
side-by-side for combined image.

So if any SoC does not support multiple independent ports (by HW or 
logically), then like Sakari mentioned we expose both x4 ports but in 
that case they both should be exposed to different video channel (DMA 
Buffer allocations).

With this SW should manage to combine captures from these 2 independent 
ports and I am not sure if this is feasible or if we ever had this case 
implemented by any SoC SW so far as SW overhead will impact as well but 
this is different issue.

But isn't most SoC, CSI RX ports are similar instances allowing parallel 
captures as any Receiver supporting multiple ports allows multiple 
streaming right?