Re: [RFC 5/5] doc_rst: media: New SDR formats SC16, SC18 & SC20

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On 11/11/2016 02:57 PM, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Friday 11 Nov 2016 14:53:58 Hans Verkuil wrote:
>> On 11/10/2016 09:08 AM, Laurent Pinchart wrote:
>>> Antti, Hans, ping ? Please see below.
>>>
>>> On Friday 04 Nov 2016 09:23:29 Ramesh Shanmugasundaram wrote:
>>>>> On 11/02/2016 10:58 PM, Laurent Pinchart wrote:
>>>>>> On Wednesday 02 Nov 2016 09:00:00 Ramesh Shanmugasundaram wrote:
>>>>>>>>> On Wednesday 12 Oct 2016 15:10:29 Ramesh Shanmugasundaram wrote:
>>>>>>>>>> This patch adds documentation for the three new SDR formats
>>>>>>>>>>
>>>>>>>>>> V4L2_SDR_FMT_SCU16BE
>>>>>>>>>> V4L2_SDR_FMT_SCU18BE
>>>>>>>>>> V4L2_SDR_FMT_SCU20BE
>>>>>>>
>>>>>>> [snip]
>>>>>>>
>>>>>>>>>> +
>>>>>>>>>> +       -  start + 0:
>>>>>>>>>> +
>>>>>>>>>> +       -  I'\ :sub:`0[D13:D6]`
>>>>>>>>>> +
>>>>>>>>>> +       -  I'\ :sub:`0[D5:D0]`
>>>>>>>>>> +
>>>>>>>>>> +    -  .. row 2
>>>>>>>>>> +
>>>>>>>>>> +       -  start + buffer_size/2:
>>>>>>>>>> +
>>>>>>>>>> +       -  Q'\ :sub:`0[D13:D6]`
>>>>>>>>>> +
>>>>>>>>>> +       -  Q'\ :sub:`0[D5:D0]`
>>>>>>>>>
>>>>>>>>> The format looks planar, does it use one V4L2 plane (as does NV12)
>>>>>>>>> or two V4L2 planes (as does NV12M) ? Same question for the other
>>>>>>>>> formats.
>>>>>>>>
>>>>>>>> Thank you for bringing up this topic. This is one of the key design
>>>>>>>> dilemma.
>>>>>>>>
>>>>>>>> The I & Q data for these three SDR formats comes from two different
>>>>>>>> DMA channels and hence two separate pointers -> we could say it is
>>>>>>>> v4l2 multi- planar. Right now, I am making it look like a single
>>>>>>>> plane by presenting the data in one single buffer ptr.
>>>>>>>>
>>>>>>>> For e.g. multi-planar SC16 format would look something like this
>>>>>>>>
>>>>>>>> <------------------------32bits---------------------->
>>>>>>>> <--I(14 bit data) + 2bit status--16bit padded zeros--> : start0 + 0
>>>>>>>> <--I(14 bit data) + 2bit status--16bit padded zeros--> : start0 + 4
>>>>>>>> ...
>>>>>>>> <--Q(14 bit data) + 2bit status--16bit padded zeros--> : start1 + 0
>>>>>>>> <--Q(14 bit data) + 2bit status--16bit padded zeros--> : start1 + 4
>>>>>>>>
>>>>>>>> My concerns are
>>>>>>>>
>>>>>>>> 1) These formats are not a standard as the video "Image Formats".
>>>>>>>> These formats are possible when we use DRIF + MAX2175 combination.
>>>>>>>> If we interface with a different tuner vendor, the above format(s)
>>>>>>>> MAY/MAY NOT be re-usable. We do not know at this point. This is the
>>>>>>>> main open item for discussion in the cover letter.
>>>>>>
>>>>>> If the formats are really device-specific then they should be
>>>>>> documented accordingly and not made generic.
>>>>>>
>>>>>>>> 2) MPLANE support within V4L2 seems specific to video. Please
>>>>>>>> correct me if this is wrong interpretation.
>>>>>>>>
>>>>>>>> - struct v4l2_format contains v4l2_sdr_format and
>>>>>>>> v4l2_pix_format_mplane as members of union. Should I create a new
>>>>>>>> v4l2_sdr_format_mplane? If I have to use v4l2_pix_format_mplane most
>>>>>>>> of the video specific members would be unused (it would be similar
>>>>>>>> to using v4l2_pix_format itself instead of v4l2_sdr_format)?
>>>>>>
>>>>>> I have no answer to that question as I'm not familiar with SDR. Antti,
>>>>>> you've added v4l2_sdr_format to the API, what's your opinion ? Hans,
>>>>>> as you've acked the patch, your input would be appreciated as well.
>>>>>
>>>>> If I understood correctly this hardware provides I and Q samples via
>>>>> different channels and driver now combines those channels as a
>>>>> sequential
>>>>> IQ sample pairs.
>>>>
>>>> The driver combines the two buffer ptrs and present as one single buffer.
>>>> For a buffer of size 200
>>>>
>>>> ptr + 0   : I I I I ... I
>>>> ptr + 100 : Q Q Q Q ... Q
>>>>
>>>>> I have never seen any other than hw which provides IQ IQ IQ IQ ... IQ.
>>>>
>>>> There are some modes where this h/w combo can also do IQ IQ IQ pattern.
>>>> Those modes are not added in the RFC patchset.
>>>>
>>>>> This is
>>>>> I I I I ... I
>>>>> Q Q Q Q ... Q
>>>>> I am not very familiar with planars, but it sounds like it is correct
>>>>> approach. So I think should be added rather than emulate packet
>>>>> sequential format.
>>>>
>>>> My understanding of V4L2 MPLANE constructs is limited to a quick code
>>>> read
>>>> only. At this point MPLANE support seems specific to video. SDR is
>>>> defined
>>>> as separate format like v4l2_pix_format. Questions would be - should we
>>>> define new SDR_MPLANE? or merge SDR format with pix format & reuse
>>>> existing
>>>> MPLANE with some SDR extensions (if possible)? These seem big design
>>>> decisions. Any suggestions please?
>>>>
>>>> For my use case, MPLANE support does not seem to add significant benefit
>>>> except it may be syntactically correct. I am doing cyclic DMA with a
>>>> small
>>>> set of h/w buffers and copying each stage to one mmapped vmalloc
>>>> vb2_buffer
>>>> at two offsets. If we add MPLANE support, it can be two non-contiguous
>>>> buffer pointers.
>>>>
>>>>>>>> - The above decision (accomodate SDR & MPLANE) needs to be
>>>>>>>> propagated across the framework. Is this the preferred approach?
>>>>>>>>
>>>>>>>> It goes back to point (1). As of today, the change set for this
>>>>>>>> combo (DRIF+MAX2175) introduces new SDR formats only. Should it add
>>>>>>>> further SDR+MPLANE support to the framework as well?
>>>>>>>>
>>>>>>>> I would appreciate your suggestions on this regard.
>>
>> Some terminology first:
>>
>> Planar formats separate the data into different memory areas: in this case
>> one part is all I and one part is all Q. This as opposed to interleaved
>> formats (IQIQIQIQ....).
>>
>> As long as both planes fit in the same buffer all is fine. Since that is
>> the case here there is no need to introduce a new MPLANE API.
>>
>> The MPLANE API was added for video to handle cases where the two planes
>> had to be in two different non-contiguous buffers.
> 
> Not only that, it can also be used for cases where storing the two planes in 
> separate buffers can be beneficial, even if a single contiguous buffer could 
> work.
> 
>> So instead of passing one buffer pointer, you need to pass two or more
>> buffer pointers.
>>
>> In hindsight we should have called it the MBUFFER API.
> 
> The name was badly chosen, yes.
> 
>> Oh well...
>>
>> Anyway, since there is no problem here apparently to keep both planes
>> in one buffer there is also no need to introduce a SDR_MPLANE.
> 
> The question here is whether there could be a benefit in separating I and Q 
> data in two buffers compared to storing them in the same buffer.
> 

The MPLANE API is very messy and introducing something like SDR_MPLANE is not
something I would promote. If we want that, then we should first make a new
v4l2_buffer struct that simplifies MPLANE handling (we discussed that before).

Regards,

	Hans
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