Re: SDR sampling rate - control or IOCTL?

[Mauro Carvalho Chehab <m.chehab@samsung.com>]

Em Thu, 21 Nov 2013 19:05:05 +0200
Antti Palosaari <crope@iki.fi> escreveu:

> Hello
> I am adding new property for sampling rate that is ideally the only 
> obligatory parameter required by SDR. It is value that could be only 
> positive and bigger the better, lets say unsigned 64 bit is quite ideal. 
> That value sets maximum radio frequency possible to receive (ideal SDR).
> 
> Valid values are not always in some single range from X to Y, there 
> could be some multiple value ranges.
> 
> For example possible values: 1000-2000, 23459, 900001-2800000
> 
> Reading possible values from device could be nice, but not necessary. 
> Reading current value is more important.
> 
> Here is what I though earlier as a requirements:
> 
> sampling rate
> *  values: 1 - infinity (unit: Hz, samples per second)
>       currently 500 MHz is more than enough
> *  operations
>       GET, inquire what HW supports
>       GET, get current value
>       SET, set desired value
> 
> 
> I am not sure what is best way to implement that kind of thing.
> IOCTL like frequency
> V4L2 Control?
> put it into stream format request?
> 
> Sampling rate is actually frequency of ADC. As there devices has almost 
> always tuner too (practical SDR) there is need for tuner frequency too. 
> As tuner is still own entity, is it possible to use same frequency 
> parameter for both ADC and RF tuner in same device?

Well, a SDR capture device will always have ADC and RF tuner.

A SDR output device will always have a DAC and a RF transmitter.

On both cases, the sampling rate and the sampling format are mandatory
arguments.

In any case, the V4L2 API has already support for setting the mandatory
parameters of the expected stream, at struct v4l2_format.

So, it makes sense do do:

 struct v4l2_format {
         __u32    type;
         union {
                 struct v4l2_pix_format          pix;     /* V4L2_BUF_TYPE_VIDEO_CAPTURE */
                 struct v4l2_pix_format_mplane   pix_mp;  /* V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE */
                 struct v4l2_window              win;     /* V4L2_BUF_TYPE_VIDEO_OVERLAY */
                 struct v4l2_vbi_format          vbi;     /* V4L2_BUF_TYPE_VBI_CAPTURE */
                 struct v4l2_sliced_vbi_format   sliced;  /* V4L2_BUF_TYPE_SLICED_VBI_CAPTURE */
+                struct v4l2_sdr_format          sdr;     /* V4L2_BUF_TYPE_SDR_CAPTURE */
                 __u8    raw_data[200];                   /* user-defined */
         } fmt;
 };

And add the mandatory parameters for SDR inside its own structure, e. g.
struct v4l2_sdr_format. Of course, the meta-data provided by a SDR device
is different than the one for video or vbi, so you'll need to add a new
streaming type for SDR anyway.

Btw, that's what I proposed here:

http://git.linuxtv.org/mchehab/experimental.git/blob/refs/heads/sdr:/include/uapi/linux/videodev2.h

With regards to the sampling rate range, my proposal there were to add a min/max
value for it, to be used by VIDIOC_G_FMT, as proposed on:
	http://git.linuxtv.org/mchehab/experimental.git/commitdiff/c3a73f84f038f043aeda5d5bfccc6fea66291451

So, the v4l2_sdr_format should be like:

+struct v4l2_sdr_format {
+       __u32                           sampleformat;
+       __u32                           sample_rate;            /* in Hz */
+       __u32                           min_sample_rate;        /* in Hz */
+       __u32                           max_sample_rate;        /* in Hz */
+
+} __attribute__ ((packed));

Where sampleformat would be something similar to FOURCC, defining the
size of each sample, its format, and if the sampling is in quadradure,
if they're plain PCM samples, or something more complex, like DPCM, RLE,
etc.

In the specific case of enumerating the sampling rate range, if the 
sampling rate can have multiple ranges, then maybe we'll need to do
something more complex like what was done on VIDIOC_ENUM_FRAMESIZES.

Regards,
Mauro