Re: [PATCH RFC] libv4lconvert: SDR conversion from U8 to FLOAT

[Antti Palosaari <crope@iki.fi>]

On 11.11.2013 15:40, Antti Palosaari wrote:
> On 11.11.2013 15:14, Hans Verkuil wrote:
>> On 11/10/2013 06:16 PM, Antti Palosaari wrote:
>>> Convert unsigned 8 to float 32 [-1 to +1], which is commonly
>>> used format for baseband signals.


> I am also going to make some tests to find out if actual float
> conversion is faster against pre-calculated LUT, in Kernel or in
> libv4lconvert and so. Worst scenario I have currently is Mirics ADC with
> 14-bit resolution => 16384 quantization levels => 32-bit float LUT will
> be 16384 * 4 = 65536 bytes. Wonder if that much big LUT is allowed to
> library - but maybe you could alloc() and populate LUT on the fly if
> needed. Or maybe native conversion is fast enough.

That integer to float conversion uses quite much CPU still, even I use 
only 2M sampling rate.

When I do it inside Kernel, in URB completion handler at the same time 
when copying data to videobuf2, using pre-calculated LUTs and using mmap 
it eats 0.5% CPU to transfer stream to app.

When I do same but using libv4lconvert as that patch, it takes ~11% CPU.

And it was only 2M sampling rate, Mirics could go something like 15M.

I wonder if I can optimize libv4lconvert to go near in Kernel LUT 
conversion...

CPU: AMD Phenom(tm) II X4 955 Processor

regards
Antti

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