Re: [PATCH 1/2] Add signal strength and bandwith to nl80211station info

[Henning Rogge <hrogge@googlemail.com>]

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On Monday 08 December 2008 20:43:35 you wrote:
> And on your second patch:
> > +static u16 nl80211_calculate_bitrate(struct rate_info *rate)
> > +{
> > +       int modulation, streams, bitrate;
> > +
> > +       if (!(rate->flags & RATE_INFO_FLAGS_MCS))
> > +               return rate->legacy;
> > +
> > +       modulation = rate->mcs & 7;
> > +       streams = rate->mcs >> 3;

> I don't think this gives correct results for MCS rates 32 through 76,
> does it? It should at least not calculate anything then. If we did it in
> userspace then programs could also actually calculate the correct rate
> as a float, rather than this approximation. Not that it'll matter, I
> guess. Doesn't anybody know how to do the actual calculation?
It had a small type (corrected in the new patches), but I think the values are 
good. I have attached a test programm to this mail that outputs the bandwith 
values calculated by the function.

Henning


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#include <stdio.h>

#define RATE_INFO_FLAGS_MCS 1
#define RATE_INFO_FLAGS_40_MHZ_WIDTH 2
#define RATE_INFO_FLAGS_SHORT_GI 4

struct rate_info {
	int flags, mcs, legacy;
};

int nl80211_calculate_bitrate(struct rate_info *rate)
{
	int modulation, streams, bitrate;

	if (!(rate->flags & RATE_INFO_FLAGS_MCS))
		return rate->legacy;

	modulation = rate->mcs & 7;
	streams = (rate->mcs >> 3) + 1;

	bitrate = (rate->flags & RATE_INFO_FLAGS_40_MHZ_WIDTH) ?
			13500000 : 6500000;

	if (modulation < 4)
		bitrate *= (modulation + 1);
	else if (modulation == 4)
		bitrate *= (modulation + 2);
	else
		bitrate *= (modulation + 3);

	bitrate *= streams;

	if (rate->flags & RATE_INFO_FLAGS_SHORT_GI)
		bitrate = (bitrate / 9) * 10;

	/* do NOT round down here */
	return (bitrate + 50000) / 100000;
}

int main(int argc, char **argv) {
	struct rate_info ri;
	int mcs;

	printf("20 Mhz wide channels\n");
	for (mcs = 0; mcs < 72; mcs++) {
		int r1, r2;
		
		ri.mcs = mcs;
		
		ri.flags = RATE_INFO_FLAGS_MCS;
		r1 = nl80211_calculate_bitrate(&ri);

		ri.flags = RATE_INFO_FLAGS_MCS | RATE_INFO_FLAGS_SHORT_GI;
		r2 = nl80211_calculate_bitrate(&ri);

		printf("mcs %d:    long gi %4d / short gi %4d\n", mcs, r1, r2);
	}
	
	printf("\n");
	printf("40 Mhz wide channels\n");

	for (mcs = 0; mcs < 72; mcs++) {
		int r1, r2;

		ri.mcs = mcs;
		
		ri.flags = RATE_INFO_FLAGS_MCS | RATE_INFO_FLAGS_40_MHZ_WIDTH;
		r1 = nl80211_calculate_bitrate(&ri);

		ri.flags = RATE_INFO_FLAGS_MCS | RATE_INFO_FLAGS_SHORT_GI | RATE_INFO_FLAGS_40_MHZ_WIDTH;
		r2 = nl80211_calculate_bitrate(&ri);

		printf("mcs %d:    long gi %4d / short gi %4d\n", mcs, r1, r2);
	}
	
}

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