On Nov 25 Stefan Richter wrote: > On Nov 25 Jean Delvare wrote: > > Hi Manu, > > > > On Sun, 24 Nov 2013 22:51:33 +0530, Manu Abraham wrote: > > > Sorry, that I came upon this patch quite late. > > > > No problem, better late than never! :) > > > > > On Mon, Jun 3, 2013 at 8:51 PM, Jean Delvare <khali@xxxxxxxxxxxx> wrote: > > > > SNR is supposed to be reported by the frontend drivers in dB, so print > > > > it that way for drivers which implement it properly. > > > > > > Not all frontends do report report the SNR in dB. Well, You can say quite > > > some frontends do report it that way. > > > > Last time I discussed this, I was told that this was the preferred way > > for frontends to report the SNR. I also referred to this document: > > http://palosaari.fi/linux/v4l-dvb/snr_2012-05-21.txt > > I don't know now up-to-date it is by now, but back then it showed a > > significant number of frontends reporting in .1 dB already, including > > the ones I'm using right now (drx-3916k and drx-3913k.) With the > > current version of femon, "femon -H" reports it as 0%, which is quite > > useless. Thus my patch. > [...] > > Hi, > > I inherited this in drivers/media/firewire/firedtv-fe.c: > > static int fdtv_read_snr(struct dvb_frontend *fe, u16 *snr) > { > struct firedtv *fdtv = fe->sec_priv; > struct firedtv_tuner_status stat; > > if (avc_tuner_status(fdtv, &stat)) > return -EINVAL; > > /* C/N[dB] = -10 * log10(snr / 65535) */ > *snr = stat.carrier_noise_ratio * 257; > return 0; > } > > As far as I understand, the comment should have been written with a "FIXME" > prefix. > > I have no documentation and no personal manufacturer contact (and the > devices are EOL). All I know from the driver source is that we do get a 16 > bits wide carrier_noise_ratio. So it appears to be something on a scale > from 0x0000 to 0xffff, and the comment makes it look like being on a linear > scale originally. Or I got it wrong and all the comment tries to tell that the value to be returned in the snr argument is meant to be linear on a scale of 0...ffff, and the code tells that we get a linear value on a scale of 0...255 from the firmware. (Cc'ing Henrik who added the code and the comment.) > I could cross-check with a Windows based TV viewer application what signal > strength value is presented there to the user with DVB-T and DVB-S2 > incarnations of FireDTV devices. Right now I don't remember how that > application presents it (i.e. as percentage or dB or whatever...). > When I looked at that application and at kaffeine some years ago, they > displayed grossly different values. I did not research back then whether > the Linux driver or kaffeine or both treated it wrong. > > Any advice for the quoted kernel driver code? > > Thanks, -- Stefan Richter -=====-===-= =-== ==--= http://arcgraph.de/sr/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
