Re: [PATCH] [media] dib7000p: Remove dead code

[Sean Young]

On Tue, Dec 04, 2018 at 09:57:14AM -0200, Mauro Carvalho Chehab wrote:
> Em Tue, 4 Dec 2018 10:26:40 +0000
> Sean Young <sean@xxxxxxxx> escreveu:
> 
> > On Mon, Sep 17, 2018 at 07:39:36PM -0300, Mauro Carvalho Chehab wrote:
> > > Em Mon, 17 Sep 2018 10:58:32 -0700
> > > Nick Desaulniers <ndesaulniers@xxxxxxxxxx> escreveu:
> > >   
> > > > On Fri, Sep 14, 2018 at 10:47 PM Nathan Chancellor
> > > > <natechancellor@xxxxxxxxx> wrote:  
> > > > >
> > > > > Clang warns that 'interleaving' is assigned to itself in this function.
> > > > >
> > > > > drivers/media/dvb-frontends/dib7000p.c:1874:15: warning: explicitly
> > > > > assigning value of variable of type 'int' to itself [-Wself-assign]
> > > > >         interleaving = interleaving;
> > > > >         ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
> > > > > 1 warning generated.
> > > > >
> > > > > It's correct. Just removing the self-assignment would sufficiently hide
> > > > > the warning but all of this code is dead because 'tmp' is zero due to
> > > > > being multiplied by zero. This doesn't appear to be an issue with
> > > > > dib8000, which this code was copied from in commit 041ad449683b
> > > > > ("[media] dib7000p: Add DVBv5 stats support").
> > > > >
> > > > > Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > > > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > > > > ---
> > > > >  drivers/media/dvb-frontends/dib7000p.c | 10 ++--------
> > > > >  1 file changed, 2 insertions(+), 8 deletions(-)
> > > > >
> > > > > diff --git a/drivers/media/dvb-frontends/dib7000p.c b/drivers/media/dvb-frontends/dib7000p.c
> > > > > index 58387860b62d..25843658fc68 100644
> > > > > --- a/drivers/media/dvb-frontends/dib7000p.c
> > > > > +++ b/drivers/media/dvb-frontends/dib7000p.c
> > > > > @@ -1800,9 +1800,8 @@ static u32 dib7000p_get_time_us(struct dvb_frontend *demod)    
> > > > 
> > > > Something looks wrong here (with this function).  The patch is no
> > > > functional change, since as you point out `interleaving` is
> > > > initialized to 0, then never updated before read, but I think there's
> > > > an underlying bug here that should be fixed differently.  Thanks for
> > > > the patch though, as it does raise the question.
> > > > 
> > > > dib7000p_get_time_us has this comment above it:
> > > > 
> > > >   1798 /* FIXME: may require changes - this one was borrowed from
> > > > dib8000 */  
> > > 
> > > The goal of dib7000p_get_time_us() is to estimate how much time it
> > > takes, with current tuning parameters, to have a certain number of
> > > DVB-T packets. This is used for block error count. That's said,
> > > on a quick look, it seems that the code is not right on many ways.
> > > 
> > > It should be aligned with the amount of data it is required for
> > > dib7000 to update the block/bit error counters. There are two kinds
> > > of implementation:
> > > 
> > > 1) the frontend has an internal counter that it is shifted and made
> > >    available to the driver after a certain amount of received data
> > >    (usually in the order of 10^5 to 10^7 bits);
> > > 
> > > 2) the frontend has an internal timer that shifts the data from its
> > >    internal counter after a certain amount of time (usually at the
> > >    seconds range).
> > > 
> > > Different vendors opt for either one of the strategy. Some updates
> > > a counter with the amount of bits taken. Unfortunately, this is not
> > > the case of those dib* frontends. So, the Kernel has to estimate
> > > it, based on the tuning parameters.
> > > 
> > > From the code, it seems that, for block errors, it waits for 1,250,000
> > > bits to arrive (e. g. about 766 packets), so, it uses type (1) strategy:
> > > 
> > >                 /* Estimate the number of packets based on bitrate */
> > >                 if (!time_us)
> > >                         time_us = dib7000p_get_time_us(demod);
> > > 
> > >                 if (time_us) {
> > >                         blocks = 1250000ULL * 1000000ULL;	// the multiply here is to convert to microsseconds...
> > >                         do_div(blocks, time_us * 8 * 204);	// As here it divides by the time in microsseconds
> > >                         c->block_count.stat[0].scale = FE_SCALE_COUNTER;
> > >                         c->block_count.stat[0].uvalue += blocks;
> > >                 }
> > > 
> > > For BER, the logic assumes that the bit error count should be divided
> > > by 10^-8:
> > > 
> > >                 c->post_bit_count.stat[0].uvalue += 100000000;
> > > 
> > > and the counter is updated every second. So, it uses (2).
> > >   
> > > > 
> > > > Wondering if it has the same bug, it seems it does not:
> > > > drivers/media/dvb-frontends/dib8000.c#dib8000_get_time_us():3986
> > > > 
> > > > dib8000_get_time_us() seems to loop over multiple layers, and then
> > > > assigns interleaving to the final layers interleaving (that looks like
> > > > loop invariant code to me).
> > > > 
> > > > Mauro, should dib7000p_get_time_us() use c->layer[???].interleaving?  
> > > 
> > > I don't think that time interleaving would affect the bit rate.
> > > I suspect that the dead code on dib8000 is just a dead code.
> > >   
> > > > I don't see a single reference to `layer` in
> > > > drivers/media/dvb-frontends/dib7000p.c.  
> > > 
> > > Layers are specific for ISDB-T, but I think DVB-T (or at least DVB-T2)
> > > may use time interleaving. 
> > > 
> > > Yet, as I said, the goal is to estimate the streaming bit rate. 
> > > 
> > > I don't remember anymore from where the dib8000 formula came.
> > > 
> > > My guts tell that time interleaving shouldn't do much changes (if any)
> > > to the bit rate. I suspect that removing the dead code is likely
> > > OK, but I'll try to see if I can find something related to where this
> > > formula came.  
> > 
> > So we have two issues. One is the clang issue and clearly the code needs
> > fixing up. The second issue is that we're not sure about the algorithm;
> > I've been reading up on mpeg-ts but I'm not anywhere near getting to an
> > answer on this.
> > 
> > How about we merge a patch which just fixes the clang issue and leave
> > the rest of the code as-is for now?
> 
> I'm ok with that, but it would be better to add a FIXME note somewhere.
> 
> > 
> > Thanks,
> > 
> > Sean
> > 
> > ---
> > From c6e4c5f514c38511d2054c69f7b103e98c520af4 Mon Sep 17 00:00:00 2001
> > From: Sean Young <sean@xxxxxxxx>
> > Date: Tue, 4 Dec 2018 09:59:10 +0000
> > Subject: [PATCH v2] media: dib7000p: Remove dead code
> > 
> > Clang warns that 'interleaving' is assigned to itself in this function.
> > 
> > drivers/media/dvb-frontends/dib7000p.c:1874:15: warning: explicitly
> > assigning value of variable of type 'int' to itself [-Wself-assign]
> >         interleaving = interleaving;
> >         ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
> > 1 warning generated.
> > 
> > Just remove the self-assign and leave existing code in place for now.
> > 
> > Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Signed-off-by: Sean Young <sean@xxxxxxxx>
> > ---
> >  drivers/media/dvb-frontends/dib7000p.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/drivers/media/dvb-frontends/dib7000p.c b/drivers/media/dvb-frontends/dib7000p.c
> > index 58387860b62d..cd84320c61c9 100644
> > --- a/drivers/media/dvb-frontends/dib7000p.c
> > +++ b/drivers/media/dvb-frontends/dib7000p.c
> > @@ -1871,8 +1871,6 @@ static u32 dib7000p_get_time_us(struct dvb_frontend *demod)
> >  		break;
> >  	}
> >  
> > -	interleaving = interleaving;
> > -
> >  	denom = bits_per_symbol * rate_num * fft_div * 384;
> 
> something like:
> 
> 	/* 
> 	 * FIXME: check if the math makes sense. If so, fill the
> 	 * interleaving var.
> 	 */
> >  
> >  	/* If calculus gets wrong, wait for 1s for the next stats */
> 

Good point. 

Sean