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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 4 Dec 2018 10:54:28 -0800
Message-ID: <CAKwvOdkx87V6Yo9Pmh44Sfr_etSBR_b77aMj79p4U-VCoCQz8Q@mail.gmail.com>
Subject: Re: [PATCH] [media] dib7000p: Remove dead code
To:     sean@mess.org
Cc:     mchehab+samsung@kernel.org,
        Nathan Chancellor <natechancellor@gmail.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
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On Tue, Dec 4, 2018 at 5:39 AM Sean Young <sean@mess.org> wrote:
>
> 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@mess.org> 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@google.com> escreveu:
> > > >
> > > > > On Fri, Sep 14, 2018 at 10:47 PM Nathan Chancellor
> > > > > <natechancellor@gmail.com> 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@google.com>
> > > > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > > > > > ---
> > > > > >  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@mess.org>
> > > 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@google.com>
> > > Signed-off-by: Sean Young <sean@mess.org>

Thanks for taking the time to revisit the warning and clean it up.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Might be nice to give Nathan some credit when applied:
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>

> > > ---
> > >  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
>
> From a31c18315830da40561db6443d3b90b8584d5232 Mon Sep 17 00:00:00 2001
> From: Sean Young <sean@mess.org>
> Date: Tue, 4 Dec 2018 09:59:10 +0000
> Subject: [PATCH v3] 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@google.com>
> Signed-off-by: Sean Young <sean@mess.org>
> ---
>  drivers/media/dvb-frontends/dib7000p.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/dvb-frontends/dib7000p.c b/drivers/media/dvb-frontends/dib7000p.c
> index 58387860b62d..2818e8def1b3 100644
> --- a/drivers/media/dvb-frontends/dib7000p.c
> +++ b/drivers/media/dvb-frontends/dib7000p.c
> @@ -1871,10 +1871,13 @@ static u32 dib7000p_get_time_us(struct dvb_frontend *demod)
>                 break;
>         }
>
> -       interleaving = interleaving;
> -
>         denom = bits_per_symbol * rate_num * fft_div * 384;
>
> +       /*
> +        * FIXME: check if the math makes sense. If so, fill the
> +        * interleaving var.
> +        */
> +
>         /* If calculus gets wrong, wait for 1s for the next stats */
>         if (!denom)
>                 return 0;
> --
> 2.19.2
>


-- 
Thanks,
~Nick Desaulniers