linux-kernel - Re: [PATCH v3 26/26] media: dvb-frontends: tda10048: Make the range of z explicit.

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Message-ID: <c4287024-8012-458d-9829-15ffbceb25cf@moroto.mountain>
Date: Fri, 3 May 2024 14:55:09 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: Ricardo Ribalda <ribalda@...omium.org>,
	Martin Tuma <martin.tuma@...iteqautomotive.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Hugues Fruchet <hugues.fruchet@...s.st.com>,
	Alain Volmat <alain.volmat@...s.st.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Chen-Yu Tsai <wens@...e.org>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Samuel Holland <samuel@...lland.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Jonathan Hunter <jonathanh@...dia.com>,
	Sowjanya Komatineni <skomatineni@...dia.com>,
	Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
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	Hans Verkuil <hverkuil@...all.nl>, Sergey Kozlov <serjk@...up.ru>,
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	Vikash Garodia <quic_vgarodia@...cinc.com>,
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	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v3 26/26] media: dvb-frontends: tda10048: Make the range
 of z explicit.

On Fri, May 03, 2024 at 11:27:58AM +0100, Mauro Carvalho Chehab wrote:
> Em Mon, 29 Apr 2024 15:05:05 +0000
> Ricardo Ribalda <ribalda@...omium.org> escreveu:
> 
> > We do not expect the sample_freq to be over 613MHz.
> > 
> > Found by cocci:
> > drivers/media/dvb-frontends/tda10048.c:345:1-7: WARNING: do_div() does a 64-by-32 division, please consider using div64_u64 instead.
> > 
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/dvb-frontends/tda10048.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/media/dvb-frontends/tda10048.c b/drivers/media/dvb-frontends/tda10048.c
> > index 3e725cdcc66b..1886f733dbbf 100644
> > --- a/drivers/media/dvb-frontends/tda10048.c
> > +++ b/drivers/media/dvb-frontends/tda10048.c
> > @@ -328,7 +328,8 @@ static int tda10048_set_wref(struct dvb_frontend *fe, u32 sample_freq_hz,
> >  			     u32 bw)
> >  {
> >  	struct tda10048_state *state = fe->demodulator_priv;
> > -	u64 t, z;
> > +	u32 z;
> > +	u64 t;
> >  
> >  	dprintk(1, "%s()\n", __func__);
> >  
> > @@ -341,6 +342,7 @@ static int tda10048_set_wref(struct dvb_frontend *fe, u32 sample_freq_hz,
> >  	/* t *= 2147483648 on 32bit platforms */
> >  	t *= (2048 * 1024);
> >  	t *= 1024;
> > +	/* Sample frequency is under 613MHz */
> 
> Are you sure about that? Some DVB devices have very high frequency 
> clocks, specially if they're also used for satellite, so I can't
> be sure by just looking at the driver's code.
> 
> Also, we had already a bunch of regressions with "fixes" like this
> that actually broke frontend drivers.

This patch preserves the existing behavior. The sample_freq_hz variable
is a u32 so, in the original code, z couldn't have been more than
U32_MAX even though it was declared as a u64.

It's possible that the original code was wrong.  We have seen that in
other places in this patchset.  Adding a note about the datasheet is
also a good idea.

regards,
dan carpenter