linux-kernel - Re: [tip: objtool/urgent] objtool, media: dib8000: Prevent divide-by-zero in dib8000_set

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Message-ID: <w636w4p7xtg4v2lbuic7dnxs7how7mqp2ipkp5xyvfbqr3idvs@oxnon2piapsl>
Date: Tue, 25 Mar 2025 18:46:16 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc: tip-bot2 for Josh Poimboeuf <tip-bot2@...utronix.de>, 
	linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org, 
	kernel test robot <lkp@...el.com>, Ingo Molnar <mingo@...nel.org>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Subject: Re: [tip: objtool/urgent] objtool, media: dib8000: Prevent
 divide-by-zero in dib8000_set_dds()

On Wed, Mar 26, 2025 at 06:42:39AM +0800, Mauro Carvalho Chehab wrote:
> > +		u32 internal = dib8000_read32(state, 23) / 1000;
> > +
> >  		ratio = 4;
> > -		unit_khz_dds_val = (1<<26) / (dib8000_read32(state, 23) / 1000);
> > +
> > +		unit_khz_dds_val = (1<<26) / (internal ?: 1);
> 
> This is theoretical, as in practice dib8096 won't likely be tuning
> if reading this register would return zero, but at least for my
> eyes, it would sound better to set unit_khz_dds_val to 1 internal
> is zero, instead of 1<<26. 

I don't pretend to understand this device, I just figured one is the
closest you can get to zero :-)

So something like this instead?

diff --git a/drivers/media/dvb-frontends/dib8000.c b/drivers/media/dvb-frontends/dib8000.c
index cfe59c3255f7..c80134ff511b 100644
--- a/drivers/media/dvb-frontends/dib8000.c
+++ b/drivers/media/dvb-frontends/dib8000.c
@@ -2705,7 +2705,7 @@ static void dib8000_set_dds(struct dib8000_state *state, s32 offset_khz)
 
 		ratio = 4;
 
-		unit_khz_dds_val = (1<<26) / (internal ?: 1);
+		unit_khz_dds_val = internal ? ((1<<26) / internal) : 1;
 		if (offset_khz < 0)
 			dds = (1 << 26) - (abs_offset_khz * unit_khz_dds_val);
 		else