netdev - Re: [PATCH net v2] ice: use proper macro for testing bit

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Date: Mon, 17 Jun 2024 20:04:03 +0100
From: Simon Horman <horms@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Petr Oros <poros@...hat.com>, netdev@...r.kernel.org,
	ivecera@...hat.com, przemyslaw.kitszel@...el.com,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Marcin Szycik <marcin.szycik@...ux.intel.com>,
	Konrad Knitter <konrad.knitter@...el.com>,
	Marcin Domagala <marcinx.domagala@...el.com>,
	"moderated list:INTEL ETHERNET DRIVERS" <intel-wired-lan@...ts.osuosl.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v2] ice: use proper macro for testing bit

On Mon, Jun 17, 2024 at 02:58:59PM +0200, Alexander Lobakin wrote:
> From: Simon Horman <horms@...nel.org>
> Date: Sat, 15 Jun 2024 16:16:41 +0100
> 
> > On Fri, Jun 14, 2024 at 11:43:38AM +0200, Petr Oros wrote:
> >> Do not use _test_bit() macro for testing bit. The proper macro for this
> >> is one without underline.
> > 
> > Hi Petr,
> > 
> > it might be nice to include a brief explanation as to
> > why test_bit() is correct.
> 
> Let me explain this as the author of all those bitops wrappers :D
> Petr is free to include either this or his own brief into v2.
> 
> _test_bit() is what test_bit() was prior to my const-optimization. It
> directly calls arch_test_bit(), i.e. the arch-specific implementation
> (or the generic one). It's strictly _internal_ and shouldn't be used
> anywhere outside the actual test_bit() macro.
> 
> test_bit() is a wrapper which checks whether the bitmap and the bit
> number are compile-time constants and if so, it calls the optimized
> function which evaluates this call to a compile-time constant as well.
> If either of them is not a compile-time constant, it just calls _test_bit().
> test_bit() is the actual function to use anywhere in the kernel.
> 
> IOW, calling _test_bit() avoids potential compile-time optimizations.
> 
> >From what I see in the code, &sensors is not a compile-time constant,
> thus most probably there are no object code changes before and after
> the patch. But anyway, we shouldn't call internal wrappers instead of
> the actual API, so this fix is correct.

Thanks for this very comprehensive description, now I know :)

> >> Fixes: 4da71a77fc3b ("ice: read internal temperature sensor")
> >> Signed-off-by: Petr Oros <poros@...hat.com>
> >> Acked-by: Ivan Vecera <ivecera@...hat.com>
> 
> To be added to v2:
> 
> Reviewed-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> 
> Thanks,
> Olek
>