linux-kernel - Re: [PATCH 5.9 14/49] net: stmmac: dwmac-meson8b: fix mask definition of the m250

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Date:   Sun, 20 Dec 2020 00:13:13 +0100
From:   Pavel Machek <pavel@....cz>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jerome Brunet <jbrunet@...libre.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 5.9 14/49] net: stmmac: dwmac-meson8b: fix mask
 definition of the m250_sel mux

On Sat 2020-12-19 23:38:25, Martin Blumenstingl wrote:
> Hi Pavel,
> 
> On Sat, Dec 19, 2020 at 10:51 PM Pavel Machek <pavel@....cz> wrote:
> [...]
> > I can't say I like this one:
> >
> >
> > >       clk_configs->m250_mux.reg = dwmac->regs + PRG_ETH0;
> > > -     clk_configs->m250_mux.shift = PRG_ETH0_CLK_M250_SEL_SHIFT;
> > > -     clk_configs->m250_mux.mask = PRG_ETH0_CLK_M250_SEL_MASK;
> > > +     clk_configs->m250_mux.shift = __ffs(PRG_ETH0_CLK_M250_SEL_MASK);
> >
> > It replaces constant with computation done at runtime; compiler can't
> > optimize it as __ffs is implemented with asm().
> what do you suggest to use instead?
> personally I don't see a problem because this is only called once at
> driver probe time.

I believe canonical solution is version before this patch, just with
fixed values....

I mean yes, computation at runtime is not end of the world, but it is
both slower and needs more code space...

Best regards,

								Pavel
-- 
http://www.livejournal.com/~pavelmachek

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