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

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Date:   Sat, 19 Dec 2020 23:38:25 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Pavel Machek <pavel@....cz>
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

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.


Best regards,
Martin