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 22:51:39 +0100
From:   Pavel Machek <pavel@....cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        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!

> From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> 
> [ Upstream commit 82ca4c922b8992013a238d65cf4e60cc33e12f36 ]
> 
> The m250_sel mux clock uses bit 4 in the PRG_ETH0 register. Fix this by
> shifting the PRG_ETH0_CLK_M250_SEL_MASK accordingly as the "mask" in
> struct clk_mux expects the mask relative to the "shift" field in the
> same struct.
> 
> While here, get rid of the PRG_ETH0_CLK_M250_SEL_SHIFT macro and use
> __ffs() to determine it from the existing PRG_ETH0_CLK_M250_SEL_MASK
> macro.

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().

Best regards,
								Pavel
-- 
http://www.livejournal.com/~pavelmachek

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