linux-kernel - Re: [PATCH v2 2/2] clk: divider: Fix divisions

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Message-ID: <66d9eaa2-4eb5-e9d1-cc76-0e0b68be44ca@collabora.com>
Date:   Mon, 29 May 2023 10:50:51 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Christopher Obbard <chris.obbard@...labora.com>,
        David Laight <David.Laight@...LAB.COM>, kernel@...labora.com
Subject: Re: [PATCH v2 2/2] clk: divider: Fix divisions

Il 26/05/23 19:10, Sebastian Reichel ha scritto:
> The clock framework handles clock rates as "unsigned long", so u32 on
> 32-bit architectures and u64 on 64-bit architectures.
> 
> The current code pointlessly casts the dividend to u64 on 32-bit
> architectures and thus pointlessly reducing the performance.
> 
> On the other hand on 64-bit architectures the divisor is masked and only
> the lower 32-bit are used. Thus requesting a frequency >= 4.3GHz results
> in incorrect values. For example requesting 4300000000 (4.3 GHz) will
> effectively request ca. 5 MHz. Requesting clk_round_rate(clk, ULONG_MAX)
> is a bit of a special case, since that still returns correct values as
> long as the parent clock is below 8.5 GHz.
> 
> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>