linux-kernel - Re: [PATCH v2 1/2] clk: composite: Fix handling of high clock rates

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Message-ID: <89b3e3bb-d725-1f92-7a0d-b5bc1109dafc@collabora.com>
Date:   Mon, 29 May 2023 10:50:50 +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,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] clk: composite: Fix handling of high clock rates

Il 26/05/23 19:10, Sebastian Reichel ha scritto:
> ULONG_MAX is used by a few drivers to figure out the highest available
> clock rate via clk_round_rate(clk, ULONG_MAX). Since abs() takes a
> signed value as input, the current logic effectively calculates with
> ULONG_MAX = -1, which results in the worst parent clock being chosen
> instead of the best one.
> 
> For example on Rockchip RK3588 the eMMC driver tries to figure out
> the highest available clock rate. There are three parent clocks
> available resulting in the following rate diffs with the existing
> logic:
> 
> GPLL:   abs(18446744073709551615 - 1188000000) = 1188000001
> CPLL:   abs(18446744073709551615 - 1500000000) = 1500000001
> XIN24M: abs(18446744073709551615 -   24000000) =   24000001
> 
> As a result the clock framework will promote a maximum supported
> clock rate of 24 MHz, even though 1.5GHz are possible. With the
> updated logic any casting between signed and unsigned is avoided
> and the numbers look like this instead:
> 
> GPLL:   18446744073709551615 - 1188000000 = 18446744072521551615
> CPLL:   18446744073709551615 - 1500000000 = 18446744072209551615
> XIN24M: 18446744073709551615 -   24000000 = 18446744073685551615
> 
> As a result the parent with the highest acceptable rate is chosen
> instead of the parent clock with the lowest one.
> 
> Cc: stable@...r.kernel.org
> Fixes: 49502408007b ("mmc: sdhci-of-dwcmshc: properly determine max clock on Rockchip")
> Tested-by: Christopher Obbard <chris.obbard@...labora.com>
> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>

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