linux-kernel - Re: [PATCH] ARM: dts: exynos: Disable frequency scaling for FSYS bus

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Date:   Mon, 20 Jul 2020 10:06:01 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Willy Wolff <willy.mh.wolff.ml@...il.com>,
        Marian Mihailescu <mihailescu2m@...il.com>,
        Chanwoo Choi <cw00.choi@...sung.com>
Subject: Re: [PATCH] ARM: dts: exynos: Disable frequency scaling for FSYS bus

On Tue, Jul 14, 2020 at 08:47:59AM +0200, Marek Szyprowski wrote:
> Commit 1019fe2c7280 ("ARM: dts: exynos: Adjust bus related OPPs to the
> values correct for Exynos5422 Odroids") changed the parameters of the
> OPPs for the FSYS bus. Besides the frequency adjustments, it also removed
> the 'shared-opp' property from the OPP table used for FSYS_APB and FSYS
> busses.
> 
> This revealed that in fact the FSYS bus frequency scaling never worked.
> When one OPP table is marked as 'opp-shared', only the first bus which
> selects the OPP sets the rate of its clock. Then OPP core assumes that
> the other busses have been changed to that OPP and no change to their
> clock rates are needed. Thus when FSYS_APB bus, which was registered
> first, set the rate for its clock, the OPP core did not change the FSYS
> bus clock later.
> 
> The mentioned commit removed that behavior, what introduced a regression
> on some OdroidXU3 boards. Frequency scaling of the FSYS bus causes
> instability of the USB host operation, what can be observed as network
> hangs. To restore old behavior, simply disable frequency scaling for the
> FSYS bus.
> 
> Reported-by: Willy Wolff <willy.mh.wolff.ml@...il.com>
> Fixes: 1019fe2c7280 ("ARM: dts: exynos: Adjust bus related OPPs to the values correct for Exynos5422 Odroids")
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  arch/arm/boot/dts/exynos5422-odroid-core.dtsi | 6 ------
>  1 file changed, 6 deletions(-)

Thanks Willy and Marek for digging into this, applied.

A follow up question would be if other boards (Exynos4412) require the
same?

Best regards,
Krzysztof