linux-kernel - Re: [PATCH v3] spmi: mtk-pmif: Serialize PMIF status check and command submission

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Message-ID: <7b407eda-702a-444a-b644-5e8dc31f3991@notapiano>
Date:   Tue, 19 Sep 2023 16:12:23 -0400
From:   Nícolas F. R. A. Prado 
        <nfraprado@...labora.com>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Chen-Yu Tsai <wenst@...gle.com>, kernel@...labora.com,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hsin-Hsiung Wang <hsin-hsiung.wang@...iatek.com>,
        James Lo <james.lo@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v3] spmi: mtk-pmif: Serialize PMIF status check and
 command submission

On Thu, Aug 10, 2023 at 06:03:53PM -0400, Nícolas F. R. A. Prado wrote:
> On Mon, Jul 24, 2023 at 11:47:33AM -0400, Nícolas F. R. A. Prado wrote:
> > Before writing the read or write command to the SPMI arbiter through the
> > PMIF interface, the current status of the channel is checked to ensure
> > it is idle. However, since the status only changes from idle when the
> > command is written, it is possible for two concurrent calls to determine
> > that the channel is idle and simultaneously send their commands. At this
> > point the PMIF interface hangs, with the status register no longer being
> > updated, and thus causing all subsequent operations to time out.
> > 
> > This was observed on the mt8195-cherry-tomato-r2 machine, particularly
> > after commit 46600ab142f8 ("regulator: Set PROBE_PREFER_ASYNCHRONOUS for
> > drivers between 5.10 and 5.15") was applied, since then the two MT6315
> > devices present on the SPMI bus would probe assynchronously and
> > sometimes (during probe or at a later point) read the bus
> > simultaneously, breaking the PMIF interface and consequently slowing
> > down the whole system.
> > 
> > To fix the issue at its root cause, introduce locking around the channel
> > status check and the command write, so that both become an atomic
> > operation, preventing race conditions between two (or more) SPMI bus
> > read/write operations. A spinlock is used since this is a fast bus, as
> > indicated by the usage of the atomic variant of readl_poll, and
> > '.fast_io = true' being used in the mt6315 driver, so spinlocks are
> > already used for the regmap access.
> > 
> > Fixes: b45b3ccef8c0 ("spmi: mediatek: Add support for MT6873/8192")
> > Signed-off-by: Nícolas F. R. A. Prado <nfraprado@...labora.com>
> 
> Hi,
> 
> gentle ping on this one. MT8195 Chromebooks sometimes boot to a broken state
> without it.

Hi Stephen,

once again kindling reminding you of this important fix for MediaTek platforms,
solving a real issue impacting basic functionality observed on the MT8195
Chromebook.

Thanks,
Nícolas