linux-kernel - Re: [PATCH 03/11] firmware: qcom-scm: atomically assign and read the global _

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Date:   Tue, 17 Oct 2023 10:29:58 +0200
From:   Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
To:     Om Prakash Singh <quic_omprsing@...cinc.com>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Alex Elder <elder@...aro.org>,
        Srini Kandagatla <srinivas.kandagatla@...aro.org>,
        kernel@...cinc.com, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 03/11] firmware: qcom-scm: atomically assign and read the
 global __scm pointer

On Tue, 17 Oct 2023 at 10:25, Om Prakash Singh
<quic_omprsing@...cinc.com> wrote:
>
>
>
> On 8/29/2023 12:54 AM, Bartosz Golaszewski wrote:
> > Checking for the availability of SCM bridge can happen from any context.
> > It's only by chance that we haven't run into concurrency issues but with
> > the upcoming SHM Bridge driver that will be initiated at the same
> > initcall level, we need to assure the assignment and readback of the
> > __scm pointer is atomic.
> >
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > ---
> >   drivers/firmware/qcom_scm.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/firmware/qcom_scm.c b/drivers/firmware/qcom_scm.c
> > index 980fcfa20b9f..422de70faff8 100644
> > --- a/drivers/firmware/qcom_scm.c
> > +++ b/drivers/firmware/qcom_scm.c
> > @@ -1331,7 +1331,7 @@ static int qcom_scm_find_dload_address(struct device *dev, u64 *addr)
> >    */
> >   bool qcom_scm_is_available(void)
> >   {
> > -     return !!__scm;
> > +     return !!READ_ONCE(__scm);
> >   }
> >   EXPORT_SYMBOL(qcom_scm_is_available);
> >
> > @@ -1477,8 +1477,8 @@ static int qcom_scm_probe(struct platform_device *pdev)
> >       if (ret)
> >               return ret;
> >
> > -     __scm = scm;
> > -     __scm->dev = &pdev->dev;
> > +     scm->dev = &pdev->dev;
> > +     WRITE_ONCE(__scm, scm);
>
> In my opinion "__scm = scm;" assignment should be done at the end of
> probe function success so that qcom_scm_is_available() return true only
> when probe is completed.
>
> Other changes may not be needed.
>

Om, this is v1. We're at v4 (soon v5).

I agree with the comment but it's outside the scope of this series. I
may address it later.

It has been like this for a long time. Typically this module is
built-in and initialized before all other drivers so it has never
caused problems.

Bart

> >       init_completion(&__scm->waitq_comp);
> >