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

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Date:   Tue, 17 Oct 2023 13:54:43 +0530
From:   Om Prakash Singh <quic_omprsing@...cinc.com>
To:     Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        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>
CC:     <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 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.

>   	init_completion(&__scm->waitq_comp);
>