2023-10-17 08:25:40

by Om Prakash Singh

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 <[email protected]>
> ---
> 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);
>

2023-10-17 08:31:03

by Bartosz Golaszewski

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
<[email protected]> 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 <[email protected]>
> > ---
> > 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);
> >