Re: [PATCH v12 7/9] firmware: qcom: scm: Fix __scm->dev assignement

[Pavan Kondeti]

On Tue, Mar 19, 2024 at 03:38:57PM +0530, Mukesh Ojha wrote:
> 
> 
> On 3/19/2024 6:47 AM, Pavan Kondeti wrote:
> > On Mon, Mar 18, 2024 at 06:38:20PM +0530, Mukesh Ojha wrote:
> > > 
> > > 
> > > On 3/3/2024 12:55 AM, Bjorn Andersson wrote:
> > > > On Tue, Feb 27, 2024 at 09:23:06PM +0530, Mukesh Ojha wrote:
> > > > > qcom_scm_is_available() gives wrong indication if __scm
> > > > > is initialized but __scm->dev is not.
> > > > > 
> > > > > Fix this appropriately by making sure if __scm is
> > > > > initialized and then it is associated with its
> > > > > device.
> > > > > 
> > > > 
> > > > This seems like a bug fix, and should as such have a Fixes: tag and
> > > > probably Cc: stable@xxxxxxxxxxxxxxx
> > > > 
> > > > > Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > > > > ---
> > > > >    drivers/firmware/qcom/qcom_scm.c | 2 +-
> > > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> > > > > index 6c252cddd44e..6f14254c0c10 100644
> > > > > --- a/drivers/firmware/qcom/qcom_scm.c
> > > > > +++ b/drivers/firmware/qcom/qcom_scm.c
> > > > > @@ -1859,6 +1859,7 @@ static int qcom_scm_probe(struct platform_device *pdev)
> > > > >    	if (!scm)
> > > > >    		return -ENOMEM;
> > > > > +	scm->dev = &pdev->dev;
> > > > >    	ret = qcom_scm_find_dload_address(&pdev->dev, &scm->dload_mode_addr);
> > > > >    	if (ret < 0)
> > > > >    		return ret;
> > > > > @@ -1895,7 +1896,6 @@ static int qcom_scm_probe(struct platform_device *pdev)
> > > > >    		return ret;
> > > > >    	__scm = scm;
> > > > > -	__scm->dev = &pdev->dev;
> > > > 
> > > > Is it sufficient to just move the line up, or do we need a barrier of
> > > > some sort here?
> > > 
> > > Would be good to use, smp_mb() before the assignment
> > >       __scm = scm
> > > along with moving below line
> > > __scm->dev = &pdev->dev
> > > 
> > 
> > Full memory barrier is not needed here. store variant is sufficient.
> > WRITE_ONCE() + smp_store_release() will fit here no?
> 
> Thanks for the comment, i again have a look at it and agree we don't
> need a full barrier here.
> 
> And we can do either of the below two ways.
> 
> -Mukesh
> 
> 
> // 1st way
> 
> diff --git a/drivers/firmware/qcom/qcom_scm.c
> b/drivers/firmware/qcom/qcom_scm.c
> index 49ddbcab0680..b638fb407fc6 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -1741,7 +1741,12 @@ static int qcom_scm_qseecom_init(struct qcom_scm
> *scm)
>   */
>  bool qcom_scm_is_available(void)
>  {
> -       return !!__scm;
> +       bool avail;
>   */
>  bool qcom_scm_is_available(void)
>  {
> -       return !!__scm;
> +       bool avail;
> +
> +       avail = !!READ_ONCE(__scm);
> +       smp_rmb();
> +
> +       return avail;
>  }
>  EXPORT_SYMBOL_GPL(qcom_scm_is_available);
> 

Your original problem statement: qcom_scm_is_available() gives wrong indication 
if __scm is initialized but __scm->dev is not.

This does not require read side barrier as there is an address
dependency. If the writer does it *correctly*, the reader would always
observe __scm->dev != NULL when __scm != NULL without any barrier.

Thanks,
Pavan