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

[Mukesh Ojha]

On 3/19/2024 3:52 PM, Pavan Kondeti wrote:
> 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.

It looks like write barrier pairs with an address-dependency barrier, a
control dependency, an acquire barrier, a release barrier, a read 
barrier, or a general barrier.

So, smp_rmb() is redundant here.

Also, for correction, we may not need smp_load_acquire() in the 1st way
and just using READ_ONCE() is enough.

-Mukesh
> Thanks,
> Pavan