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

[Mukesh Ojha]

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);

@@ -1822,10 +1827,12 @@ 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;

+       init_completion(&scm->waitq_comp);
        mutex_init(&scm->scm_bw_lock);

        scm->path = devm_of_icc_get(&pdev->dev, NULL);
@@ -1857,10 +1864,8 @@ static int qcom_scm_probe(struct platform_device 
*pdev)
        if (ret)
                return ret;

-       __scm = scm;
-       __scm->dev = &pdev->dev;
-
-       init_completion(&__scm->waitq_comp);
+       smp_wmb();
+       WRITE_ONCE(__scm, scm);


// 2nd way

diff --git a/drivers/firmware/qcom/qcom_scm.c 
b/drivers/firmware/qcom/qcom_scm.c
index 49ddbcab0680..911699123f9f 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -1741,7 +1741,7 @@ static int qcom_scm_qseecom_init(struct qcom_scm *scm)
  */
 bool qcom_scm_is_available(void)
 {
-	return !!__scm;
+	return !!smp_load_acquire(&__scm);
 }
 EXPORT_SYMBOL_GPL(qcom_scm_is_available);

@@ -1822,10 +1822,12 @@ 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;

+	init_completion(&scm->waitq_comp);
 	mutex_init(&scm->scm_bw_lock);

 	scm->path = devm_of_icc_get(&pdev->dev, NULL);
@@ -1857,10 +1859,8 @@ static int qcom_scm_probe(struct platform_device 
*pdev)
 	if (ret)
 		return ret;

-	__scm = scm;
-	__scm->dev = &pdev->dev;
-
-	init_completion(&__scm->waitq_comp);
+	/* Let all above stores available after this. */
+	smp_store_release(&__scm, scm);

 	irq = platform_get_irq_optional(pdev, 0);
 	if (irq < 0) {
--
2.7.4



> Thanks,
> Pavan