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

[Mukesh Ojha <quic_mojha@quicinc.com>]

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@vger.kernel.org
>>>
>>>> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
>>>> ---
>>>>    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