linux-kernel - Re: [PATCH] soc: qcom: llcc: Add llcc device availability check

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Message-ID: <96aa13e2-828e-b961-996e-7260cd564589@quicinc.com>
Date: Mon, 26 Feb 2024 16:30:03 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Sahil Chandna <quic_chandna@...cinc.com>, <andersson@...nel.org>,
        <konrad.dybcio@...aro.org>
CC: <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soc: qcom: llcc: Add llcc device availability check



On 2/26/2024 4:19 PM, Sahil Chandna wrote:
> On 2/26/2024 4:02 PM, Mukesh Ojha wrote:
>>
>>
>> On 2/22/2024 11:37 PM, Sahil Chandna wrote:
>>> On 2/20/2024 5:58 PM, Mukesh Ojha wrote:
>>>> When llcc driver is enabled  and llcc device is not
>>>> physically there on the SoC, client can get
>>>> -EPROBE_DEFER on calling llcc_slice_getd() and it
>>>> is possible they defer forever.
>>>>
>>>> Let's add a check device availabilty and set the
>>>> appropriate applicable error in drv_data.
>>>>
>>>> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>>>> ---
>>>>   drivers/soc/qcom/llcc-qcom.c | 23 ++++++++++++++++++++++-
>>>>   1 file changed, 22 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/soc/qcom/llcc-qcom.c 
>>>> b/drivers/soc/qcom/llcc-qcom.c
>>>> index 4ca88eaebf06..cb336b183bba 100644
>>>> --- a/drivers/soc/qcom/llcc-qcom.c
>>>> +++ b/drivers/soc/qcom/llcc-qcom.c
>>>> @@ -769,6 +769,27 @@ static const struct qcom_sct_config 
>>>> x1e80100_cfgs = {
>>>>   };
>>>>   static struct llcc_drv_data *drv_data = (void *) -EPROBE_DEFER;
>>>> +static DEFINE_MUTEX(dev_avail);
>>> what is the requirement for mutex lock here? Since we are only trying 
>>> to find if node present or not
>>
>> I was trying to avoid two parallel call from llcc_slice_getd() calling
>> parallel call to of_find_node_by_name() as it should be one time 
>> search for device presence to find a node and check if device is 
>> present or
>> not.
>>
>> -Mukesh
>>
> Got it, but of_find_node_by_name () is holding a raw_spin_lock_irqsave 
> () for concurrency, right ? please correct me if understanding is wrong.

Even though, of_find_node_by_name () is holding spin_lock but nothing
is preventing the 2nd call from happening. Here, mutex and check on
!ptr ensures, we don't make 2nd call.

-Mukesh
>>>> +
>>>> +static bool is_llcc_device_available(void)
>>>> +{
>>>> +    static struct llcc_drv_data *ptr;
>>>> +
>>>> +    mutex_lock(&dev_avail);
>>>> +    if (!ptr) {
>>>> +        struct device_node *node;
>>>> +
>>>> +        node = of_find_node_by_name(NULL, "system-cache-controller");
>>>> +        if (!of_device_is_available(node)) {
>>>> +            pr_warn("llcc-qcom: system-cache-controller node not 
>>>> found\n");
>>>> +            drv_data = ERR_PTR(-ENODEV);
>>>> +        }
>>>> +        of_node_put(node);
>>>> +        ptr = drv_data;
>>>> +    }
>>>> +    mutex_unlock(&dev_avail);
>>>> +    return (PTR_ERR(ptr) != -ENODEV) ? true : false;
>>>> +}
>>>>   /**
>>>>    * llcc_slice_getd - get llcc slice descriptor
>>>> @@ -783,7 +804,7 @@ struct llcc_slice_desc *llcc_slice_getd(u32 uid)
>>>>       struct llcc_slice_desc *desc;
>>>>       u32 sz, count;
>>>> -    if (IS_ERR(drv_data))
>>>> +    if (!is_llcc_device_available() || IS_ERR(drv_data))
> Also, thinking about this, should the status of device present or not be 
> saved in static variable instead of function call for each client ?
>>>>           return ERR_CAST(drv_data);
>>>>       cfg = drv_data->cfg;
>>>
> 
> Regards,
> Sahil