linux-kernel - Re: [PATCH v11 1/4] firmware: qcom: scm: provide a read-modify-write function

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Date: Tue, 9 Jan 2024 16:52:58 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Pavan Kondeti <quic_pkondeti@...cinc.com>
CC: <andersson@...nel.org>, <konrad.dybcio@...aro.org>,
        <linus.walleij@...aro.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v11 1/4] firmware: qcom: scm: provide a read-modify-write
 function



On 1/9/2024 10:34 AM, Pavan Kondeti wrote:
> On Mon, Jan 08, 2024 at 08:57:31PM +0530, Mukesh Ojha wrote:
>> It was realized by Srinivas K. that there is a need of
>> read-modify-write scm exported function so that it can
>> be used by multiple clients.
>>
>> Let's introduce qcom_scm_io_rmw() which masks out the bits
>> and write the passed value to that bit-offset.
>>
>> Suggested-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>> Tested-by: Kathiravan Thirumoorthy <quic_kathirav@...cinc.com> # IPQ9574 and IPQ5332
>> ---
>>   drivers/firmware/qcom/qcom_scm.c       | 26 ++++++++++++++++++++++++++
>>   include/linux/firmware/qcom/qcom_scm.h |  1 +
>>   2 files changed, 27 insertions(+)
>>
>> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
>> index 520de9b5633a..25549178a30f 100644
>> --- a/drivers/firmware/qcom/qcom_scm.c
>> +++ b/drivers/firmware/qcom/qcom_scm.c
>> @@ -19,6 +19,7 @@
>>   #include <linux/of_irq.h>
>>   #include <linux/of_platform.h>
>>   #include <linux/platform_device.h>
>> +#include <linux/spinlock.h>
>>   #include <linux/reset-controller.h>
>>   #include <linux/types.h>
>>   
>> @@ -41,6 +42,8 @@ struct qcom_scm {
>>   	int scm_vote_count;
>>   
>>   	u64 dload_mode_addr;
>> +	/* Atomic context only */
>> +	spinlock_t lock;
> 
> IMHO, this comment can be confusing later. one might think that
> qcom_scm_call_atomic() needs to be called with this lock, but that does
> not seems to be the intention here.
> 
>>   };
>>   
>>   struct qcom_scm_current_perm_info {
>> @@ -481,6 +484,28 @@ static int qcom_scm_disable_sdi(void)
>>   	return ret ? : res.result[0];
>>   }
>>   
>> +int qcom_scm_io_rmw(phys_addr_t addr, unsigned int mask, unsigned int val)
>> +{
>> +	unsigned int old, new;
>> +	int ret;
>> +
>> +	if (!__scm)
>> +		return -EINVAL;
>> +
>> +	spin_lock(&__scm->lock);
> 
> So, this function can't be called from hardirq context. If that ever
> happens, with this new spinlock (without disabling interrupts), can
> result in deadlock.

Ok, let's make it fully atomic with spin_lock_irqsave();

-Mukesh
> 
>> +	ret = qcom_scm_io_readl(addr, &old);
>> +	if (ret)
>> +		goto unlock;
>> +
>> +	new = (old & ~mask) | (val & mask);
>> +
>> +	ret = qcom_scm_io_writel(addr, new);
>> +unlock:
>> +	spin_unlock(&__scm->lock);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(qcom_scm_io_rmw);
> 
> Thanks,
> Pavan