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

[Mukesh Ojha]

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@xxxxxxxxxx>
> > Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > Tested-by: Kathiravan Thirumoorthy <quic_kathirav@xxxxxxxxxxx> # 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