[PATCH] bus: mhi: host: Disable preemption while processing data events

[PATCH] bus: mhi: host: Disable preemption while processing data events

[Qiang Yu]

If data processing of an event is scheduled out because core
is busy handling multiple irqs, this can starves the processing
of MHI M0 state change event on another core. Fix this issue by
disabling irq on the core processing data events.

Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
---
 drivers/bus/mhi/host/main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
index f3aef77a..b58698d 100644
--- a/drivers/bus/mhi/host/main.c
+++ b/drivers/bus/mhi/host/main.c
@@ -1027,13 +1027,14 @@ int mhi_process_data_event_ring(struct mhi_controller *mhi_cntrl,
 
 void mhi_ev_task(unsigned long data)
 {
+	unsigned long flags;
 	struct mhi_event *mhi_event = (struct mhi_event *)data;
 	struct mhi_controller *mhi_cntrl = mhi_event->mhi_cntrl;
 
 	/* process all pending events */
-	spin_lock_bh(&mhi_event->lock);
+	spin_lock_irqsave(&mhi_event->lock, flags);
 	mhi_event->process_event(mhi_cntrl, mhi_event, U32_MAX);
-	spin_unlock_bh(&mhi_event->lock);
+	spin_unlock_irqrestore(&mhi_event->lock, flags);
 }
 
 void mhi_ctrl_ev_task(unsigned long data)
-- 
2.7.4

Re: [PATCH] bus: mhi: host: Disable preemption while processing data events

[Manivannan Sadhasivam]

On Thu, Nov 03, 2022 at 03:55:11PM +0800, Qiang Yu wrote:
> If data processing of an event is scheduled out because core
> is busy handling multiple irqs, this can starves the processing

s/starves/starve

> of MHI M0 state change event on another core. Fix this issue by
> disabling irq on the core processing data events.
> 
> Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
> ---
>  drivers/bus/mhi/host/main.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> index f3aef77a..b58698d 100644
> --- a/drivers/bus/mhi/host/main.c
> +++ b/drivers/bus/mhi/host/main.c
> @@ -1027,13 +1027,14 @@ int mhi_process_data_event_ring(struct mhi_controller *mhi_cntrl,
>  
>  void mhi_ev_task(unsigned long data)
>  {
> +	unsigned long flags;
>  	struct mhi_event *mhi_event = (struct mhi_event *)data;
>  	struct mhi_controller *mhi_cntrl = mhi_event->mhi_cntrl;
>  
>  	/* process all pending events */
> -	spin_lock_bh(&mhi_event->lock);
> +	spin_lock_irqsave(&mhi_event->lock, flags);

Please add a comment stating why we are disabling local IRQs here. This will
help in understanding the code better.

Thanks,
Mani

>  	mhi_event->process_event(mhi_cntrl, mhi_event, U32_MAX);
> -	spin_unlock_bh(&mhi_event->lock);
> +	spin_unlock_irqrestore(&mhi_event->lock, flags);
>  }
>  
>  void mhi_ctrl_ev_task(unsigned long data)
> -- 
> 2.7.4
> 
> 

-- 
மணிவண்ணன் சதாசிவம்

Re: [PATCH] bus: mhi: host: Disable preemption while processing data events

[Jeffrey Hugo]

On 11/3/2022 1:55 AM, Qiang Yu wrote:
> If data processing of an event is scheduled out because core
> is busy handling multiple irqs, this can starves the processing
> of MHI M0 state change event on another core. Fix this issue by
> disabling irq on the core processing data events.

Can you elaborate on the scenario more?

If a device hasn't entered M0, then what event could be processed?

Re: [PATCH] bus: mhi: host: Disable preemption while processing data events

[Qiang Yu]

On 11/7/2022 11:00 PM, Jeffrey Hugo wrote:
> On 11/3/2022 1:55 AM, Qiang Yu wrote:
>> If data processing of an event is scheduled out because core
>> is busy handling multiple irqs, this can starves the processing
>> of MHI M0 state change event on another core. Fix this issue by
>> disabling irq on the core processing data events.
>
> Can you elaborate on the scenario more?
>
> If a device hasn't entered M0, then what event could be processed?
The scenario is following:
1. On Core 2 IPCR channel 20 transfer completion event MSI arrived which 
is processed by holding Event ring 1 lock by calling spin_lock_bh() in 
normal priority tasklet.
2. On Core 2 multiple irqs come in and scheduled out the event ring 
processing done by #1. Due to that event ring lock remained acquired but 
event ring processing for IPCR got scheduled out.
3. On Core 1, primary event ring MSI arrived for M0, which is handled by 
a separate MHI hi-priority tasklet. As part of M0 state transition,
tasklet goes over event ring 1 and tries to acquire event ring 1 lock in 
order to ring the event ring DB, but just gets stuck in busy wait due to 
#2.
At this point tasklet is at the mercy of the muiltiple irqs handle 
(which scheduled out the MHI tasklet of #1) to finish and let the MHI 
normal priority tasklet to run.