linux-kernel - Re: [PATCH v2 RESEND] bus: mhi: core: Wait for ready state after reset

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Date:   Tue, 16 Mar 2021 13:28:41 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     hemantk@...eaurora.org, bbhatt@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 RESEND] bus: mhi: core: Wait for ready state after
 reset

On 3/16/2021 12:14 AM, Manivannan Sadhasivam wrote:
> On Wed, Mar 10, 2021 at 01:41:58PM -0700, Jeffrey Hugo wrote:
>> After the device has signaled the end of reset by clearing the reset bit,
>> it will automatically reinit MHI and the internal device structures.  Once
>> That is done, the device will signal it has entered the ready state.
>>
>> Signaling the ready state involves sending an interrupt (MSI) to the host
>> which might cause IOMMU faults if it occurs at the wrong time.
>>
>> If the controller is being powered down, and possibly removed, then the
>> reset flow would only wait for the end of reset.  At which point, the host
>> and device would start a race.  The host may complete its reset work, and
>> remove the interrupt handler, which would cause the interrupt to be
>> disabled in the IOMMU.  If that occurs before the device signals the ready
>> state, then the IOMMU will fault since it blocked an interrupt.  While
>> harmless, the fault would appear like a serious issue has occurred so let's
>> silence it by making sure the device hits the ready state before the host
>> completes its reset processing.
>>
>> Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.org>
>> ---
>>   drivers/bus/mhi/core/pm.c | 17 ++++++++++++++++-
>>   1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/bus/mhi/core/pm.c b/drivers/bus/mhi/core/pm.c
>> index adb0e80..414da4f 100644
>> --- a/drivers/bus/mhi/core/pm.c
>> +++ b/drivers/bus/mhi/core/pm.c
>> @@ -467,7 +467,7 @@ static void mhi_pm_disable_transition(struct mhi_controller *mhi_cntrl)
>>   
>>   	/* Trigger MHI RESET so that the device will not access host memory */
>>   	if (!MHI_PM_IN_FATAL_STATE(mhi_cntrl->pm_state)) {
>> -		u32 in_reset = -1;
>> +		u32 in_reset = -1, ready = 0;
>>   		unsigned long timeout = msecs_to_jiffies(mhi_cntrl->timeout_ms);
>>   
>>   		dev_dbg(dev, "Triggering MHI Reset in device\n");
>> @@ -490,6 +490,21 @@ static void mhi_pm_disable_transition(struct mhi_controller *mhi_cntrl)
>>   		 * hence re-program it
>>   		 */
>>   		mhi_write_reg(mhi_cntrl, mhi_cntrl->bhi, BHI_INTVEC, 0);
>> +
>> +		if (!MHI_IN_PBL(mhi_get_exec_env(mhi_cntrl))) {
>> +			/* wait for ready to be set */
>> +			ret = wait_event_timeout(mhi_cntrl->state_event,
>> +						 mhi_read_reg_field(mhi_cntrl,
>> +							mhi_cntrl->regs,
>> +							MHISTATUS,
>> +							MHISTATUS_READY_MASK,
>> +							MHISTATUS_READY_SHIFT,
>> +							&ready)
>> +						 || ready, timeout);
>> +			if (!ret || !ready)
>> +				dev_warn(dev,
>> +					"Device failed to enter READY state\n");
> 
> Wouldn't dev_err be more appropriate here provided that we might get IOMMU fault
> anytime soon?

I supposed.  Didn't feel like a "true" error because nothing has 
actually failed, the chance of the IOMMU fault is low, and I couldn't 
enumerate what would be the expected action for the system user to take 
if they saw this as an error.

I don't have a particularly strong opinion one way or the other.  I 
figured warn was the more conservative option here.

Will change.

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.