Re: [PATCH] net: qrtr: mhi: synchronize qrtr and mhi preparation

[Chris Lew <quic_clew@quicinc.com>]

On 11/7/2024 3:27 AM, Manivannan Sadhasivam wrote:
> On Mon, Nov 04, 2024 at 05:29:37PM -0800, Chris Lew wrote:
>> From: Bhaumik Bhatt <bbhatt@codeaurora.org>
>>
>> The call to qrtr_endpoint_register() was moved before
>> mhi_prepare_for_transfer_autoqueue() to prevent a case where a dl
>> callback can occur before the qrtr endpoint is registered.
>>
>> Now the reverse can happen where qrtr will try to send a packet
>> before the channels are prepared. Add a wait in the sending path to
>> ensure the channels are prepared before trying to do a ul transfer.
>>
>> Fixes: 68a838b84eff ("net: qrtr: start MHI channel after endpoit creation")
>> Reported-by: Johan Hovold <johan@kernel.org>
>> Closes: https://lore.kernel.org/linux-arm-msm/ZyTtVdkCCES0lkl4@hovoldconsulting.com/
>> Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
>> Signed-off-by: Chris Lew <quic_clew@quicinc.com>
> 
> I think we need to have the check in 'mhi_queue()' instead of waiting for the
> channels in client drivers. Would it be a problem if qrtr returns -EAGAIN from
> qcom_mhi_qrtr_send() instead of waiting for the channel?
> 

The packet would get dropped which usually ends up causing some 
functional problem down the line.

I can add retry handling for EAGAIN in qcom_mhi_qrtr_send().

Downstream we had also seen some issue where we received EAGAIN because 
the ring buffer was full. I think we saw issues doing a dumb retry so we 
triggered the retry on getting a ul_callback().

We would need to differentiate between this kind of EAGAIN from a 
ringbuf full EAGAIN.

> - Mani
> 
>> ---
>>   net/qrtr/mhi.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/net/qrtr/mhi.c b/net/qrtr/mhi.c
>> index 69f53625a049..5b7268868bbd 100644
>> --- a/net/qrtr/mhi.c
>> +++ b/net/qrtr/mhi.c
>> @@ -15,6 +15,7 @@ struct qrtr_mhi_dev {
>>   	struct qrtr_endpoint ep;
>>   	struct mhi_device *mhi_dev;
>>   	struct device *dev;
>> +	struct completion prepared;
>>   };
>>   
>>   /* From MHI to QRTR */
>> @@ -53,6 +54,10 @@ static int qcom_mhi_qrtr_send(struct qrtr_endpoint *ep, struct sk_buff *skb)
>>   	if (skb->sk)
>>   		sock_hold(skb->sk);
>>   
>> +	rc = wait_for_completion_interruptible(&qdev->prepared);
>> +	if (rc)
>> +		goto free_skb;
>> +
>>   	rc = skb_linearize(skb);
>>   	if (rc)
>>   		goto free_skb;
>> @@ -85,6 +90,7 @@ static int qcom_mhi_qrtr_probe(struct mhi_device *mhi_dev,
>>   	qdev->mhi_dev = mhi_dev;
>>   	qdev->dev = &mhi_dev->dev;
>>   	qdev->ep.xmit = qcom_mhi_qrtr_send;
>> +	init_completion(&qdev->prepared);
>>   
>>   	dev_set_drvdata(&mhi_dev->dev, qdev);
>>   	rc = qrtr_endpoint_register(&qdev->ep, QRTR_EP_NID_AUTO);
>> @@ -97,6 +103,7 @@ static int qcom_mhi_qrtr_probe(struct mhi_device *mhi_dev,
>>   		qrtr_endpoint_unregister(&qdev->ep);
>>   		return rc;
>>   	}
>> +	complete_all(&qdev->prepared);
>>   
>>   	dev_dbg(qdev->dev, "Qualcomm MHI QRTR driver probed\n");
>>   
>>
>> ---
>> base-commit: 1ffec08567f426a1c593e038cadc61bdc38cb467
>> change-id: 20241104-qrtr_mhi-dfec353030af
>>
>> Best regards,
>> -- 
>> Chris Lew <quic_clew@quicinc.com>
>>
>