Re: [PATCH 3/4] net: mhi_net: Implement runtime PM support

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

On 12/3/2025 3:07 AM, Bjorn Andersson wrote:
> On Mon, Dec 01, 2025 at 06:13:19PM +0530, Krishna Chaitanya Chundru wrote:
>> Add runtime power management support to the mhi_net driver to align with
>> the updated MHI framework, which expects runtime PM to be enabled by client
>> drivers. This ensures the controller remains active during data transfers
>> and can autosuspend when idle.
> This last sentence hints at there being an actual problem with the
> current code. Perhaps we do this because it's the right thing to do,
> perhaps we're making this change because devices are crashing and
> burning?
we are doing because its right thing to do, with current code we are not 
seeing
any issues.
> Start your commit message with making the reason for your change clear.
>
> Ask yourself https://en.wikipedia.org/wiki/Five_whys when you come up
> with your problem description.
ack.
>> The driver now uses pm_runtime_get() and pm_runtime_put() around
>> transmit, receive, and buffer refill operations. Runtime PM is initialized
>> during probe with autosuspend enabled and a 100ms delay. The device is
>> marked with pm_runtime_no_callbacks() to notify PM framework that there
>> are no callbacks for this driver.
> This looks like an AI prompt, does it add value to the commit message?
>
> It can mostly be summarized as "Implement pm_runtime in the driver". The
> only part that's not obvious is the 100ms autosuspend delay, but you
> skipped explaining why you did choose this number.
100ms taken from pci port bus driver reference, I will add the reason 
for 100ms
in next patch.
>> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
>> ---
>>   drivers/net/mhi_net.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/net/mhi_net.c b/drivers/net/mhi_net.c
>> index ae169929a9d8e449b5a427993abf68e8d032fae2..c5c697f29e69e9bc874b6cfff4699de12a4af114 100644
>> --- a/drivers/net/mhi_net.c
>> +++ b/drivers/net/mhi_net.c
>> @@ -9,6 +9,7 @@
>>   #include <linux/mod_devicetable.h>
>>   #include <linux/module.h>
>>   #include <linux/netdevice.h>
>> +#include <linux/pm_runtime.h>
>>   #include <linux/skbuff.h>
>>   #include <linux/u64_stats_sync.h>
>>   
>> @@ -76,6 +77,7 @@ static netdev_tx_t mhi_ndo_xmit(struct sk_buff *skb, struct net_device *ndev)
>>   	struct mhi_device *mdev = mhi_netdev->mdev;
>>   	int err;
>>   
>> +	pm_runtime_get(&mdev->dev);
> What happened to your error handling?
ack I will add error handling.

- Krishna Chaitanya.
>
> Regards,
> Bjorn
>
>>   	err = mhi_queue_skb(mdev, DMA_TO_DEVICE, skb, skb->len, MHI_EOT);
>>   	if (unlikely(err)) {
>>   		net_err_ratelimited("%s: Failed to queue TX buf (%d)\n",
>> @@ -94,6 +96,7 @@ static netdev_tx_t mhi_ndo_xmit(struct sk_buff *skb, struct net_device *ndev)
>>   	u64_stats_inc(&mhi_netdev->stats.tx_dropped);
>>   	u64_stats_update_end(&mhi_netdev->stats.tx_syncp);
>>   
>> +	pm_runtime_put_autosuspend(&mdev->dev);
>>   	return NETDEV_TX_OK;
>>   }
>>   
>> @@ -261,6 +264,7 @@ static void mhi_net_ul_callback(struct mhi_device *mhi_dev,
>>   	}
>>   	u64_stats_update_end(&mhi_netdev->stats.tx_syncp);
>>   
>> +	pm_runtime_put_autosuspend(&mdev->dev);
>>   	if (netif_queue_stopped(ndev) && !mhi_queue_is_full(mdev, DMA_TO_DEVICE))
>>   		netif_wake_queue(ndev);
>>   }
>> @@ -277,6 +281,7 @@ static void mhi_net_rx_refill_work(struct work_struct *work)
>>   
>>   	size = mhi_netdev->mru ? mhi_netdev->mru : READ_ONCE(ndev->mtu);
>>   
>> +	pm_runtime_get_sync(&mdev->dev);
>>   	while (!mhi_queue_is_full(mdev, DMA_FROM_DEVICE)) {
>>   		skb = netdev_alloc_skb(ndev, size);
>>   		if (unlikely(!skb))
>> @@ -296,6 +301,7 @@ static void mhi_net_rx_refill_work(struct work_struct *work)
>>   		cond_resched();
>>   	}
>>   
>> +	pm_runtime_put_autosuspend(&mdev->dev);
>>   	/* If we're still starved of rx buffers, reschedule later */
>>   	if (mhi_get_free_desc_count(mdev, DMA_FROM_DEVICE) == mhi_netdev->rx_queue_sz)
>>   		schedule_delayed_work(&mhi_netdev->rx_refill, HZ / 2);
>> @@ -362,12 +368,19 @@ static int mhi_net_probe(struct mhi_device *mhi_dev,
>>   
>>   	SET_NETDEV_DEV(ndev, &mhi_dev->dev);
>>   
>> +	pm_runtime_no_callbacks(&mhi_dev->dev);
>> +	devm_pm_runtime_set_active_enabled(&mhi_dev->dev);
>> +	pm_runtime_set_autosuspend_delay(&mhi_dev->dev, 100);
>> +	pm_runtime_use_autosuspend(&mhi_dev->dev);
>> +	pm_runtime_get(&mhi_dev->dev);
>>   	err = mhi_net_newlink(mhi_dev, ndev);
>>   	if (err) {
>>   		free_netdev(ndev);
>> +		pm_runtime_put_autosuspend(&mhi_dev->dev);
>>   		return err;
>>   	}
>>   
>> +	pm_runtime_put_autosuspend(&mhi_dev->dev);
>>   	return 0;
>>   }
>>   
>>
>> -- 
>> 2.34.1
>>
>>