wait queues and work queues, do they always go together?

I’ve been trying to refresh my understanding of sleeping in the kernel with regards to wait queues. So started browsing the source code for bcmgenet.c (kernel version 4.4) which is the driver responsible for driving the 7xxx series of Broadcom SoC for their set top box solution.

As part of the probe callback, this driver initializes a work queue which is part of the driver’s private structure and adds itself to the Q. But I do not see any blocking of any kind anywhere. Then it goes on to initialize a work queue with a function to call when woken up.

Now coming to the ISR0 for the driver, within that is an explicit call to the scheduler as part of the ISR (bcmgenet_isr0) if certain conditions are met. Now AFAIK, this call is used to defer work to a later time, much like a tasklet does.

Post this we check some MDIO status flags and if the conditions are met, we wake up the process which was blocked in process context. But where exactly is the process blocked?

Also, most of the time, wait queues seem to be used in conjunction with work queues. Is that the typical way to use them?

> As part of the probe callback, this driver initializes a work queue which is part of the driver’s private structure and adds itself to the Q. But I do not see any blocking of any kind anywhere.

I think you meant the wait queue head, not the work queue. I do not see any evidence of the probe adding itself to the queue; it is merely initializing the queue.

The queue is used by the calls to the wait_event_timeout() macro in the bcmgenet_mii_read() and bcmgenet_mii_write() functions in bcmmii.c. These calls will block until either the condition they are waiting for becomes true or the timeout period elapses. They are woken up by the wake_up(&priv->wq); call in the ISR0 interrupt handler.

> Then it goes on to initialize a work queue with a function to call when woken up.

It is initializing a work item, not a work queue. The function will be called from a kernel thread as a result of the work item being added to the system work queue.

> Now coming to the ISR0 for the driver, within that is an explicit call to the scheduler as part of the ISR (bcmgenet_isr0) if certain conditions are met. Now AFAIK, this call is used to defer work to a later time, much like a tasklet does.

You are referring to the schedule_work(&priv->bcmgenet_irq_work); call in the ISR0 interrupt handler. This is adding the previously mentioned work item to the system work queue. It is similar to as tasklet, but tasklets are run in a softirq context whereas work items are run in a process context.

> Post this we check some MDIO status flags and if the conditions are met, we wake up the process which was blocked in process context. But where exactly is the process blocked?

As mentioned above, the process is blocked in the bcmgenet_mii_read() and bcmgenet_mii_write() functions, although they use a timeout to avoid blocking for long periods. (This timeout is especially important for those versions of GENET that do not support MDIO-related interrupts!)

> Also, most of the time, wait queues seem to be used in conjunction with work queues. Is that the typical way to use them?

Not especially. This particular driver uses both a wait queue and a work item, but I wouldn't describe them as being used "in conjunction" since they are being used to handle different interrupt conditions.