Re: [PATCH] uio: eliminate extraneous irq thread under CONFIG_PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-08-19 20:48:51 [+0000], Chris Packham wrote:
> Hi Sebastian,
Hi Chris,

> On 20/08/2026 03:06, Sebastian Andrzej Siewior wrote:
> > Chris, what was the use case to split this?
> 
> I was dealing with having a UIO interrupt for an input on a PCA9539. 
> There's some more info on the original email thread[1] but basically 
> because the PCA9539 inputs are nested __setup_irq() enforces that anyone 
> requesting them does so with a threaded interrupt.
> 
> https://lore.kernel.org/all/20240408234050.2056374-3-chris.packham@alliedtelesis.co.nz/

so you wrote:

|    Prior to this the UIO registration fails with:
|    
|      [    6.484699] uio_pdrv_genirq detect-gpio-9: unable to register uio device
|      [    6.484722] uio_pdrv_genirq detect-gpio-9: probe with driver uio_pdrv_genirq failed with error -22
|    
|    The -EINVAL ultimately comes from __setup_irq() where it knows the
|    interrupt descriptor is nested but we haven't provided a thread_fn.

So we talk about this piece of kernel/irq/manage.c:

|         /*
|          * Check whether the interrupt nests into another interrupt
|          * thread.
|          */
|         nested = irq_settings_is_nested_thread(desc);
|         if (nested) {
|                 if (!new->thread_fn) {
|                         ret = -EINVAL;
|                         goto out_mput;
|                 }
|                 /*
|                  * Replace the primary handler which was provided from
|                  * the driver for non nested interrupt handling by the
|                  * dummy function which warns when called.
|                  */
|                 new->handler = irq_nested_primary_handler;
|         } else {

If so, you realize that uio_interrupt_handler() isn't called at all?

Sebastian