media: dw100: Fix kernel oops with PREEMPT_RT enabled

media: dw100: Fix kernel oops with PREEMPT_RT enabled

On kernels with PREEMPT_RT enabled, a "BUG: scheduling while atomic"
kernel oops occurs inside dw100_irq_handler -> vb2_buffer_done. This is
because vb2_buffer_done takes a spinlock which is not allowed within
interrupt context on PREEMPT_RT.

The first attempt to fix this was to just drop the IRQF_ONESHOT so that
the interrupt is handled threaded on PREEMPT_RT systems. This introduced
a new issue. The dw100 has an internal timeout counter that is gated by
the DW100_BUS_CTRL_AXI_MASTER_ENABLE bit. Depending on the time it takes
for the threaded handler to run and the geometry of the data being
processed it is possible to reach the timeout resulting in
DW100_INTERRUPT_STATUS_INT_ERR_TIME_OUT being set and "dw100
32e30000.dwe: Interrupt error: 0x1" errors in dmesg.

To properly fix that, split the interrupt into two halves, reset the
DW100_BUS_CTRL_AXI_MASTER_ENABLE bit in the hard interrupt handler and
do the v4l2 buffer handling in the threaded half. The IRQF_ONESHOT can
still be dropped as the interrupt gets disabled in the hard handler and
will only be reenabled on the next dw100_device_run which will not be
called before the current job has finished.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Xavier Roumegue <xavier.roumegue@oss.nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patch.msgid.link/20260304-sklug-v6-16-topic-dw100-v3-1-dev-v5-3-1a7e1f721b50@ideasonboard.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>