When stopping the stream, the driver needs to ensure that ongoing DMA
completes and that no new DMA is started. It does so using a feature of
the PPI that can be stopped on a frame boundary. The downside of this
mechanism is that the DMA can't be stopped independently of the source,
which prevents usage of multiple contexts for the same source (to handle
CSI-2 virtual channels or data types).

Rework the stream stop mechanism to stop the write DMA without disabling
the PPI first. The new mechanism relies on the combination of a state
machine in the driver and shadowing of the CAL_WR_DMA_CTRL_j.MODE field
in the hardware. When a stop is requested, the DMA start interrupt
handler will request the hardware to stop at the end of the current
frame by disabling the write DMA context in the shadowed register, and
flag that a stop is in progress. The next DMA end interrupt will then
report that the stop is complete.

This makes it possible to stop the PPI after stopping the DMA, and fold
the cal_camerarx_ppi_disable() call into cal_camerarx_stop().
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>