staging: comedi: adl_pci9118: try and avoid unnecessary DMA restart

staging: comedi: adl_pci9118: try and avoid unnecessary DMA restart

`interrupt_pci9118_ai_dma()` is called on interrupt to transfer data
from DMA buffers into the comedi async data buffer.  Currently it always
restarts DMA.  If double buffering, it restarts DMA on the next DMA
buffer before processing the current DMA buffer, otherwise it restarts
DMA on the same DMA buffer after it has been processed.

For single buffering we can avoid restarting the DMA transfer by
checking the async event flags after the current buffer has been
processed, which is easy.

For double buffering, we need to know how many valid samples there are
in the current buffer before it has been processed and determine whether
there is enough to complete the acquisition.  Call new function
`valid_samples_in_act_dma_buf()` to determine the number of valid
samples in the current DMA buffer, and compare that with the result of
`comedi_nsamples_left()` to determine if DMA needs to be restarted.
(`comedi_nsamples_left()` needs an upper bound to clamp to, so use the
number of valid samples in the DMA buffer plus one for our test.)

It is still possible for DMA to be restarted unnecessarily in the double
buffer case if a `COMEDI_CB_OVERFLOW` event occurs while copying to the
comedi async buffer, but it doesn't really matter.  The ongoing DMA
operation will get disabled when the subdevice's `cancel()` handler is
called when the events are handled later in the interrupt service
routine (as it does currently).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>