spi: spi-fsl-dspi: Implement .max_message_size method for EOQ mode

When it gets set, End Of Queue Flag halts the DSPI controller and forces
the chip select signal to deassert.

This operating mode is not ideal, but it is used for the DSPI
instantiations where there is no other notification from the controller
that the data in the FIFO has finished transmission. So in practice, it
means that transmitting buffers larger than the FIFO size will yield
unpredictable results.

The only controller that operates in EOQ mode is MCF5441X (Coldfire). I
would say that the way EOQ is used (and documented in the reference
manual, too) on this chip is incorrect, and I would personally migrate
it to TCFQ, but that's notably worse in terms of performance (it can
only use 1 entry of the 16-deep FIFO) and if this limitation didn't
bother any Coldfire DSPI user so far, it's likely that we just need to
throw an error for larger buffers to make sure that callers are aware
their transfers are getting truncated/split.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-7-olteanv@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-dspi.c

@@ -1084,6 +1084,22 @@ static int dspi_slave_abort(struct spi_master *master)
 	return 0;
 }
 
+/*
+ * EOQ mode will inevitably deassert its PCS signal on last word in a queue
+ * (hardware limitation), so we need to inform the spi_device that larger
+ * buffers than the FIFO size are going to have the chip select randomly
+ * toggling, so it has a chance to adapt its message sizes.
+ */
+static size_t dspi_max_message_size(struct spi_device *spi)
+{
+	struct fsl_dspi *dspi = spi_controller_get_devdata(spi->controller);
+
+	if (dspi->devtype_data->trans_mode == DSPI_EOQ_MODE)
+		return dspi->devtype_data->fifo_size;
+
+	return SIZE_MAX;
+}
+
 static int dspi_probe(struct platform_device *pdev)
 {
 	struct device_node *np = pdev->dev.of_node;
@@ -1105,6 +1121,7 @@ static int dspi_probe(struct platform_device *pdev)
 
 	ctlr->setup = dspi_setup;
 	ctlr->transfer_one_message = dspi_transfer_one_message;
+	ctlr->max_message_size = dspi_max_message_size;
 	ctlr->dev.of_node = pdev->dev.of_node;
 
 	ctlr->cleanup = dspi_cleanup;