spi: atmel: introduce probe deferring

Return probe defer if requesting a dma channel without a dma controller
probed.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-atmel.c

@@ -427,14 +427,23 @@ static int atmel_spi_configure_dma(struct atmel_spi *as)
 	dma_cap_zero(mask);
 	dma_cap_set(DMA_SLAVE, mask);
 
-	as->dma.chan_tx = dma_request_slave_channel(dev, "tx");
-	if (!as->dma.chan_tx) {
+	as->dma.chan_tx = dma_request_slave_channel_reason(dev, "tx");
+	if (IS_ERR(as->dma.chan_tx)) {
+		err = PTR_ERR(as->dma.chan_tx);
+		if (err == -EPROBE_DEFER) {
+			dev_warn(dev, "no DMA channel available at the moment\n");
+			return err;
+		}
 		dev_err(dev,
 			"DMA TX channel not available, SPI unable to use DMA\n");
 		err = -EBUSY;
 		goto error;
 	}
 
+	/*
+	 * No reason to check EPROBE_DEFER here since we have already requested
+	 * tx channel. If it fails here, it's for another reason.
+	 */
 	as->dma.chan_rx = dma_request_slave_channel(dev, "rx");
 
 	if (!as->dma.chan_rx) {
@@ -456,7 +465,7 @@ static int atmel_spi_configure_dma(struct atmel_spi *as)
 error:
 	if (as->dma.chan_rx)
 		dma_release_channel(as->dma.chan_rx);
-	if (as->dma.chan_tx)
+	if (!IS_ERR(as->dma.chan_tx))
 		dma_release_channel(as->dma.chan_tx);
 	return err;
 }
@@ -1328,8 +1337,11 @@ static int atmel_spi_probe(struct platform_device *pdev)
 	as->use_dma = false;
 	as->use_pdc = false;
 	if (as->caps.has_dma_support) {
-		if (atmel_spi_configure_dma(as) == 0)
+		ret = atmel_spi_configure_dma(as);
+		if (ret == 0)
 			as->use_dma = true;
+		else if (ret == -EPROBE_DEFER)
+			return ret;
 	} else {
 		as->use_pdc = true;
 	}