This diff is the result of replacing the downstream I2S driver with the @HiassofT's modified upstream I2S driver after a simple 2835->2708 textual substitution, because I find it makes it easier to understand the consequences of switching. As you can see, the two drivers are virtually identical.

pr1163.patch

diff --git a/sound/soc/bcm/bcm2708-i2s.c b/sound/soc/bcm/bcm2708-i2s.c
index ad9ca2c..1eeb131 100644
--- a/sound/soc/bcm/bcm2708-i2s.c
+++ b/sound/soc/bcm/bcm2708-i2s.c
@@ -31,8 +31,6 @@
  * General Public License for more details.
  */
 
-#include "bcm2708-i2s.h"
-
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/device.h>
@@ -40,9 +38,7 @@
 #include <linux/delay.h>
 #include <linux/io.h>
 #include <linux/clk.h>
-#ifndef CONFIG_ARCH_BCM2835
-#include <mach/gpio.h>
-#endif
+#include <linux/of_address.h>
 
 #include <sound/core.h>
 #include <sound/pcm.h>
@@ -51,8 +47,6 @@
 #include <sound/soc.h>
 #include <sound/dmaengine_pcm.h>
 
-#include <asm/system_info.h>
-
 /* Clock registers */
 #define BCM2708_CLK_PCMCTL_REG  0x00
 #define BCM2708_CLK_PCMDIV_REG  0x04
@@ -165,14 +159,6 @@ static const unsigned int bcm2708_clk_freq[BCM2708_CLK_SRC_HDMI+1] = {
 #define BCM2708_I2S_INT_RXR		BIT(1)
 #define BCM2708_I2S_INT_TXW		BIT(0)
 
-/* I2S DMA interface */
-#define BCM2708_I2S_FIFO_PHYSICAL_ADDR	0x7E203004
-#define BCM2708_DMA_DREQ_PCM_TX		2
-#define BCM2708_DMA_DREQ_PCM_RX		3
-
-/* I2S pin configuration */
-static int bcm2708_i2s_gpio=BCM2708_I2S_GPIO_AUTO;
-
 /* General device struct */
 struct bcm2708_i2s_dev {
 	struct device				*dev;
@@ -184,12 +170,6 @@ struct bcm2708_i2s_dev {
 	struct regmap *clk_regmap;
 };
 
-void bcm2708_i2s_set_gpio(int gpio) {
-	bcm2708_i2s_gpio=gpio;
-}
-EXPORT_SYMBOL(bcm2708_i2s_set_gpio);
-
-
 static void bcm2708_i2s_start_clock(struct bcm2708_i2s_dev *dev)
 {
 	/* Start the clock if in master mode */
@@ -321,71 +301,6 @@ static int bcm2708_i2s_set_dai_bclk_ratio(struct snd_soc_dai *dai,
 	return 0;
 }
 
-
-#ifdef CONFIG_ARCH_BCM2835
-static void bcm2708_i2s_setup_gpio(void) { }
-#else
-static int bcm2708_i2s_set_function(unsigned offset, int function)
-{
-	#define GPIOFSEL(x)  (0x00+(x)*4)
-	void __iomem *gpio = __io_address(GPIO_BASE);
-	unsigned alt = function <= 3 ? function + 4: function == 4 ? 3 : 2;
-	unsigned gpiodir;
-	unsigned gpio_bank = offset / 10;
-	unsigned gpio_field_offset = (offset - 10 * gpio_bank) * 3;
-
-	if (offset >= BCM2708_NR_GPIOS)
-		return -EINVAL;
-
-	gpiodir = readl(gpio + GPIOFSEL(gpio_bank));
-	gpiodir &= ~(7 << gpio_field_offset);
-	gpiodir |= alt << gpio_field_offset;
-	writel(gpiodir, gpio + GPIOFSEL(gpio_bank));
-	return 0;
-}
-
-static void bcm2708_i2s_setup_gpio(void)
-{
-	/*
-	 * This is the common way to handle the GPIO pins for
-	 * the Raspberry Pi.
-	 * TODO Better way would be to handle
-	 * this in the device tree!
-	 */
-	int pin,pinconfig,startpin,alt;
-
-	/* SPI is on different GPIOs on different boards */
-        /* for Raspberry Pi B+, this is pin GPIO18-21, for original on 28-31 */
-	if (bcm2708_i2s_gpio==BCM2708_I2S_GPIO_AUTO) {
-		if ((system_rev & 0xffffff) >= 0x10) {
-			/* Model B+ */
-			pinconfig=BCM2708_I2S_GPIO_PIN18;
-		} else {
-			/* original */
-			pinconfig=BCM2708_I2S_GPIO_PIN28;
-		}
-	} else {
-		pinconfig=bcm2708_i2s_gpio;
-	}
-
-	if (pinconfig==BCM2708_I2S_GPIO_PIN18) {
-		startpin=18;
-		alt=BCM2708_I2S_GPIO_PIN18_ALT;
-	} else if (pinconfig==BCM2708_I2S_GPIO_PIN28) {
-		startpin=28;
-		alt=BCM2708_I2S_GPIO_PIN28_ALT;
-	} else {
-		printk(KERN_INFO "Can't configure I2S GPIOs, unknown pin mode for I2S: %i\n",pinconfig);
-		return;
-	}
-
-	/* configure I2S pins to correct ALT mode */
-	for (pin = startpin; pin <= startpin+3; pin++) {
-		bcm2708_i2s_set_function(pin, alt);
-	}
-}
-#endif
-
 static int bcm2708_i2s_hw_params(struct snd_pcm_substream *substream,
 				 struct snd_pcm_hw_params *params,
 				 struct snd_soc_dai *dai)
@@ -415,9 +330,6 @@ static int bcm2708_i2s_hw_params(struct snd_pcm_substream *substream,
 	if (csreg & (BCM2708_I2S_TXON | BCM2708_I2S_RXON))
 		return 0;
 
-	if (!dev->dev->of_node)
-		bcm2708_i2s_setup_gpio();
-
 	/*
 	 * Adjust the data length according to the format.
 	 * We prefill the half frame length with an integer
@@ -792,8 +704,9 @@ static int bcm2708_i2s_dai_probe(struct snd_soc_dai *dai)
 {
 	struct bcm2708_i2s_dev *dev = snd_soc_dai_get_drvdata(dai);
 
-	dai->playback_dma_data = &dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK];
-	dai->capture_dma_data = &dev->dma_data[SNDRV_PCM_STREAM_CAPTURE];
+	snd_soc_dai_init_dma_data(dai,
+			&dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK],
+			&dev->dma_data[SNDRV_PCM_STREAM_CAPTURE]);
 
 	return 0;
 }
@@ -899,7 +812,6 @@ static const struct snd_dmaengine_pcm_config bcm2708_dmaengine_pcm_config = {
 	.prealloc_buffer_size = 256 * PAGE_SIZE,
 };
 
-
 static int bcm2708_i2s_probe(struct platform_device *pdev)
 {
 	struct bcm2708_i2s_dev *dev;
@@ -907,6 +819,15 @@ static int bcm2708_i2s_probe(struct platform_device *pdev)
 	int ret;
 	struct regmap *regmap[2];
 	struct resource *mem[2];
+	const __be32 *addr;
+	dma_addr_t dma_reg_base;
+
+	addr = of_get_address(pdev->dev.of_node, 0, NULL, NULL);
+	if (!addr) {
+		dev_err(&pdev->dev, "could not get DMA-register address\n");
+		return -ENODEV;
+	}
+	dma_reg_base = be32_to_cpup(addr);
 
 	if (of_property_read_bool(pdev->dev.of_node, "brcm,enable-mmap"))
 		bcm2708_pcm_hardware.info |=
@@ -924,32 +845,24 @@ static int bcm2708_i2s_probe(struct platform_device *pdev)
 
 		regmap[i] = devm_regmap_init_mmio(&pdev->dev, base,
 					    &bcm2708_regmap_config[i]);
-		if (IS_ERR(regmap[i])) {
-			dev_err(&pdev->dev, "I2S probe: regmap init failed\n");
+		if (IS_ERR(regmap[i]))
 			return PTR_ERR(regmap[i]);
-		}
 	}
 
 	dev = devm_kzalloc(&pdev->dev, sizeof(*dev),
 			   GFP_KERNEL);
-	if (IS_ERR(dev))
-		return PTR_ERR(dev);
+	if (!dev)
+		return -ENOMEM;
 
 	dev->i2s_regmap = regmap[0];
 	dev->clk_regmap = regmap[1];
 
 	/* Set the DMA address */
 	dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK].addr =
-		(dma_addr_t)BCM2708_I2S_FIFO_PHYSICAL_ADDR;
+		dma_reg_base + BCM2708_I2S_FIFO_A_REG;
 
 	dev->dma_data[SNDRV_PCM_STREAM_CAPTURE].addr =
-		(dma_addr_t)BCM2708_I2S_FIFO_PHYSICAL_ADDR;
-
-	/* Set the DREQ */
-	dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK].slave_id =
-		BCM2708_DMA_DREQ_PCM_TX;
-	dev->dma_data[SNDRV_PCM_STREAM_CAPTURE].slave_id =
-		BCM2708_DMA_DREQ_PCM_RX;
+		dma_reg_base + BCM2708_I2S_FIFO_A_REG;
 
 	/* Set the bus width */
 	dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK].addr_width =
@@ -968,34 +881,24 @@ static int bcm2708_i2s_probe(struct platform_device *pdev)
 	dev->dev = &pdev->dev;
 	dev_set_drvdata(&pdev->dev, dev);
 
-	ret = snd_soc_register_component(&pdev->dev,
+	ret = devm_snd_soc_register_component(&pdev->dev,
 			&bcm2708_i2s_component, &bcm2708_i2s_dai, 1);
-
 	if (ret) {
 		dev_err(&pdev->dev, "Could not register DAI: %d\n", ret);
-		ret = -ENOMEM;
 		return ret;
 	}
 
-	ret = snd_dmaengine_pcm_register(&pdev->dev,
-				&bcm2708_dmaengine_pcm_config,
-				SND_DMAENGINE_PCM_FLAG_COMPAT);
+	ret = devm_snd_dmaengine_pcm_register(&pdev->dev,
+			&bcm2708_dmaengine_pcm_config,
+			SND_DMAENGINE_PCM_FLAG_COMPAT);
 	if (ret) {
 		dev_err(&pdev->dev, "Could not register PCM: %d\n", ret);
-		snd_soc_unregister_component(&pdev->dev);
 		return ret;
 	}
 
 	return 0;
 }
 
-static int bcm2708_i2s_remove(struct platform_device *pdev)
-{
-	snd_dmaengine_pcm_unregister(&pdev->dev);
-	snd_soc_unregister_component(&pdev->dev);
-	return 0;
-}
-
 static const struct of_device_id bcm2708_i2s_of_match[] = {
 	{ .compatible = "brcm,bcm2708-i2s", },
 	{},
@@ -1004,10 +907,8 @@ MODULE_DEVICE_TABLE(of, bcm2708_i2s_of_match);
 
 static struct platform_driver bcm2708_i2s_driver = {
 	.probe		= bcm2708_i2s_probe,
-	.remove		= bcm2708_i2s_remove,
 	.driver		= {
 		.name	= "bcm2708-i2s",
-		.owner	= THIS_MODULE,
 		.of_match_table = bcm2708_i2s_of_match,
 	},
 };