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+How to port a SPI driver to driver model
+========================================
+
+Here is a rough step-by-step guide. It is based around converting the
+exynos SPI driver to driver model (DM) and the example code is based
+around U-Boot v2014.10-rc2 (commit be9f643).
+
+It is quite long since it includes actual code examples.
+
+Before driver model, SPI drivers have their own private structure which
+contains 'struct spi_slave'. With driver model, 'struct spi_slave' still
+exists, but now it is 'per-child data' for the SPI bus. Each child of the
+SPI bus is a SPI slave. The information that was stored in the
+driver-specific slave structure can now be port in private data for the
+SPI bus.
+
+For example, struct tegra_spi_slave looks like this:
+
+struct tegra_spi_slave {
+	struct spi_slave slave;
+	struct tegra_spi_ctrl *ctrl;
+};
+
+In this case 'slave' will be in per-child data, and 'ctrl' will be in the
+SPI's buses private data.
+
+
+0. How long does this take?
+
+You should be able to complete this within 2 hours, including testing but
+excluding preparing the patches. The API is basically the same as before
+with only minor changes:
+
+- methods to set speed and mode are separated out
+- cs_info is used to get information on a chip select
+
+
+1. Enable driver mode for SPI and SPI flash
+
+Add these to your board config:
+
+#define CONFIG_DM_SPI
+#define CONFIG_DM_SPI_FLASH
+
+
+2. Add the skeleton
+
+Put this code at the bottom of your existing driver file:
+
+struct spi_slave *spi_setup_slave(unsigned int busnum, unsigned int cs,
+			unsigned int max_hz, unsigned int mode)
+{
+	return NULL;
+}
+
+struct spi_slave *spi_setup_slave_fdt(const void *blob, int slave_node,
+				      int spi_node)
+{
+	return NULL;
+}
+
+static int exynos_spi_ofdata_to_platdata(struct udevice *dev)
+{
+	return -ENODEV;
+}
+
+static int exynos_spi_probe(struct udevice *dev)
+{
+	return -ENODEV;
+}
+
+static int exynos_spi_remove(struct udevice *dev)
+{
+	return -ENODEV;
+}
+
+static int exynos_spi_claim_bus(struct udevice *dev)
+{
+
+	return -ENODEV;
+}
+
+static int exynos_spi_release_bus(struct udevice *dev)
+{
+
+	return -ENODEV;
+}
+
+static int exynos_spi_xfer(struct udevice *dev, unsigned int bitlen,
+			    const void *dout, void *din, unsigned long flags)
+{
+
+	return -ENODEV;
+}
+
+static int exynos_spi_set_speed(struct udevice *dev, uint speed)
+{
+	return -ENODEV;
+}
+
+static int exynos_spi_set_mode(struct udevice *dev, uint mode)
+{
+	return -ENODEV;
+}
+
+static int exynos_cs_info(struct udevice *bus, uint cs,
+			  struct spi_cs_info *info)
+{
+	return -ENODEV;
+}
+
+static const struct dm_spi_ops exynos_spi_ops = {
+	.claim_bus	= exynos_spi_claim_bus,
+	.release_bus	= exynos_spi_release_bus,
+	.xfer		= exynos_spi_xfer,
+	.set_speed	= exynos_spi_set_speed,
+	.set_mode	= exynos_spi_set_mode,
+	.cs_info	= exynos_cs_info,
+};
+
+static const struct udevice_id exynos_spi_ids[] = {
+	{ .compatible = "samsung,exynos-spi" },
+	{ }
+};
+
+U_BOOT_DRIVER(exynos_spi) = {
+	.name	= "exynos_spi",
+	.id	= UCLASS_SPI,
+	.of_match = exynos_spi_ids,
+	.ops	= &exynos_spi_ops,
+	.ofdata_to_platdata = exynos_spi_ofdata_to_platdata,
+	.probe	= exynos_spi_probe,
+	.remove	= exynos_spi_remove,
+};
+
+
+3. Replace 'exynos' in the above code with your driver name
+
+
+4. #ifdef out all of the code in your driver except for the above
+
+This will allow you to get it building, which means you can work
+incrementally. Since all the methods return an error initially, there is
+less chance that you will accidentally leave something in.
+
+Also, even though your conversion is basically a rewrite, it might help
+reviewers if you leave functions in the same place in the file,
+particularly for large drivers.
+
+
+5. Add some includes
+
+Add these includes to your driver:
+
+#include <dm.h>
+#include <errno.h>
+
+
+6. Build
+
+At this point you should be able to build U-Boot for your board with the
+empty SPI driver. You still have empty methods in your driver, but we will
+write these one by one.
+
+If you have spi_init() functions or the like that are called from your
+board then the build will fail. Remove these calls and make a note of the
+init that needs to be done.
+
+
+7. Set up your platform data structure
+
+This will hold the information your driver to operate, like its hardware
+address or maximum frequency.
+
+You may already have a struct like this, or you may need to create one
+from some of the #defines or global variables in the driver.
+
+Note that this information is not the run-time information. It should not
+include state that changes. It should be fixed throughout the live of
+U-Boot. Run-time information comes later.
+
+Here is what was in the exynos spi driver:
+
+struct spi_bus {
+	enum periph_id periph_id;
+	s32 frequency;		/* Default clock frequency, -1 for none */
+	struct exynos_spi *regs;
+	int inited;		/* 1 if this bus is ready for use */
+	int node;
+	uint deactivate_delay_us;	/* Delay to wait after deactivate */
+};
+
+Of these, inited is handled by DM and node is the device tree node, which
+DM tells you. The name is not quite right. So in this case we would use:
+
+struct exynos_spi_platdata {
+	enum periph_id periph_id;
+	s32 frequency;		/* Default clock frequency, -1 for none */
+	struct exynos_spi *regs;
+	uint deactivate_delay_us;	/* Delay to wait after deactivate */
+};
+
+
+8a. Write ofdata_to_platdata()   [for device tree only]
+
+This method will convert information in the device tree node into a C
+structure in your driver (called platform data). If you are not using
+device tree, go to 8b.
+
+DM will automatically allocate the struct for us when we are using device
+tree, but we need to tell it the size:
+
+U_BOOT_DRIVER(spi_exynos) = {
+...
+	.platdata_auto_alloc_size = sizeof(struct exynos_spi_platdata),
+
+
+Here is a sample function. It gets a pointer to the platform data and
+fills in the fields from device tree.
+
+static int exynos_spi_ofdata_to_platdata(struct udevice *bus)
+{
+	struct exynos_spi_platdata *plat = bus->platdata;
+	const void *blob = gd->fdt_blob;
+	int node = bus->of_offset;
+
+	plat->regs = (struct exynos_spi *)fdtdec_get_addr(blob, node, "reg");
+	plat->periph_id = pinmux_decode_periph_id(blob, node);
+
+	if (plat->periph_id == PERIPH_ID_NONE) {
+		debug("%s: Invalid peripheral ID %d\n", __func__,
+			plat->periph_id);
+		return -FDT_ERR_NOTFOUND;
+	}
+
+	/* Use 500KHz as a suitable default */
+	plat->frequency = fdtdec_get_int(blob, node, "spi-max-frequency",
+					500000);
+	plat->deactivate_delay_us = fdtdec_get_int(blob, node,
+					"spi-deactivate-delay", 0);
+	debug("%s: regs=%p, periph_id=%d, max-frequency=%d, deactivate_delay=%d\n",
+	      __func__, plat->regs, plat->periph_id, plat->frequency,
+              plat->deactivate_delay_us);
+
+	return 0;
+}
+
+
+8b. Add the platform data  [non-device-tree only]
+
+Specify this data in a U_BOOT_DEVICE() declaration in your board file:
+
+struct exynos_spi_platdata platdata_spi0 = {
+	.periph_id = ...
+	.frequency = ...
+	.regs = ...
+	.deactivate_delay_us = ...
+};
+
+U_BOOT_DEVICE(board_spi0) = {
+	.name = "exynos_spi",
+	.platdata = &platdata_spi0,
+};
+
+You will unfortunately need to put the struct into a header file in this
+case so that your board file can use it.
+
+
+9. Add the device private data
+
+Most devices have some private data which they use to keep track of things
+while active. This is the run-time information and needs to be stored in
+a structure. There is probably a structure in the driver that includes a
+'struct spi_slave', so you can use that.
+
+struct exynos_spi_slave {
+	struct spi_slave slave;
+	struct exynos_spi *regs;
+	unsigned int freq;		/* Default frequency */
+	unsigned int mode;
+	enum periph_id periph_id;	/* Peripheral ID for this device */
+	unsigned int fifo_size;
+	int skip_preamble;
+	struct spi_bus *bus;		/* Pointer to our SPI bus info */
+	ulong last_transaction_us;	/* Time of last transaction end */
+};
+
+
+We should rename this to make its purpose more obvious, and get rid of
+the slave structure, so we have:
+
+struct exynos_spi_priv {
+	struct exynos_spi *regs;
+	unsigned int freq;		/* Default frequency */
+	unsigned int mode;
+	enum periph_id periph_id;	/* Peripheral ID for this device */
+	unsigned int fifo_size;
+	int skip_preamble;
+	ulong last_transaction_us;	/* Time of last transaction end */
+};
+
+
+DM can auto-allocate this also:
+
+U_BOOT_DRIVER(spi_exynos) = {
+...
+	.priv_auto_alloc_size = sizeof(struct exynos_spi_priv),
+
+
+Note that this is created before the probe method is called, and destroyed
+after the remove method is called. It will be zeroed when the probe
+method is called.
+
+
+10. Add the probe() and remove() methods
+
+Note: It's a good idea to build repeatedly as you are working, to avoid a
+huge amount of work getting things compiling at the end.
+
+The probe method is supposed to set up the hardware. U-Boot used to use
+spi_setup_slave() to do this. So take a look at this function and see
+what you can copy out to set things up.
+
+
+static int exynos_spi_probe(struct udevice *bus)
+{
+	struct exynos_spi_platdata *plat = dev_get_platdata(bus);
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+
+	priv->regs = plat->regs;
+	if (plat->periph_id == PERIPH_ID_SPI1 ||
+	    plat->periph_id == PERIPH_ID_SPI2)
+		priv->fifo_size = 64;
+	else
+		priv->fifo_size = 256;
+
+	priv->skip_preamble = 0;
+	priv->last_transaction_us = timer_get_us();
+	priv->freq = plat->frequency;
+	priv->periph_id = plat->periph_id;
+
+	return 0;
+}
+
+This implementation doesn't actually touch the hardware, which is somewhat
+unusual for a driver. In this case we will do that when the device is
+claimed by something that wants to use the SPI bus.
+
+For remove we could shut down the clocks, but in this case there is
+nothing to do. DM frees any memory that it allocated, so we can just
+remove exynos_spi_remove() and its reference in U_BOOT_DRIVER.
+
+
+11. Implement set_speed()
+
+This should set up clocks so that the SPI bus is running at the right
+speed. With the old API spi_claim_bus() would normally do this and several
+of the following functions, so let's look at that function:
+
+int spi_claim_bus(struct spi_slave *slave)
+{
+	struct exynos_spi_slave *spi_slave = to_exynos_spi(slave);
+	struct exynos_spi *regs = spi_slave->regs;
+	u32 reg = 0;
+	int ret;
+
+	ret = set_spi_clk(spi_slave->periph_id,
+					spi_slave->freq);
+	if (ret < 0) {
+		debug("%s: Failed to setup spi clock\n", __func__);
+		return ret;
+	}
+
+	exynos_pinmux_config(spi_slave->periph_id, PINMUX_FLAG_NONE);
+
+	spi_flush_fifo(slave);
+
+	reg = readl(&regs->ch_cfg);
+	reg &= ~(SPI_CH_CPHA_B | SPI_CH_CPOL_L);
+
+	if (spi_slave->mode & SPI_CPHA)
+		reg |= SPI_CH_CPHA_B;
+
+	if (spi_slave->mode & SPI_CPOL)
+		reg |= SPI_CH_CPOL_L;
+
+	writel(reg, &regs->ch_cfg);
+	writel(SPI_FB_DELAY_180, &regs->fb_clk);
+
+	return 0;
+}
+
+
+It sets up the speed, mode, pinmux, feedback delay and clears the FIFOs.
+With DM these will happen in separate methods.
+
+
+Here is an example for the speed part:
+
+static int exynos_spi_set_speed(struct udevice *bus, uint speed)
+{
+	struct exynos_spi_platdata *plat = bus->platdata;
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+	int ret;
+
+	if (speed > plat->frequency)
+		speed = plat->frequency;
+	ret = set_spi_clk(priv->periph_id, speed);
+	if (ret)
+		return ret;
+	priv->freq = speed;
+	debug("%s: regs=%p, speed=%d\n", __func__, priv->regs, priv->freq);
+
+	return 0;
+}
+
+
+12. Implement set_mode()
+
+This should adjust the SPI mode (polarity, etc.). Again this code probably
+comes from the old spi_claim_bus(). Here is an example:
+
+
+static int exynos_spi_set_mode(struct udevice *bus, uint mode)
+{
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+	uint32_t reg;
+
+	reg = readl(&priv->regs->ch_cfg);
+	reg &= ~(SPI_CH_CPHA_B | SPI_CH_CPOL_L);
+
+	if (mode & SPI_CPHA)
+		reg |= SPI_CH_CPHA_B;
+
+	if (mode & SPI_CPOL)
+		reg |= SPI_CH_CPOL_L;
+
+	writel(reg, &priv->regs->ch_cfg);
+	priv->mode = mode;
+	debug("%s: regs=%p, mode=%d\n", __func__, priv->regs, priv->mode);
+
+	return 0;
+}
+
+
+13. Implement claim_bus()
+
+This is where a client wants to make use of the bus, so claims it first.
+At this point we need to make sure everything is set up ready for data
+transfer. Note that this function is wholly internal to the driver - at
+present the SPI uclass never calls it.
+
+Here again we look at the old claim function and see some code that is
+needed. It is anything unrelated to speed and mode:
+
+static int exynos_spi_claim_bus(struct udevice *bus)
+{
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+
+	exynos_pinmux_config(priv->periph_id, PINMUX_FLAG_NONE);
+	spi_flush_fifo(priv->regs);
+
+	writel(SPI_FB_DELAY_180, &priv->regs->fb_clk);
+
+	return 0;
+}
+
+The spi_flush_fifo() function is in the removed part of the code, so we
+need to expose it again (perhaps with an #endif before it and '#if 0'
+after it). It only needs access to priv->regs which is why we have
+passed that in:
+
+/**
+ * Flush spi tx, rx fifos and reset the SPI controller
+ *
+ * @param regs	Pointer to SPI registers
+ */
+static void spi_flush_fifo(struct exynos_spi *regs)
+{
+	clrsetbits_le32(&regs->ch_cfg, SPI_CH_HS_EN, SPI_CH_RST);
+	clrbits_le32(&regs->ch_cfg, SPI_CH_RST);
+	setbits_le32(&regs->ch_cfg, SPI_TX_CH_ON | SPI_RX_CH_ON);
+}
+
+
+14. Implement release_bus()
+
+This releases the bus - in our example the old code in spi_release_bus()
+is a call to spi_flush_fifo, so we add:
+
+static int exynos_spi_release_bus(struct udevice *bus)
+{
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+
+	spi_flush_fifo(priv->regs);
+
+	return 0;
+}
+
+
+15. Implement xfer()
+
+This is the final method that we need to create, and it is where all the
+work happens. The method parameters are the same as the old spi_xfer() with
+the addition of a 'struct udevice' so conversion is pretty easy. Start
+by copying the contents of spi_xfer() to your new xfer() method and proceed
+from there.
+
+If (flags & SPI_XFER_BEGIN) is non-zero then xfer() normally calls an
+activate function, something like this:
+
+void spi_cs_activate(struct spi_slave *slave)
+{
+	struct exynos_spi_slave *spi_slave = to_exynos_spi(slave);
+
+	/* If it's too soon to do another transaction, wait */
+	if (spi_slave->bus->deactivate_delay_us &&
+	    spi_slave->last_transaction_us) {
+		ulong delay_us;		/* The delay completed so far */
+		delay_us = timer_get_us() - spi_slave->last_transaction_us;
+		if (delay_us < spi_slave->bus->deactivate_delay_us)
+			udelay(spi_slave->bus->deactivate_delay_us - delay_us);
+	}
+
+	clrbits_le32(&spi_slave->regs->cs_reg, SPI_SLAVE_SIG_INACT);
+	debug("Activate CS, bus %d\n", spi_slave->slave.bus);
+	spi_slave->skip_preamble = spi_slave->mode & SPI_PREAMBLE;
+}
+
+The new version looks like this:
+
+static void spi_cs_activate(struct udevice *dev)
+{
+	struct udevice *bus = dev->parent;
+	struct exynos_spi_platdata *pdata = dev_get_platdata(bus);
+	struct exynos_spi_priv *priv = dev_get_priv(bus);
+
+	/* If it's too soon to do another transaction, wait */
+	if (pdata->deactivate_delay_us &&
+	    priv->last_transaction_us) {
+		ulong delay_us;		/* The delay completed so far */
+		delay_us = timer_get_us() - priv->last_transaction_us;
+		if (delay_us < pdata->deactivate_delay_us)
+			udelay(pdata->deactivate_delay_us - delay_us);
+	}
+
+	clrbits_le32(&priv->regs->cs_reg, SPI_SLAVE_SIG_INACT);
+	debug("Activate CS, bus '%s'\n", bus->name);
+	priv->skip_preamble = priv->mode & SPI_PREAMBLE;
+}
+
+All we have really done here is change the pointers and print the device name
+instead of the bus number. Other local static functions can be treated in
+the same way.
+
+
+16. Set up the per-child data and child pre-probe function
+
+To minimise the pain and complexity of the SPI subsystem while the driver
+model change-over is in place, struct spi_slave is used to reference a
+SPI bus slave, even though that slave is actually a struct udevice. In fact
+struct spi_slave is the device's child data. We need to make sure this space
+is available. It is possible to allocate more space that struct spi_slave
+needs, but this is the minimum.
+
+U_BOOT_DRIVER(exynos_spi) = {
+...
+	.per_child_auto_alloc_size	= sizeof(struct spi_slave),
+}
+
+
+17. Optional: Set up cs_info() if you want it
+
+Sometimes it is useful to know whether a SPI chip select is valid, but this
+is not obvious from outside the driver. In this case you can provide a
+method for cs_info() to deal with this. If you don't provide it, then the
+device tree will be used to determine what chip selects are valid.
+
+Return -ENODEV if the supplied chip select is invalid, or 0 if it is valid.
+If you don't provide the cs_info() method, -ENODEV is assumed for all
+chip selects that do not appear in the device tree.
+
+
+18. Test it
+
+Now that you have the code written and it compiles, try testing it using
+the 'sf test' command. You may need to enable CONFIG_CMD_SF_TEST for your
+board.
+
+
+19. Prepare patches and send them to the mailing lists
+
+You can use 'tools/patman/patman' to prepare, check and send patches for
+your work. See the README for details.