[PATCH v2 17/24] powerpc/mpc512x: improve DIU related clock setup

[Gerhard Sittig <gsi-ynQEQJNshbs@public.gmane.org>]

adapt the DIU clock initialization to the COMMON_CLK approach: device
tree based clock lookup, prepare and unprepare for clocks, work with
frequencies not dividers, call the appropriate clk_*() routines and
don't access CCM registers, remove the pre-enable workaround in the
platform's clock driver

the "best clock" determination now completely relies on the platform's
clock driver to pick a frequency close to what the caller requests, and
merely checks whether the desired frequency was met (is acceptable since
it meets the tolerance of the monitor) -- this approach shall succeed
upon first try in the usual case, will test a few less desirable yet
acceptable frequencies in edge cases, and will fallback to "best effort"
if none of the previously tried frequencies pass the test

Signed-off-by: Gerhard Sittig <gsi-ynQEQJNshbs@public.gmane.org>
---
 arch/powerpc/platforms/512x/clock-commonclk.c |    1 -
 arch/powerpc/platforms/512x/mpc512x_shared.c  |  165 +++++++++++++------------
 2 files changed, 88 insertions(+), 78 deletions(-)

diff --git a/arch/powerpc/platforms/512x/clock-commonclk.c b/arch/powerpc/platforms/512x/clock-commonclk.c
index 99d197d..b8963b7 100644
--- a/arch/powerpc/platforms/512x/clock-commonclk.c
+++ b/arch/powerpc/platforms/512x/clock-commonclk.c
@@ -692,7 +692,6 @@ static void mpc512x_clk_setup_clock_tree(int busfreq)
 	/* some are not yet acquired by their respective drivers */
 	clk_prepare_enable(clks[MPC512x_CLK_PSC3_MCLK]);/* serial console */
 	clk_prepare_enable(clks[MPC512x_CLK_FEC]);	/* network, NFS */
-	clk_prepare_enable(clks[MPC512x_CLK_DIU]);	/* display */
 	clk_prepare_enable(clks[MPC512x_CLK_I2C]);
 	/*
 	 * some have their individual clock subtree with separate clock
diff --git a/arch/powerpc/platforms/512x/mpc512x_shared.c b/arch/powerpc/platforms/512x/mpc512x_shared.c
index a82a41b..3381eea 100644
--- a/arch/powerpc/platforms/512x/mpc512x_shared.c
+++ b/arch/powerpc/platforms/512x/mpc512x_shared.c
@@ -12,6 +12,7 @@
  * (at your option) any later version.
  */
 
+#include <linux/clk.h>
 #include <linux/kernel.h>
 #include <linux/io.h>
 #include <linux/irq.h>
@@ -70,98 +71,108 @@ struct fsl_diu_shared_fb {
 	bool		in_use;
 };
 
-#define DIU_DIV_MASK	0x000000ff
+/* receives a pixel clock spec in pico seconds, adjusts the DIU clock rate */
 void mpc512x_set_pixel_clock(unsigned int pixclock)
 {
-	unsigned long bestval, bestfreq, speed, busfreq;
-	unsigned long minpixclock, maxpixclock, pixval;
-	struct mpc512x_ccm __iomem *ccm;
 	struct device_node *np;
-	u32 temp;
-	long err;
-	int i;
+	struct clk *clk_diu;
+	unsigned long epsilon, minpixclock, maxpixclock;
+	unsigned long offset, want, got, delta;
 
-	np = of_find_compatible_node(NULL, NULL, "fsl,mpc5121-clock");
+	/* lookup and enable the DIU clock */
+	np = of_find_compatible_node(NULL, NULL, "fsl,mpc5121-diu");
 	if (!np) {
-		pr_err("Can't find clock control module.\n");
+		pr_err("Could not find DIU device tree node.\n");
 		return;
 	}
-
-	ccm = of_iomap(np, 0);
+	clk_diu = of_clk_get_by_name(np, "per");
 	of_node_put(np);
-	if (!ccm) {
-		pr_err("Can't map clock control module reg.\n");
+	if (IS_ERR(clk_diu)) {
+		pr_err("Could not lookup DIU clock.\n");
 		return;
 	}
-
-	np = of_find_node_by_type(NULL, "cpu");
-	if (np) {
-		const unsigned int *prop =
-			of_get_property(np, "bus-frequency", NULL);
-
-		of_node_put(np);
-		if (prop) {
-			busfreq = *prop;
-		} else {
-			pr_err("Can't get bus-frequency property\n");
-			return;
-		}
-	} else {
-		pr_err("Can't find 'cpu' node.\n");
+	if (clk_prepare_enable(clk_diu)) {
+		pr_err("Could not enable DIU clock.\n");
 		return;
 	}
 
-	/* Pixel Clock configuration */
-	pr_debug("DIU: Bus Frequency = %lu\n", busfreq);
-	speed = busfreq * 4; /* DIU_DIV ratio is 4 * CSB_CLK / DIU_CLK */
-
-	/* Calculate the pixel clock with the smallest error */
-	/* calculate the following in steps to avoid overflow */
-	pr_debug("DIU pixclock in ps - %d\n", pixclock);
-	temp = (1000000000 / pixclock) * 1000;
-	pixclock = temp;
-	pr_debug("DIU pixclock freq - %u\n", pixclock);
-
-	temp = temp / 20; /* pixclock * 0.05 */
-	pr_debug("deviation = %d\n", temp);
-	minpixclock = pixclock - temp;
-	maxpixclock = pixclock + temp;
-	pr_debug("DIU minpixclock - %lu\n", minpixclock);
-	pr_debug("DIU maxpixclock - %lu\n", maxpixclock);
-	pixval = speed/pixclock;
-	pr_debug("DIU pixval = %lu\n", pixval);
-
-	err = LONG_MAX;
-	bestval = pixval;
-	pr_debug("DIU bestval = %lu\n", bestval);
-
-	bestfreq = 0;
-	for (i = -1; i <= 1; i++) {
-		temp = speed / (pixval+i);
-		pr_debug("DIU test pixval i=%d, pixval=%lu, temp freq. = %u\n",
-			i, pixval, temp);
-		if ((temp < minpixclock) || (temp > maxpixclock))
-			pr_debug("DIU exceeds monitor range (%lu to %lu)\n",
-				minpixclock, maxpixclock);
-		else if (abs(temp - pixclock) < err) {
-			pr_debug("Entered the else if block %d\n", i);
-			err = abs(temp - pixclock);
-			bestval = pixval + i;
-			bestfreq = temp;
-		}
+	/*
+	 * convert the picoseconds spec into the desired clock rate,
+	 * determine the acceptable clock range for the monitor (+/- 5%),
+	 * do the calculation in steps to avoid integer overflow
+	 */
+	pr_debug("DIU pixclock in ps - %u\n", pixclock);
+	pixclock = (1000000000 / pixclock) * 1000;
+	pr_debug("DIU pixclock freq  - %u\n", pixclock);
+	epsilon = pixclock / 20; /* pixclock * 0.05 */
+	pr_debug("DIU deviation      - %lu\n", epsilon);
+	minpixclock = pixclock - epsilon;
+	maxpixclock = pixclock + epsilon;
+	pr_debug("DIU minpixclock    - %lu\n", minpixclock);
+	pr_debug("DIU maxpixclock    - %lu\n", maxpixclock);
+
+	/*
+	 * check whether the DIU supports the desired pixel clock
+	 *
+	 * - simply request the desired clock and see what the
+	 *   platform's clock driver will make of it, assuming that it
+	 *   will setup the best approximation of the requested value
+	 * - try other candidate frequencies in the order of decreasing
+	 *   preference (i.e. with increasing distance from the desired
+	 *   pixel clock, and checking the lower frequency before the
+	 *   higher frequency to not overload the hardware) until the
+	 *   first match is found -- any potential subsequent match
+	 *   would only be as good as the former match or typically
+	 *   would be less preferrable
+	 *
+	 * the offset increment of pixelclock divided by 64 is an
+	 * arbitrary choice -- it's simple to calculate, in the typical
+	 * case we expect the first check to succeed already, in the
+	 * worst case seven frequencies get tested (the exact center and
+	 * three more values each to the left and to the right) before
+	 * the 5% tolerance window is exceeded, resulting in fast enough
+	 * execution yet high enough probability of finding a suitable
+	 * value, while the error rate will be in the order of single
+	 * percents
+	 */
+	for (offset = 0; offset <= epsilon; offset += pixclock / 64) {
+		want = pixclock - offset;
+		pr_debug("DIU checking clock - %lu\n", want);
+		clk_set_rate(clk_diu, want);
+		got = clk_get_rate(clk_diu);
+		delta = abs(pixclock - got);
+		if (delta < epsilon)
+			break;
+		if (!offset)
+			continue;
+		want = pixclock + offset;
+		pr_debug("DIU checking clock - %lu\n", want);
+		clk_set_rate(clk_diu, want);
+		got = clk_get_rate(clk_diu);
+		delta = abs(pixclock - got);
+		if (delta < epsilon)
+			break;
 	}
+	if (offset <= epsilon) {
+		pr_debug("DIU clock accepted - %lu\n", want);
+		pr_debug("DIU pixclock want %u, got %lu, delta %lu, eps %lu\n",
+			 pixclock, got, delta, epsilon);
+		return;
+	}
+	pr_warn("DIU pixclock auto search unsuccessful\n");
 
-	pr_debug("DIU chose = %lx\n", bestval);
-	pr_debug("DIU error = %ld\n NomPixClk ", err);
-	pr_debug("DIU: Best Freq = %lx\n", bestfreq);
-	/* Modify DIU_DIV in CCM SCFR1 */
-	temp = in_be32(&ccm->scfr1);
-	pr_debug("DIU: Current value of SCFR1: 0x%08x\n", temp);
-	temp &= ~DIU_DIV_MASK;
-	temp |= (bestval & DIU_DIV_MASK);
-	out_be32(&ccm->scfr1, temp);
-	pr_debug("DIU: Modified value of SCFR1: 0x%08x\n", temp);
-	iounmap(ccm);
+	/*
+	 * what is the most appropriate action to take when the search
+	 * for an available pixel clock which is acceptable to the
+	 * monitor has failed?  disable the DIU (clock) or just provide
+	 * a "best effort"?  we go with the latter
+	 */
+	pr_warn("DIU pixclock best effort fallback (backend's choice)\n");
+	clk_set_rate(clk_diu, pixclock);
+	got = clk_get_rate(clk_diu);
+	delta = abs(pixclock - got);
+	pr_debug("DIU pixclock want %u, got %lu, delta %lu, eps %lu\n",
+		 pixclock, got, delta, epsilon);
 }
 
 enum fsl_diu_monitor_port
-- 
1.7.10.4