nvmem: imx-ocotp: Change TIMING calculation to u-boot algorithm

The RELAX field of the OCOTP block is turning out as a zero on i.MX8MM.
This messes up the subsequent re-load of the fuse shadow registers.

After some discussion with people @ NXP its clear we have missed a trick
here in Linux.

The OCOTP fuse programming time has a physical minimum 'burn time' that is
not related to the ipg_clk.

We need to define the RELAX, STROBE_READ and STROBE_PROG fields in terms of
desired timings to allow for the burn-in to safely complete. Right now only
the RELAX field is calculated in terms of an absolute time and we are
ending up with a value of zero.

This patch inherits the u-boot timings for the OCOTP_TIMING calculation on
the i.MX6 and i.MX8. Those timings are known to work and critically specify
values such as STROBE_PROG as a minimum timing.

Fixes: 0642bac7 ("nvmem: imx-ocotp: add write support")
Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Suggested-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/imx-ocotp.c

@@ -44,7 +44,9 @@
 #define IMX_OCOTP_BM_CTRL_ERROR		0x00000200
 #define IMX_OCOTP_BM_CTRL_REL_SHADOWS	0x00000400
 
-#define DEF_RELAX			20	/* > 16.5ns */
+#define TIMING_STROBE_PROG_US		10	/* Min time to blow a fuse */
+#define TIMING_STROBE_READ_NS		37	/* Min time before read */
+#define TIMING_RELAX_NS			17
 #define DEF_FSOURCE			1001	/* > 1000 ns */
 #define DEF_STROBE_PROG			10000	/* IPG clocks */
 #define IMX_OCOTP_WR_UNLOCK		0x3E770000
@@ -176,12 +178,38 @@ static void imx_ocotp_set_imx6_timing(struct ocotp_priv *priv)
 	 * fields with timing values to match the current frequency of the
 	 * ipg_clk. OTP writes will work at maximum bus frequencies as long
 	 * as the HW_OCOTP_TIMING parameters are set correctly.
+	 *
+	 * Note: there are minimum timings required to ensure an OTP fuse burns
+	 * correctly that are independent of the ipg_clk. Those values are not
+	 * formally documented anywhere however, working from the minimum
+	 * timings given in u-boot we can say:
+	 *
+	 * - Minimum STROBE_PROG time is 10 microseconds. Intuitively 10
+	 *   microseconds feels about right as representative of a minimum time
+	 *   to physically burn out a fuse.
+	 *
+	 * - Minimum STROBE_READ i.e. the time to wait post OTP fuse burn before
+	 *   performing another read is 37 nanoseconds
+	 *
+	 * - Minimum RELAX timing is 17 nanoseconds. This final RELAX minimum
+	 *   timing is not entirely clear the documentation says "This
+	 *   count value specifies the time to add to all default timing
+	 *   parameters other than the Tpgm and Trd. It is given in number
+	 *   of ipg_clk periods." where Tpgm and Trd refer to STROBE_PROG
+	 *   and STROBE_READ respectively. What the other timing parameters
+	 *   are though, is not specified. Experience shows a zero RELAX
+	 *   value will mess up a re-load of the shadow registers post OTP
+	 *   burn.
 	 */
 	clk_rate = clk_get_rate(priv->clk);
 
-	relax = clk_rate / (1000000000 / DEF_RELAX) - 1;
-	strobe_prog = clk_rate / (1000000000 / 10000) + 2 * (DEF_RELAX + 1) - 1;
-	strobe_read = clk_rate / (1000000000 / 40) + 2 * (DEF_RELAX + 1) - 1;
+	relax = DIV_ROUND_UP(clk_rate * TIMING_RELAX_NS, 1000000000) - 1;
+	strobe_read = DIV_ROUND_UP(clk_rate * TIMING_STROBE_READ_NS,
+				   1000000000);
+	strobe_read += 2 * (relax + 1) - 1;
+	strobe_prog = DIV_ROUND_CLOSEST(clk_rate * TIMING_STROBE_PROG_US,
+					1000000);
+	strobe_prog += 2 * (relax + 1) - 1;
 
 	timing = readl(priv->base + IMX_OCOTP_ADDR_TIMING) & 0x0FC00000;
 	timing |= strobe_prog & 0x00000FFF;