powerpc/t4240qds: Add VDD override

Allow VDD voltage overriding with a command. This is an add-on feasture of
VID. To override VDD, use command vdd_override with the value of voltage
in mV, for example

vdd_override <voltage in mV, eg. 1050>

The above example will set the VDD to 1.050 volt. Any wrong value out of
range of 0.8188 to 1.2125 volt or invalid string is ignored.

In addition to the command, if overriding VDD is needed earlier in booting
process, save an variable and reboot:

setenv t4240qds_vdd_mv <voltage in mV>
saveenv

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/doc/README.t4240qds b/doc/README.t4240qds
index 677d120a80..19e8a8ae1f 100644
--- a/doc/README.t4240qds
+++ b/doc/README.t4240qds
@@ -96,3 +96,27 @@ The addresses in brackets are physical addresses.
 0x0_ffff_f000 (0x0_7fff_fff0) - 0x0_ffff_ffff   4KB Boot page translation for secondary cores
 
 The physical address of the last (boot page translation) varies with the actual DDR size.
+
+Voltage ID and VDD override
+--------------------
+T4240 has a VID feature. U-boot reads the VID efuses and adjust the voltage
+accordingly. The voltage can also be override by command vdd_override. The
+syntax is
+
+vdd_override <voltage in mV>, eg. 1050 is for 1.050v.
+
+Upon success, the actual voltage will be read back. The value is checked
+for safety and any invalid value will not adjust the voltage.
+
+Another way to override VDD is to use environmental variable, in case of using
+command is too late for some debugging. The syntax is
+
+setenv t4240qds_vdd_mv <voltage in mV>
+saveenv
+reset
+
+The override voltage takes effect when booting.
+
+Note: voltage adjustment needs to be done step by step. Changing voltage too
+rapidly may cause current surge. The voltage stepping is done by software.
+Users can set the final voltage directly.