env_mmc: allow negative CONFIG_ENV_OFFSET

A negative value of CONFIG_ENV_OFFSET is treated as a backwards offset
from the end of the eMMC device/partition, rather than a forwards offset
from the start.

This is useful when a single board may be stuffed with different eMMC
devices, each of which has a different capacity, and you always want the
environment to be stored at the very end of the device (or eMMC boot
partition for example).

One example of this case is NVIDIA's Ventana reference board.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/README b/README
index f1485bac13..6852080c43 100644
--- a/README
+++ b/README
@@ -3666,6 +3666,14 @@ but it can not erase, write this NOR flash by SRIO or PCIE interface.
 	  These two #defines specify the offset and size of the environment
 	  area within the specified MMC device.
 
+	  If offset is positive (the usual case), it is treated as relative to
+	  the start of the MMC partition. If offset is negative, it is treated
+	  as relative to the end of the MMC partition. This can be useful if
+	  your board may be fitted with different MMC devices, which have
+	  different sizes for the MMC partitions, and you always want the
+	  environment placed at the very end of the partition, to leave the
+	  maximum possible space before it, to store other data.
+
 	  These two values are in units of bytes, but must be aligned to an
 	  MMC sector boundary.
 
@@ -3676,6 +3684,9 @@ but it can not erase, write this NOR flash by SRIO or PCIE interface.
 	  valid backup copy in case the other copy is corrupted, e.g. due
 	  to a power failure during a "saveenv" operation.
 
+	  This value may also be positive or negative; this is handled in the
+	  same way as CONFIG_ENV_OFFSET.
+
 	  This value is also in units of bytes, but must also be aligned to
 	  an MMC sector boundary.