ref-manual/variables.rst: document the IMAGE_ROOTFS_MAXSIZE variable

This variable specifies the maximum allowed size
of the generated image in kilobytes.

CC: Yoann Congal <yoann.congal@smile.fr>
CC: Randy MacLeod <randy.macleod@windriver.com>
CC: Antonin Godard <antonin.godard@bootlin.com>
CC: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: f3ddc92081363aa7ef7d4fc2c3b918f32f7bda05)

Signed-off-by: Christos Gavros <gavrosc@yahoo.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -4299,9 +4299,33 @@ system and gives an overview of their function and contents.
          IMAGE_ROOTFS_EXTRA_SPACE = "41943040"
 
    :term:`IMAGE_ROOTFS_MAXSIZE`
-      Defines the maximum size in Kbytes for the generated image. If the
-      generated image size is above that, the build will fail. It's a good
-      idea to set this variable for images that need to fit on a limited
+      Defines the maximum allowed size of the generated image in kilobytes.
+      The build will fail if the generated image size exceeds this value.
+
+      The generated image size undergoes several calculation steps before being
+      compared to :term:`IMAGE_ROOTFS_MAXSIZE`.
+      In the first step, the size of the directory pointed to by :term:`IMAGE_ROOTFS`
+      is calculated.
+      In the second step, the result from the first step is multiplied
+      by :term:`IMAGE_OVERHEAD_FACTOR`.
+      In the third step, the result from the second step is compared with
+      :term:`IMAGE_ROOTFS_SIZE`. The larger value of these is added to
+      :term:`IMAGE_ROOTFS_EXTRA_SPACE`.
+      In the fourth step, the result from the third step is checked for
+      a decimal part. If it has one, it is rounded up to the next integer.
+      If it does not, it is simply converted into an integer.
+      In the fifth step, the :term:`IMAGE_ROOTFS_ALIGNMENT` is added to the result
+      from the fourth step and "1" is subtracted.
+      In the sixth step, the remainder of the division between the result
+      from the fifth step and :term:`IMAGE_ROOTFS_ALIGNMENT` is subtracted from the
+      result of the fifth step. In this way, the result from the fourth step is
+      rounded up to the nearest multiple of :term:`IMAGE_ROOTFS_ALIGNMENT`.
+
+      Thus, if the :term:`IMAGE_ROOTFS_MAXSIZE` is set, is compared with the result
+      of the above calculations and is independent of the final image type.
+      No default value is set for :term:`IMAGE_ROOTFS_MAXSIZE`.
+
+      It's a good idea to set this variable for images that need to fit on a limited
       space (e.g. SD card, a fixed-size partition, ...).
 
    :term:`IMAGE_ROOTFS_SIZE`