udlfb: Add module option to do without shadow framebuffer

By default, udlfb allocates a 2nd buffer to shadow what's across
the bus on the USB device.  It can operate without this shadow,
but then it cannot tell which pixels have changed, and must send all.

Saves host memory, but worsens the USB 2.0 bus bottleneck.

This option allows users in very low memory situations (e.g.
bifferboard) to optionally turn off this shadow framebuffer.

Signed-off-by: Stuart Hopkins <stuart@linux-depot.com>
Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/fb/udlfb.txt b/Documentation/fb/udlfb.txt
index 7fdde2a02a2..473ceed9e4e 100644
--- a/Documentation/fb/udlfb.txt
+++ b/Documentation/fb/udlfb.txt
@@ -105,6 +105,11 @@ console		Allow fbcon to attach to udlfb provided framebuffers. This
                 the first framebuffer it finds, which isn't usually what the
                 user wants in the case of USB displays.
 
+shadow          Allocate a 2nd framebuffer to shadow what's currently across
+                the USB bus in device memory. If any pixels are unchanged,
+                do not transmit. Spends host memory to save USB transfers.
+                Enabled by default. Only disable on very low memory systems.
+
 Sysfs Attributes
 ================