dix: adds support for none root window background

It lets the driver notify the server whether it can draw a background when
'-background none' option is used by the system platform. Use cases for that
could be video drivers performing mode-setting in kernel time, before X is up,
so a seamless transition would happen until X clients start to show up.

If the driver can copy the framebuffer cleanly then it can set the flag
(canDoBGNoneRoot), otherwise the server will fallback to the normal behaviour.
The system must explicit indicates willingness of doing so through
'-background none'. We could do this option as default; in such case,
malicious users would be able to steal the framebuffer with a bit of tricks.

For instance, I can see the content of my nVidia Quadro FX 580 framebuffer
old X session modifying a bit nv driver:

     xf86DPMSInit(pScreen, xf86DPMSSet, 0);

-    /* Clear the screen */
-    if(pNv->xaa) {
-        /* Use the acceleration engine */
-        pNv->xaa->SetupForSolidFill(pScrn, 0, GXcopy, ~0);
-        pNv->xaa->SubsequentSolidFillRect(pScrn,
-            0, 0, pScrn->displayWidth, pNv->offscreenHeight);
-        G80DmaKickoff(pNv);
-    } else {
-        /* Use a slow software clear path */
-        memset(pNv->mem, 0, pitch * pNv->offscreenHeight);
-    }
+    pScreen->canDoBGNoneRoot = TRUE;

The commit is originally based on discussions happened on xorg-devel:
http://lists.freedesktop.org/archives/xorg-devel/2010-June/009755.html

Signed-off-by: Tiago Vignatti <tiago.vignatti@nokia.com>
Reviewed-by: Rami Ylimäki <rami.ylimaki@vincit.fi>
Acked-by: Pauli Nieminen <ext-pauli.nieminen@nokia.com>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/os/utils.c b/os/utils.c
index c2158dc72..afdff0c5a 100644
--- a/os/utils.c
+++ b/os/utils.c
@@ -498,6 +498,7 @@ void UseMsg(void)
     ErrorF("-nolock                disable the locking mechanism\n");
     ErrorF("-nolisten string       don't listen on protocol\n");
     ErrorF("-noreset               don't reset after last client exists\n");
+    ErrorF("-background [none]     create root window with no background\n");
     ErrorF("-reset                 reset after last client exists\n");
     ErrorF("-p #                   screen-saver pattern duration (minutes)\n");
     ErrorF("-pn                    accept failure to listen on all ports\n");
@@ -828,6 +829,14 @@ ProcessCommandLine(int argc, char *argv[])
 	    defaultBackingStore = WhenMapped;
         else if ( strcmp( argv[i], "-wr") == 0)
             whiteRoot = TRUE;
+        else if ( strcmp( argv[i], "-background") == 0) {
+            if(++i < argc) {
+                if (!strcmp ( argv[i], "none"))
+                    bgNoneRoot = TRUE;
+                else
+                    UseMsg();
+            }
+        }
         else if ( strcmp( argv[i], "-maxbigreqsize") == 0) {
              if(++i < argc) {
                  long reqSizeArg = atol(argv[i]);