When a device sits in recovery mode waiting for boot media to be plugged in, it periodically polls USB/SD/etc ports to see if a new storage device was plugged in.

In every poll cycle it invokes the vbDisplayScreen() function, which re-draws the screen. This is unnecessary, could cause flickering on some slow devices.


the below patch is one way of fixing the problem:


diff --git a/firmware/lib/vboot_api_kernel.c b/firmware/lib/vboot_api_kernel.c
index c336278..b8e0830 100644
--- a/firmware/lib/vboot_api_kernel.c
+++ b/firmware/lib/vboot_api_kernel.c
@@ -499,6 +499,7 @@ VbError_t VbBootRecovery(VbCommonParams *cparams, LoadKernelParams *p)
 {
        VbSharedDataHeader *shared =
                (VbSharedDataHeader *)cparams->shared_data_blob;
+       uint32_t previous_retval = VBERROR_SUCCESS;
        uint32_t retval;
        uint32_t key;
        int i;
@@ -552,10 +553,14 @@ VbError_t VbBootRecovery(VbCommonParams *cparams, LoadKernelParams *p)
                if (VBERROR_SUCCESS == retval)
                        break; /* Found a recovery kernel */
 
-               VbDisplayScreen(cparams, VBERROR_NO_DISK_FOUND == retval ?
-                               VB_SCREEN_RECOVERY_INSERT :
-                               VB_SCREEN_RECOVERY_NO_GOOD,
-                               0, &vnc);
+               if (retval != previous_retval) {
+                       previous_retval = retval;
+                       VbDisplayScreen(cparams,
+                                       VBERROR_NO_DISK_FOUND == retval ?
+                                       VB_SCREEN_RECOVERY_INSERT :
+                                       VB_SCREEN_RECOVERY_NO_GOOD,
+                                       0, &vnc);
+               }
 
                /*
                 * Scan keyboard more frequently than media, since x86



it was considered not comprehensive enough.