arm/efi: Handle Xen bootargs from both xen.cfg and DT

Currently the Xen UEFI stub can accept Xen boot arguments from
the Xen configuration file using the "options=" keyword, but also
directly from the device tree specifying xen,xen-bootargs
property.

When the configuration file is used, device tree boot arguments
are ignored and overwritten even if the keyword "options=" is
not used.

This patch handle this case, so if the Xen configuration file is not
specifying boot arguments, the device tree boot arguments will be
used, if they are present.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit f3999bc2e0)

docs/misc/efi.pandoc

@@ -249,6 +249,10 @@ UEFI stub for module loading.
 When adding DomU modules to device tree, also add the property
 xen,uefi-cfg-load under chosen for Xen to load the Xen config file.
 Otherwise, Xen will skip the config file and rely on device tree alone.
+When using the Xen configuration file in conjunction with the device tree, you
+can specify the Xen boot arguments in the configuration file with the "options="
+keyword or in the device tree with the "xen,xen-bootargs" property, but be
+aware that the Xen configuration file value has a precedence over the DT value.
 
 Example 1 of how to boot a true dom0less configuration:
 

xen/arch/arm/efi/efi-boot.h

@@ -514,11 +514,26 @@ static void __init efi_arch_handle_cmdline(CHAR16 *image_name,
 
     if ( cfgfile_options )
     {
+        PrintMessage(L"Using bootargs from Xen configuration file.");
         prop_len += snprintf(buf + prop_len,
                                EFI_PAGE_SIZE - prop_len, " %s", cfgfile_options);
         if ( prop_len >= EFI_PAGE_SIZE )
             blexit(L"FDT string overflow");
     }
+    else
+    {
+        /* Get xen,xen-bootargs in /chosen if it is specified */
+        const char *dt_bootargs_prop = fdt_getprop(fdt, chosen,
+                                                   "xen,xen-bootargs", NULL);
+        if ( dt_bootargs_prop )
+        {
+            PrintMessage(L"Using bootargs from device tree.");
+            prop_len += snprintf(buf + prop_len, EFI_PAGE_SIZE - prop_len,
+                                 " %s", dt_bootargs_prop);
+            if ( prop_len >= EFI_PAGE_SIZE )
+                blexit(L"FDT string overflow");
+        }
+    }
 
     if ( cmdline_options )
     {