dtc: Warning rather than error on possible truncation of cell values

We always evaluate integer values in cell arrays as 64-bit quantities, then
truncate to the size of the array cells (32-bit by default).  However to
detect accidental truncation of meaningful values, we give an error if the
truncated portion isn't either all 0 or all 1 bits.  However, this can
still give counterintuitive errors.  For if the user is thinking in 2's
complement 32-bit arithmetic (which would be quite natural), then they'd
expect the expression (-0xffffffff-2) to evaluate to -1 (0xffffffff).
However in 64-bit it evaluates to 0xfffffffeffffffff which does truncate
to the expected value but trips this error message.

Because of this reduce the error to only a warnings, with a somewhat more
helpful message.

Fixes: #74

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-parser.y

@@ -404,9 +404,14 @@ arrayprefix:
 				 * within the mask to one (i.e. | in the
 				 * mask), all bits are one.
 				 */
-				if (($2 > mask) && (($2 | mask) != -1ULL))
-					ERROR(&@2, "Value out of range for"
-					      " %d-bit array element", $1.bits);
+				if (($2 > mask) && (($2 | mask) != -1ULL)) {
+					char *loc = srcpos_string(&@2);
+					fprintf(stderr,
+						"WARNING: %s: Value 0x%016" PRIx64
+						" truncated to 0x%0*" PRIx64 "\n",
+						loc, $2, $1.bits / 4, ($2 & mask));
+					free(loc);
+				}
 			}
 
 			$$.data = data_append_integer($1.data, $2, $1.bits);

tests/cell-overflow-results.dts

@@ -0,0 +1,7 @@
+/dts-v1/;
+
+/ {
+	prop1 = < 0 >;
+	prop2 = < 0xffffffff >;
+	prop3 = < 0 >;
+};

tests/cell-overflow.dts

@@ -0,0 +1,7 @@
+/dts-v1/;
+
+/ {
+	prop1 = < (-0xffffffff - 1) >;
+	prop2 = < (-0xffffffff - 2) >;
+	prop3 = < ((-0xffffffff - 1) * 2) >;
+};

tests/run_tests.sh

@@ -518,6 +518,11 @@ libfdt_tests () {
     ## https://github.com/dgibson/dtc/issues/64
     check_tests "$SRCDIR/phandle-args-overflow.dts" clocks_property
 
+    ## https://github.com/dgibson/dtc/issues/74
+    run_dtc_test -I dts -O dtb -o cell-overflow-results.test.dtb cell-overflow-results.dts
+    run_dtc_test -I dts -O dtb -o cell-overflow.test.dtb cell-overflow.dts
+    run_test dtbs_equal_ordered cell-overflow.test.dtb cell-overflow-results.test.dtb
+
     # check full tests
     for good in test_tree1.dtb; do
 	run_test check_full $good