[v2] Make the dcache (code/stack cache) handle line reading errors better

diff --git a/gdb/dcache.c b/gdb/dcache.c
index d3b546b..9780f4d 100644
--- a/gdb/dcache.c
+++ b/gdb/dcache.c
@@ -497,8 +497,11 @@  dcache_read_memory_partial (struct target_ops *ops, DCACHE *dcache,
 
   if (i == 0)
     {
-      /* FIXME: We lose the real error status.  */
-      return TARGET_XFER_E_IO;
+      /* Even though reading the whole line failed, we may be able to
+	 read a piece starting where the caller wanted.  */
+      return ops->to_xfer_partial (ops, TARGET_OBJECT_MEMORY, NULL,
+				   myaddr, NULL, memaddr, len,
+				   xfered_len);
     }
   else
     {
diff --git a/gdb/testsuite/gdb.base/dcache-line-read-error.c b/gdb/testsuite/gdb.base/dcache-line-read-error.c
new file mode 100644
index 0000000..4120593
--- /dev/null
+++ b/gdb/testsuite/gdb.base/dcache-line-read-error.c
@@ -0,0 +1,107 @@ 
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2012-2014 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <string.h>
+
+size_t pg_size;
+void *first_mapped_page;
+void *last_mapped_page;
+
+void
+breakpt (void)
+{
+  /* Nothing. */
+}
+
+int
+main (void)
+{
+  void *p;
+  int pg_count;
+  size_t i;
+
+  /* Map 6 contiguous pages, and then unmap all second first, and
+     second last.
+
+     From GDB we will disassemble each of the _mapped_ pages, with a
+     code-cache (dcache) line size bigger than the page size (twice
+     bigger).  This makes GDB try to read one page before the mapped
+     page once, and the page after another time.  GDB should give no
+     error in either case.
+
+     That is, depending on where the kernel aligns the pages, we get
+     either:
+
+      .---.---.---.---.---.---.
+      | U | M | U | U | M | U |
+      '---'---'---'---'---'---.
+      |       |       |       |  <- line alignment
+       ^^^^^^^         ^^^^^^^
+          |               |
+          + line1         + line2
+
+     Or:
+
+      .---.---.---.---.---.---.
+      | U | M | U | U | M | U |
+      '---'---'---'---'---'---.
+          |       |       |      <- line alignment
+           ^^^^^^^ ^^^^^^^
+              |       |
+        line1 +       + line2
+
+    Note we really want to test that dcache behaves correctly when
+    reading a cache line fails.  We're just using unmapped memory as
+    proxy for any kind of error.  */
+
+  pg_size = getpagesize ();
+  pg_count = 6;
+
+  p = mmap (0, pg_count * pg_size, PROT_READ|PROT_WRITE,
+	    MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
+  if (p == MAP_FAILED)
+    {
+      perror ("mmap");
+      return EXIT_FAILURE;
+    }
+
+  /* Leave memory zero-initialized.  Disassembling 0s should behave on
+     all targets.  */
+
+  for (i = 0; i < pg_count; i++)
+    {
+      if (i == 1 || i == 4)
+	continue;
+
+      if (munmap (p + (i * pg_size), pg_size) == -1)
+	{
+	  perror ("munmap");
+	  return EXIT_FAILURE;
+	}
+    }
+
+  first_mapped_page = p + 1 * pg_size;;
+  last_mapped_page = p + 4 * pg_size;
+
+  breakpt ();
+
+  return EXIT_SUCCESS;
+}
diff --git a/gdb/testsuite/gdb.base/dcache-line-read-error.exp b/gdb/testsuite/gdb.base/dcache-line-read-error.exp
new file mode 100644
index 0000000..7e75460
--- /dev/null
+++ b/gdb/testsuite/gdb.base/dcache-line-read-error.exp
@@ -0,0 +1,68 @@ 
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that dcache behaves correctly when reading a cache line fails.
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" ${testfile}] } {
+    return -1
+}
+
+if ![runto breakpt] {
+    return -1
+}
+
+# Issue the "delete mem" command.  This makes GDB ignore the
+# target-provided list, if any.
+
+proc delete_mem {} {
+    global gdb_prompt
+
+    set test "delete mem"
+    gdb_test_multiple $test $test {
+	-re "Delete all memory regions.*y or n.*$" {
+	    send_gdb "y\n"
+	    exp_continue
+	}
+	-re "$gdb_prompt $" {
+	    pass $test
+	}
+    }
+}
+
+# Make the dcache line size bigger than the pagesize.
+set pagesize [get_integer_valueof "pg_size" -1]
+set linesize [expr $pagesize * 2]
+
+gdb_test_no_output "set dcache line-size $linesize" \
+    "set dcache line size to twice the pagesize"
+
+gdb_test "info dcache" \
+    "Dcache 4096 lines of $linesize bytes each.\r\nNo data cache available."
+
+# Make sure dcache doesn't automatically skip unmapped regions.
+delete_mem
+
+gdb_test "info mem" \
+    "Using user-defined memory regions.\r\nThere are no memory regions defined\."
+
+# Given the line size is bigger than the page size, we have
+# alternating mapped and unmapped pages, these make dcache fail to
+# fill in the cache line.  GDB used to have a bug where that failure
+# would end up as user-visible error.  The range being disassembled is
+# wholly available, so GDB should succeed.
+gdb_test "disassemble first_mapped_page, +10" "End of assembler dump\."
+gdb_test "disassemble last_mapped_page, +10" "End of assembler dump\."