drm/xe: fix devcoredump chunk alignmnent calculation

The device core dumps are copied in 1.5GB chunks, which leads to a
link-time error on 32-bit builds because of the 64-bit division not
getting trivially turned into mask and shift operations:

ERROR: modpost: "__moddi3" [drivers/gpu/drm/xe/xe.ko] undefined!

On top of this, I noticed that the ALIGN_DOWN() usage here cannot
work because that is only defined for power-of-two alignments.
Change ALIGN_DOWN into an explicit div_u64_rem() that avoids the
link error and hopefully produces the right results.

Doing a 1.5GB kvmalloc() does seem a bit suspicious as well, e.g.
this will clearly fail on any 32-bit platform and is also likely
to run out of memory on 64-bit systems under memory pressure, so
using a much smaller power-of-two chunk size might be a good idea
instead.

v2:
 - Always call div_u64_rem (Matt)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504251238.JsNgFeFc-lkp@intel.com/
Fixes: c4a2e5f865b7 ("drm/xe: Add devcoredump chunking")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250501012545.1045247-1-matthew.brost@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

diff --git a/drivers/gpu/drm/xe/xe_devcoredump.c b/drivers/gpu/drm/xe/xe_devcoredump.c
index a9e618abf8ac1e2843a50cc4f02f7929f7071407..7a8af231131808235d9f7bbc8632169e1f194411 100644 (file)
--- a/drivers/gpu/drm/xe/xe_devcoredump.c
+++ b/drivers/gpu/drm/xe/xe_devcoredump.c
@@ -177,6 +177,8 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t offset,
        struct xe_devcoredump *coredump = data;
        struct xe_devcoredump_snapshot *ss;
        ssize_t byte_copied;
+       u32 chunk_offset;
+       ssize_t new_chunk_position;
 
        if (!coredump)
                return -ENODEV;
@@ -201,10 +203,14 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t offset,
                return 0;
        }
 
+       new_chunk_position = div_u64_rem(offset,
+                                        XE_DEVCOREDUMP_CHUNK_MAX,
+                                        &chunk_offset);
+
        if (offset >= ss->read.chunk_position + XE_DEVCOREDUMP_CHUNK_MAX ||
            offset < ss->read.chunk_position) {
-               ss->read.chunk_position =
-                       ALIGN_DOWN(offset, XE_DEVCOREDUMP_CHUNK_MAX);
+               ss->read.chunk_position = new_chunk_position *
+                       XE_DEVCOREDUMP_CHUNK_MAX;
 
                __xe_devcoredump_read(ss->read.buffer,
                                      XE_DEVCOREDUMP_CHUNK_MAX,
@@ -213,8 +219,7 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t offset,
 
        byte_copied = count < ss->read.size - offset ? count :
                ss->read.size - offset;
-       memcpy(buffer, ss->read.buffer +
-              (offset % XE_DEVCOREDUMP_CHUNK_MAX), byte_copied);
+       memcpy(buffer, ss->read.buffer + chunk_offset, byte_copied);
 
        mutex_unlock(&coredump->lock);