[PATCH] drm/xe: fix devcoredump chunk alignmnent calculation

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

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.

Fixes: c4a2e5f865b7 ("drm/xe: Add devcoredump chunking")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
Please test this with multi-gigabyte buffers, the original code
was clearly not right, but I don't trust my version either.
---
 drivers/gpu/drm/xe/xe_devcoredump.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_devcoredump.c b/drivers/gpu/drm/xe/xe_devcoredump.c
index a9e618abf8ac..4eb70e2d9f68 100644
--- a/drivers/gpu/drm/xe/xe_devcoredump.c
+++ b/drivers/gpu/drm/xe/xe_devcoredump.c
@@ -177,6 +177,7 @@ 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;
 
 	if (!coredump)
 		return -ENODEV;
@@ -203,8 +204,9 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t 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 = div_u64_rem(offset,
+			XE_DEVCOREDUMP_CHUNK_MAX, &chunk_offset)
+			* XE_DEVCOREDUMP_CHUNK_MAX;
 
 		__xe_devcoredump_read(ss->read.buffer,
 				      XE_DEVCOREDUMP_CHUNK_MAX,
@@ -213,8 +215,7 @@ 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);
 
-- 
2.39.5