[PATCH] media: v4l2-tpg: reduce stack usage for kasan builds

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

tpg_fill_plane_buffer() is a rather complex function. While there is
nothing wrong with it per se, I have run into corner cases with clang-22
on s390 using KASAN that makes it run out of registers and blow the
stack warning limit from excessive spills:

drivers/media/common/v4l2-tpg/v4l2-tpg-core.c:2629:6: error: stack frame size (1560) exceeds limit (1536)
      in 'tpg_fill_plane_buffer' [-Werror,-Wframe-larger-than]
 2629 | void tpg_fill_plane_buffer(struct tpg_data *tpg, v4l2_std_id std,

Forcing the two largest callees out of line completely avoids the problem
and prevents all the register spills, with the stack usage for each function
going down to a few bytes for the local variables.

Arguably this is a problem caused by clang rather than the code, but
a noinline_for_stack annotation is an easy workaround.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
If anyone thinks we should track this as an llvm bug, I can help
come up with a reproducer
---
 drivers/media/common/v4l2-tpg/v4l2-tpg-core.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c b/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
index 931e5dc453b9..e1d5c220f738 100644
--- a/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
+++ b/drivers/media/common/v4l2-tpg/v4l2-tpg-core.c
@@ -2346,9 +2346,11 @@ static void tpg_fill_params_extras(const struct tpg_data *tpg,
 			(params->is_60hz ? V4L2_FIELD_TOP : V4L2_FIELD_BOTTOM);
 }
 
-static void tpg_fill_plane_extras(const struct tpg_data *tpg,
-				  const struct tpg_draw_params *params,
-				  unsigned p, unsigned h, u8 *vbuf)
+/* noinline to work around clang KASAN issues */
+static noinline_for_stack void
+tpg_fill_plane_extras(const struct tpg_data *tpg,
+		      const struct tpg_draw_params *params,
+		      unsigned p, unsigned h, u8 *vbuf)
 {
 	unsigned twopixsize = params->twopixsize;
 	unsigned img_width = params->img_width;
@@ -2483,9 +2485,9 @@ static void tpg_fill_plane_extras(const struct tpg_data *tpg,
 	}
 }
 
-static void tpg_fill_plane_pattern(const struct tpg_data *tpg,
-				   const struct tpg_draw_params *params,
-				   unsigned p, unsigned h, u8 *vbuf)
+static noinline_for_stack void
+tpg_fill_plane_pattern(const struct tpg_data *tpg, const struct tpg_draw_params *params,
+		       unsigned p, unsigned h, u8 *vbuf)
 {
 	unsigned twopixsize = params->twopixsize;
 	unsigned img_width = params->img_width;
-- 
2.39.5