media: vivid: work around high stack usage with clang

Building a KASAN-enabled kernel with clang ends up in a case where too much is inlined into vivid_thread_vid_cap() and the stack usage grows a lot, possibly when the register allocation fails to produce efficient code and spills a lot of temporaries to the stack. This uses more than twice the amount of stack than the sum of the individual functions when they are not inlined:

drivers/media/platform/vivid/vivid-kthread-cap.c:766:12: error: stack frame size of 2208 bytes in function 'vivid_thread_vid_cap' [-Werror,-Wframe-larger-than=]

Marking two of the key functions in here as 'noinline_for_stack' avoids the pathological case in clang without any apparent downside for gcc.

Signed-off-by Arnd Bergmann <[email protected]>

---
Not sure how much effort we want to put into fixing clang to not get into this case. I could open an llvm bug report if something thinks this has a chance of getting fixed there.

---
 drivers/media/platform/vivid/vivid-kthread-cap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

drivers/media/platform/vivid/vivid-kthread-cap.c +4 -4

index 6cf495a7d5cc..4f94897e6303 100644
@@ -232,8 +232,8 @@ static void *plane_vaddr(struct tpg_data *tpg, struct vivid_buffer *buf,
        return vbuf;
 }
 
-static int vivid_copy_buffer(struct vivid_dev *dev, unsigned p, u8 *vcapbuf,
-               struct vivid_buffer *vid_cap_buf)
+static noinline_for_stack int vivid_copy_buffer(struct vivid_dev *dev, unsigned p,
+               u8 *vcapbuf, struct vivid_buffer *vid_cap_buf)
 {
        bool blank = dev->must_blank[vid_cap_buf->vb.vb2_buf.index];
        struct tpg_data *tpg = &dev->tpg;

@@ -670,7 +670,8 @@ static void vivid_cap_update_frame_period(struct vivid_dev *dev)
        dev->cap_frame_period = f_period;
 }
 
-static void vivid_thread_vid_cap_tick(struct vivid_dev *dev, int dropped_bufs)
+static noinline_for_stack void vivid_thread_vid_cap_tick(struct vivid_dev *dev,
+                                                        int dropped_bufs)
 {
        struct vivid_buffer *vid_cap_buf = NULL;
        struct vivid_buffer *vbi_cap_buf = NULL;
--

2.20.0

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