[PATCH 1/2] media: rkvdec: reduce excessive stack usage in assemble_hw_pps()

[Arnd Bergmann]

From: Arnd Bergmann <arnd at arndb.de>

The rkvdec_pps had a large set of bitfields, all of which
as misaligned. This causes clang-21 and likely other versions to
produce absolutely awful object code and a warning about very
large stack usage, on targets without unaligned access:

drivers/media/platform/rockchip/rkvdec/rkvdec-vp9.c:966:12: error: stack frame size (1472) exceeds limit (1280) in 'rkvdec_vp9_start' [-Werror,-Wframe-larger-than]

Part of the problem here is how all the bitfield accesses are
inlined into a function that already has large structures on
the stack.

Mark set_field_order_cnt() as noinline_for_stack, and split out
the following accesses in assemble_hw_pps() into another noinline
function, both of which now using around 800 bytes of stack in the
same configuration.

There is clearly still something wrong with clang here, but
splitting it into multiple functions reduces the risk of stack
overflow.

Fixes: fde24907570d ("media: rkvdec: Add H264 support for the VDPU383 variant")
Link: https://godbolt.org/z/acP1eKeq9
Signed-off-by: Arnd Bergmann <arnd at arndb.de>
---
 .../rockchip/rkvdec/rkvdec-vdpu383-h264.c     | 50 ++++++++++---------
 1 file changed, 27 insertions(+), 23 deletions(-)

diff --git a/drivers/media/platform/rockchip/rkvdec/rkvdec-vdpu383-h264.c b/drivers/media/platform/rockchip/rkvdec/rkvdec-vdpu383-h264.c
index 6ab3167addc8..ef69f2a36478 100644
--- a/drivers/media/platform/rockchip/rkvdec/rkvdec-vdpu383-h264.c
+++ b/drivers/media/platform/rockchip/rkvdec/rkvdec-vdpu383-h264.c
@@ -130,7 +130,7 @@ struct rkvdec_h264_ctx {
 	struct vdpu383_regs_h26x regs;
 };
 
-static void set_field_order_cnt(struct rkvdec_pps *pps, const struct v4l2_h264_dpb_entry *dpb)
+static noinline_for_stack void set_field_order_cnt(struct rkvdec_pps *pps, const struct v4l2_h264_dpb_entry *dpb)
 {
 	pps->top_field_order_cnt0 = dpb[0].top_field_order_cnt;
 	pps->bot_field_order_cnt0 = dpb[0].bottom_field_order_cnt;
@@ -166,6 +166,31 @@ static void set_field_order_cnt(struct rkvdec_pps *pps, const struct v4l2_h264_d
 	pps->bot_field_order_cnt15 = dpb[15].bottom_field_order_cnt;
 }
 
+static noinline_for_stack void set_dec_params(struct rkvdec_pps *pps, const struct v4l2_ctrl_h264_decode_params *dec_params)
+{
+	const struct v4l2_h264_dpb_entry *dpb = dec_params->dpb;
+
+	for (int i = 0; i < ARRAY_SIZE(dec_params->dpb); i++) {
+		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM)
+			pps->is_longterm |= (1 << i);
+		pps->ref_field_flags |=
+		 (!!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_FIELD)) << i;
+		pps->ref_colmv_use_flag |=
+		 (!!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_ACTIVE)) << i;
+		pps->ref_topfield_used |=
+		 (!!(dpb[i].fields & V4L2_H264_TOP_FIELD_REF)) << i;
+		pps->ref_botfield_used |=
+			(!!(dpb[i].fields & V4L2_H264_BOTTOM_FIELD_REF)) << i;
+	}
+	pps->pic_field_flag =
+		!!(dec_params->flags & V4L2_H264_DECODE_PARAM_FLAG_FIELD_PIC);
+	pps->pic_associated_flag =
+		!!(dec_params->flags & V4L2_H264_DECODE_PARAM_FLAG_BOTTOM_FIELD);
+
+	pps->cur_top_field = dec_params->top_field_order_cnt;
+	pps->cur_bot_field = dec_params->bottom_field_order_cnt;
+}
+
 static void assemble_hw_pps(struct rkvdec_ctx *ctx,
 			    struct rkvdec_h264_run *run)
 {
@@ -177,7 +202,6 @@ static void assemble_hw_pps(struct rkvdec_ctx *ctx,
 	struct rkvdec_h264_priv_tbl *priv_tbl = h264_ctx->priv_tbl.cpu;
 	struct rkvdec_sps_pps *hw_ps;
 	u32 pic_width, pic_height;
-	u32 i;
 
 	/*
 	 * HW read the SPS/PPS information from PPS packet index by PPS id.
@@ -261,28 +285,8 @@ static void assemble_hw_pps(struct rkvdec_ctx *ctx,
 		!!(pps->flags & V4L2_H264_PPS_FLAG_SCALING_MATRIX_PRESENT);
 
 	set_field_order_cnt(&hw_ps->pps, dpb);
+	set_dec_params(&hw_ps->pps, dec_params);
 
-	for (i = 0; i < ARRAY_SIZE(dec_params->dpb); i++) {
-		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM)
-			hw_ps->pps.is_longterm |= (1 << i);
-
-		hw_ps->pps.ref_field_flags |=
-			(!!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_FIELD)) << i;
-		hw_ps->pps.ref_colmv_use_flag |=
-			(!!(dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_ACTIVE)) << i;
-		hw_ps->pps.ref_topfield_used |=
-			(!!(dpb[i].fields & V4L2_H264_TOP_FIELD_REF)) << i;
-		hw_ps->pps.ref_botfield_used |=
-			(!!(dpb[i].fields & V4L2_H264_BOTTOM_FIELD_REF)) << i;
-	}
-
-	hw_ps->pps.pic_field_flag =
-		!!(dec_params->flags & V4L2_H264_DECODE_PARAM_FLAG_FIELD_PIC);
-	hw_ps->pps.pic_associated_flag =
-		!!(dec_params->flags & V4L2_H264_DECODE_PARAM_FLAG_BOTTOM_FIELD);
-
-	hw_ps->pps.cur_top_field = dec_params->top_field_order_cnt;
-	hw_ps->pps.cur_bot_field = dec_params->bottom_field_order_cnt;
 }
 
 static void rkvdec_write_regs(struct rkvdec_ctx *ctx)
-- 
2.39.5