linux-kernel - [PATCH] drm/tiny: pixpaper: Prevent inlining of send helpers to reduce stack usage

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Message-Id: <20251031-fix_202510270858-v1-1-6b111d475ce2@gmail.com>
Date: Fri, 31 Oct 2025 16:06:30 +0800
From: LiangCheng Wang <zaq14760@...il.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, kernel test robot <lkp@...el.com>, 
 LiangCheng Wang <zaq14760@...il.com>
Subject: [PATCH] drm/tiny: pixpaper: Prevent inlining of send helpers to
 reduce stack usage

Clang reports that pixpaper_panel_hw_init() exceeds the 8 KB stack
frame limit:

    drivers/gpu/drm/tiny/pixpaper.c:579:12: warning:
      stack frame size (20024) exceeds limit (8192)
      in 'pixpaper_panel_hw_init'

This warning occurs because the compiler aggressively inlines
pixpaper_send_cmd() and pixpaper_send_data() into
pixpaper_panel_hw_init(), which inflates the estimated stack usage.

Mark these two helper functions as 'noinline' to prevent inlining.
This significantly reduces the reported stack usage without changing
runtime behavior.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510270858.1GzE6iQg-lkp@intel.com/
Fixes: c9e70639f591 ("drm: tiny: Add support for Mayqueen Pixpaper e-ink panel")
Signed-off-by: LiangCheng Wang <zaq14760@...il.com>
---
When building with Clang and frame size warnings enabled
(-Wframe-larger-than=8192), the compiler reports that
pixpaper_panel_hw_init() consumes over 20 KB of stack space:

    drivers/gpu/drm/tiny/pixpaper.c:579:12: warning:
      stack frame size (20024) exceeds limit (8192)
      in 'pixpaper_panel_hw_init'

This happens because Clang aggressively inlines
pixpaper_send_cmd() and pixpaper_send_data() into
pixpaper_panel_hw_init(), causing the calculated stack usage
to balloon far beyond the warning threshold.

The fix is straightforward: mark these two helper functions as
'noinline' to prevent inlining. This reduces the reported stack
usage to within normal limits without changing runtime behavior.
---
 drivers/gpu/drm/tiny/pixpaper.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/tiny/pixpaper.c b/drivers/gpu/drm/tiny/pixpaper.c
index 32598fb2fee7fcdea3ea0696c2bf54404fcffa2e..70e3239adfd0f86f92551991872486380489fb9b 100644
--- a/drivers/gpu/drm/tiny/pixpaper.c
+++ b/drivers/gpu/drm/tiny/pixpaper.c
@@ -536,7 +536,7 @@ static void pixpaper_spi_sync(struct spi_device *spi, struct spi_message *msg,
 		err->errno_code = ret;
 }
 
-static void pixpaper_send_cmd(struct pixpaper_panel *panel, u8 cmd,
+static noinline void pixpaper_send_cmd(struct pixpaper_panel *panel, u8 cmd,
 			      struct pixpaper_error_ctx *err)
 {
 	if (err->errno_code)
@@ -556,7 +556,7 @@ static void pixpaper_send_cmd(struct pixpaper_panel *panel, u8 cmd,
 	pixpaper_spi_sync(panel->spi, &msg, err);
 }
 
-static void pixpaper_send_data(struct pixpaper_panel *panel, u8 data,
+static noinline void pixpaper_send_data(struct pixpaper_panel *panel, u8 data,
 			       struct pixpaper_error_ctx *err)
 {
 	if (err->errno_code)

---
base-commit: d127176862a93c4b3216bda533d2bee170af5e71
change-id: 20251031-fix_202510270858-2e4643b00545

Best regards,
-- 
LiangCheng Wang <zaq14760@...il.com>