[PATCH v2] drm/sysfb: efidrm: Avoid clang -Wsometimes-uninitialized in efidrm_device_create()

[Nathan Chancellor <nathan@kernel.org>]

Clang warns (or errors with CONFIG_WERROR=y):

  drivers/gpu/drm/sysfb/efidrm.c:353:11: error: variable 'screen_base' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    353 |         else if (mem_flags & EFI_MEMORY_WB)
        |                  ^~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/gpu/drm/sysfb/efidrm.c:356:7: note: uninitialized use occurs here
    356 |         if (!screen_base)
        |              ^~~~~~~~~~~
  drivers/gpu/drm/sysfb/efidrm.c:353:7: note: remove the 'if' if its condition is always true
    353 |         else if (mem_flags & EFI_MEMORY_WB)
        |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    354 |                 screen_base = devm_memremap(&pdev->dev, mem->start, resource_size(mem),
  drivers/gpu/drm/sysfb/efidrm.c:261:27: note: initialize the variable 'screen_base' to silence this warning
    261 |         void __iomem *screen_base;
        |                                  ^
        |                                   = NULL

efidrm_get_mem_flags() can only return a mask that has at least one of
the tested values set so the else case is impossible but clang's static
analysis runs before inlining so it cannot know that.

Initialize screen_base to NULL and add a defensive error message in case
mem_flags were ever returned without one of the four valid values.

Fixes: 32ae90c66fb6 ("drm/sysfb: Add efidrm for EFI displays")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
Changes in v2:
- Upgrade drm_warn() to drm_err() and adjust message (Thomas).
- Carry forward Thomas's reviewed-by.
- Link to v1: https://lore.kernel.org/r/20250409-efidrm-avoid-uninit-screen_info-warning-v1-1-67babb19d831@kernel.org
---
 drivers/gpu/drm/sysfb/efidrm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sysfb/efidrm.c b/drivers/gpu/drm/sysfb/efidrm.c
index af90064a4c04..3cfd5d2cbf48 100644
--- a/drivers/gpu/drm/sysfb/efidrm.c
+++ b/drivers/gpu/drm/sysfb/efidrm.c
@@ -258,7 +258,7 @@ static struct efidrm_device *efidrm_device_create(struct drm_driver *drv,
 	struct drm_sysfb_device *sysfb;
 	struct drm_device *dev;
 	struct resource *mem = NULL;
-	void __iomem *screen_base;
+	void __iomem *screen_base = NULL;
 	struct drm_plane *primary_plane;
 	struct drm_crtc *crtc;
 	struct drm_encoder *encoder;
@@ -353,6 +353,8 @@ static struct efidrm_device *efidrm_device_create(struct drm_driver *drv,
 	else if (mem_flags & EFI_MEMORY_WB)
 		screen_base = devm_memremap(&pdev->dev, mem->start, resource_size(mem),
 					    MEMREMAP_WB);
+	else
+		drm_err(dev, "invalid mem_flags: 0x%llx\n", mem_flags);
 	if (!screen_base)
 		return ERR_PTR(-ENOMEM);
 	iosys_map_set_vaddr_iomem(&sysfb->fb_addr, screen_base);

---
base-commit: e8bf4a1bdaeadb28d13b9a2bcfd5910fda06eede
change-id: 20250409-efidrm-avoid-uninit-screen_info-warning-d62ef67ce3c5

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>