drm/ast: Clear preserved bits from register output value

Preserve the I/O register bits in __ast_write8_i_masked() as specified
by preserve_mask. Accidentally OR-ing the output value into these will
overwrite the register's previous settings.

Fixes display output on the AST2300, where the screen can go blank at
boot. The driver's original commit 312fec1405dd ("drm: Initial KMS
driver for AST (ASpeed Technologies) 2000 series (v2)") already added
the broken code. Commit 6f719373b943 ("drm/ast: Blank with VGACR17 sync
enable, always clear VGACRB6 sync off") triggered the bug.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Peter Schneider <pschneider1968@googlemail.com>
Closes: https://lore.kernel.org/dri-devel/a40caf8e-58ad-4f9c-af7f-54f6f69c29bb@googlemail.com/
Tested-by: Peter Schneider <pschneider1968@googlemail.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Fixes: 6f719373b943 ("drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off")
Fixes: 312fec1405dd ("drm: Initial KMS driver for AST (ASpeed Technologies) 2000 series (v2)")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Nick Bowler <nbowler@draconx.ca>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v3.5+
Link: https://patch.msgid.link/20251024073626.129032-1-tzimmermann@suse.de

diff --git a/drivers/gpu/drm/ast/ast_drv.h b/drivers/gpu/drm/ast/ast_drv.h
index c15aef014f698c9134437787609dc7e376973430..d41bd876167c9d85d75adc249dab3408f8a706e3 100644 (file)
--- a/drivers/gpu/drm/ast/ast_drv.h
+++ b/drivers/gpu/drm/ast/ast_drv.h
@@ -282,13 +282,13 @@ static inline void __ast_write8_i(void __iomem *addr, u32 reg, u8 index, u8 val)
        __ast_write8(addr, reg + 1, val);
 }
 
-static inline void __ast_write8_i_masked(void __iomem *addr, u32 reg, u8 index, u8 read_mask,
+static inline void __ast_write8_i_masked(void __iomem *addr, u32 reg, u8 index, u8 preserve_mask,
                                         u8 val)
 {
-       u8 tmp = __ast_read8_i_masked(addr, reg, index, read_mask);
+       u8 tmp = __ast_read8_i_masked(addr, reg, index, preserve_mask);
 
-       tmp |= val;
-       __ast_write8_i(addr, reg, index, tmp);
+       val &= ~preserve_mask;
+       __ast_write8_i(addr, reg, index, tmp | val);
 }
 
 static inline u32 ast_read32(struct ast_device *ast, u32 reg)