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 312fec1 ("drm: Initial KMS
driver for AST (ASpeed Technologies) 2000 series (v2)") already added
the broken code. Commit 6f71937 ("drm/ast: Blank with VGACR17 sync
enable, always clear VGACRB6 sync off") triggered the bug.

Signed-off-by: Thomas Zimmermann <[email protected]>
Reported-by: Peter Schneider <[email protected]>
Closes: https://lore.kernel.org/dri-devel/[email protected]/
Tested-by: Peter Schneider <[email protected]>
Reviewed-by: Jocelyn Falempe <[email protected]>
Fixes: 6f71937 ("drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off")
Fixes: 312fec1 ("drm: Initial KMS driver for AST (ASpeed Technologies) 2000 series (v2)")
Cc: Thomas Zimmermann <[email protected]>
Cc: Nick Bowler <[email protected]>
Cc: Douglas Anderson <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Jocelyn Falempe <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v3.5+
Link: https://patch.msgid.link/[email protected]

Author: tdz

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)