mtd: spi-nor: avoid holes in struct spi_mem_op

gcc gets confused when -ftrivial-auto-var-init=pattern is used on sparse
bit fields such as 'struct spi_mem_op', which caused the previous false
positive warning about an uninitialized variable:

drivers/mtd/spi-nor/spansion.c: error: 'op' is used uninitialized [-Werror=uninitialized]

In fact, the variable is fully initialized and gcc does not see it being
used, so the warning is entirely bogus. The problem appears to be
a misoptimization in the initialization of single bit fields when the
rest of the bytes are not initialized.

A previous workaround added another initialization, which ended up
shutting up the warning in spansion.c, though it apparently still happens
in other files as reported by Peter Foley in the gcc bugzilla. The
workaround of adding a fake initialization seems particularly bad
because it would set values that can never be correct but prevent the
compiler from warning about actually missing initializations.

Revert the broken workaround and instead pad the structure to only
have bitfields that add up to full bytes, which should avoid this
behavior in all drivers.

I also filed a new bug against gcc with what I found, so this can
hopefully be addressed in future gcc releases. At the moment, only
gcc-12 and gcc-13 are affected.

Cc: Peter Foley <[e-mail]>
Cc: Pedro Falcato <[e-mail]>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110743
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108402
Link: https://godbolt.org/z/efMMsG1Kx
Fixes: 420c4495b5e56 ("mtd: spi-nor: spansion: make sure local struct does not contain garbage")
Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Mark Brown <[e-mail]>
Acked-by: Tudor Ambarus <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/spi-nor/spansion.c

···
  */
 static int cypress_nor_set_addr_mode_nbytes(struct spi_nor *nor)
 {
-	struct spi_mem_op op = {};
+	struct spi_mem_op op;
 	u8 addr_mode;
 	int ret;
 
···
 			  const struct sfdp_parameter_header *bfpt_header,
 			  const struct sfdp_bfpt *bfpt)
 {
-	struct spi_mem_op op = {};
+	struct spi_mem_op op;
 	int ret;
 
 	ret = cypress_nor_set_addr_mode_nbytes(nor);

include/linux/spi/spi-mem.h

···
 		u8 nbytes;
 		u8 buswidth;
 		u8 dtr : 1;
+		u8 __pad : 7;
 		u16 opcode;
 	} cmd;
 
···
 		u8 nbytes;
 		u8 buswidth;
 		u8 dtr : 1;
+		u8 __pad : 7;
 		u64 val;
 	} addr;
 
···
 		u8 nbytes;
 		u8 buswidth;
 		u8 dtr : 1;
+		u8 __pad : 7;
 	} dummy;
 
 	struct {
 		u8 buswidth;
 		u8 dtr : 1;
 		u8 ecc : 1;
+		u8 __pad : 6;
 		enum spi_mem_data_dir dir;
 		unsigned int nbytes;
 		union {