[PATCH] firmware: arm_scmi: Eliminate local db variable in SCMI_PERF_FC_RING_DB

[PATCH] firmware: arm_scmi: Eliminate local db variable in SCMI_PERF_FC_RING_DB

[Nathan Chancellor]

clang warns four times:

drivers/firmware/arm_scmi/perf.c:320:24: warning: variable 'db' is
uninitialized when used within its own initialization [-Wuninitialized]
                SCMI_PERF_FC_RING_DB(db, 64);
                ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
drivers/firmware/arm_scmi/perf.c:300:31: note: expanded from macro
'SCMI_PERF_FC_RING_DB'
        struct scmi_fc_db_info *db = doorbell;          \
                                ~~   ^~~~~~~~

This happens because the doorbell identifier becomes db after
preprocessing:

        if (db->width == 1)
                do {
                        u8 val = 0;
                        struct scmi_fc_db_info *db = db;
                        if (db->mask)
                                val = ioread8(db->addr) & db->mask;
                        iowrite8((u8)db->set | val, db->addr);
                } while (0);

We could swap the doorbell and db identifiers within the macro and that
would resolve the issue; however, there doesn't appear to be a good
reason for having two copies of the same variable. Eliminate the one in
the do while loop to prevent this warning and make the code clearer.

Fixes: 8f12cbcb6abc ("firmware: arm_scmi: Make use SCMI v2.0 fastchannel for performance protocol")
Link: https://github.com/ClangBuiltLinux/linux/issues/635
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
 drivers/firmware/arm_scmi/perf.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/firmware/arm_scmi/perf.c b/drivers/firmware/arm_scmi/perf.c
index 2c5201c8354c..ab946ef6b914 100644
--- a/drivers/firmware/arm_scmi/perf.c
+++ b/drivers/firmware/arm_scmi/perf.c
@@ -294,10 +294,9 @@ scmi_perf_describe_levels_get(const struct scmi_handle *handle, u32 domain,
 	return ret;
 }
 
-#define SCMI_PERF_FC_RING_DB(doorbell, w)		\
+#define SCMI_PERF_FC_RING_DB(db, w)			\
 do {							\
 	u##w val = 0;					\
-	struct scmi_fc_db_info *db = doorbell;		\
 							\
 	if (db->mask)					\
 		val = ioread##w(db->addr) & db->mask;	\
-- 
2.23.0.rc2

Re: [PATCH] firmware: arm_scmi: Eliminate local db variable in SCMI_PERF_FC_RING_DB

[Sudeep Holla]

On Fri, Aug 09, 2019 at 09:49:10PM -0700, Nathan Chancellor wrote:
> clang warns four times:
>
> drivers/firmware/arm_scmi/perf.c:320:24: warning: variable 'db' is
> uninitialized when used within its own initialization [-Wuninitialized]
>                 SCMI_PERF_FC_RING_DB(db, 64);
>                 ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> drivers/firmware/arm_scmi/perf.c:300:31: note: expanded from macro
> 'SCMI_PERF_FC_RING_DB'
>         struct scmi_fc_db_info *db = doorbell;          \
>                                 ~~   ^~~~~~~~
>
> This happens because the doorbell identifier becomes db after
> preprocessing:
>
>         if (db->width == 1)
>                 do {
>                         u8 val = 0;
>                         struct scmi_fc_db_info *db = db;
>                         if (db->mask)
>                                 val = ioread8(db->addr) & db->mask;
>                         iowrite8((u8)db->set | val, db->addr);
>                 } while (0);
>
> We could swap the doorbell and db identifiers within the macro and that
> would resolve the issue; however, there doesn't appear to be a good
> reason for having two copies of the same variable. Eliminate the one in
> the do while loop to prevent this warning and make the code clearer.
>

I originally had exactly what we will after this patch applied. I think
one of the tool complained about argument 'db' reused in the macro
might have possible side-effects. That's the reason I moved it. I will
dig it up and fold this in the original patch as before.

--
Regards,
Sudeep