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SubjectRe: [PATCH] scsi: aic94xx: Fix fall-through warning for Clang
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On Wed, 2021-06-30 at 20:04 -0500, Gustavo A. R. Silva wrote:
> In preparation to enable -Wimplicit-fallthrough for Clang, fix a
> warning by explicitly adding a fallthrough; statement.
>
> Notice that this seems to be a Duff device for performance[1]. So,
> although the code looks a bit _funny_, I didn't want to refactor
> or modify it beyond merely adding a fallthrough marking, which
> might be the least disruptive way to fix this issue.

If you read the surrounding calling code, it appears it's
not performant.

* @start has to be the _base_ element start, since the
* linked list entries's offset is from this pointer.
* Some linked list entries use only the first id, in which case
* you can pass 0xFF for the second.

> diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
[]
> @@ -718,10 +718,12 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)
>   do {
>   switch (id1) {
>   default:
> - if (el->id1 == id1)
> + if (el->id1 == id1) {
> + fallthrough;
>   case 0xFF:
>   if (el->id0 == id0)
>   return el;
> + }
>   }
>   el = start + le16_to_cpu(el->next);
>   } while (el != start);

And this is still horrible code to read.

I think the below is rather more sensible and it doesn't change
the defconfig with aic94xx object size.

$ size drivers/scsi/aic94xx/aic94xx_sds.o*
text data bss dec hex filename
10010 192 0 10202 27da drivers/scsi/aic94xx/aic94xx_sds.o.new
10010 192 0 10202 27da drivers/scsi/aic94xx/aic94xx_sds.o.old
---
drivers/scsi/aic94xx/aic94xx_sds.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
index 297a66770260c..069278f6d6cea 100644
--- a/drivers/scsi/aic94xx/aic94xx_sds.c
+++ b/drivers/scsi/aic94xx/aic94xx_sds.c
@@ -717,11 +717,14 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)

do {
switch (id1) {
- default:
- if (el->id1 == id1)
case 0xFF:
- if (el->id0 == id0)
- return el;
+ if (el->id0 == id0)
+ return el;
+ break;
+ default:
+ if (el->id0 == id0 && el->id1 == id1)
+ return el;
+ break;
}
el = start + le16_to_cpu(el->next);
} while (el != start);
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