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Subject[PATCH] Re: cciss: warning: right shift count >= width of type
On 08/12/2007 02:28 AM, Jesper Juhl wrote:

> I've been building some randconfig kernels lately and I've noticed
> this in a few builds :
>
> drivers/block/cciss.c:2614: warning: right shift count >= width of type
> drivers/block/cciss.c:2615: warning: right shift count >= width of type
> drivers/block/cciss.c:2616: warning: right shift count >= width of type
>
> The code in question is this :
>
> static void do_cciss_request(struct request_queue *q)
> {
> ...
> sector_t start_blk;
> ...
> c->Request.CDB[2]= (start_blk >> 56) & 0xff; //MSB
> c->Request.CDB[3]= (start_blk >> 48) & 0xff;
> c->Request.CDB[4]= (start_blk >> 40) & 0xff;
> ...
> }
>
>
> The problem stems from these lines in include/linux/types.h :
> ...
> #ifdef CONFIG_LBD
> typedef u64 sector_t;
> #else
> typedef unsigned long sector_t;
> #endif
> ...
>
> So on a 32bit arch without CONFIG_LBD, sector_t is going to be 32 bits wide.
> Thus it seems gcc is absolutely right in complaining about those
> 56, 48 & 40 bit shifts, since they are indeed wider than the type
> in the "!CONFIG_LBD on a 32bit arch" case.
>
>
> I must admit that I have no idear what the proper way to deal with
> that is, so I'll just report it so hopefully someone else can fix it.
>
> By the way; I'm building current Linus git tree, head at commit
> ac07860264bd2b18834d3fa3be47032115524cea

Well, given that it's explicitly treating start_blk as a 64-bit value, the
proper fix is probably to cast start_blk to an actual (guaranteed) 64-bit
value. Untested, uncompiled, and someone else may disagree:

Rene.
diff --git a/drivers/block/cciss.c b/drivers/block/cciss.c
index 5acc6c4..fa2eec8 100644
--- a/drivers/block/cciss.c
+++ b/drivers/block/cciss.c
@@ -2609,13 +2609,13 @@ static void do_cciss_request(request_queue_t *q)
} else {
c->Request.CDBLen = 16;
c->Request.CDB[1]= 0;
- c->Request.CDB[2]= (start_blk >> 56) & 0xff; //MSB
- c->Request.CDB[3]= (start_blk >> 48) & 0xff;
- c->Request.CDB[4]= (start_blk >> 40) & 0xff;
- c->Request.CDB[5]= (start_blk >> 32) & 0xff;
- c->Request.CDB[6]= (start_blk >> 24) & 0xff;
- c->Request.CDB[7]= (start_blk >> 16) & 0xff;
- c->Request.CDB[8]= (start_blk >> 8) & 0xff;
+ c->Request.CDB[2]= ((u64)start_blk >> 56) & 0xff; //MSB
+ c->Request.CDB[3]= ((u64)start_blk >> 48) & 0xff;
+ c->Request.CDB[4]= ((u64)start_blk >> 40) & 0xff;
+ c->Request.CDB[5]= ((u64)start_blk >> 32) & 0xff;
+ c->Request.CDB[6]= ((u64)start_blk >> 24) & 0xff;
+ c->Request.CDB[7]= ((u64)start_blk >> 16) & 0xff;
+ c->Request.CDB[8]= ((u64)start_blk >> 8) & 0xff;
c->Request.CDB[9]= start_blk & 0xff;
c->Request.CDB[10]= (creq->nr_sectors >> 24) & 0xff;
c->Request.CDB[11]= (creq->nr_sectors >> 16) & 0xff;
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