[PATCH] Re: cciss: warning: right shift count >= width of type

Rene Herman
2007-08-12T01:42:39Z

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On 08/12/2007 02:56 AM, Jesper Juhl wrote:

> (whoops, forgot to add maintainer to Cc - now added)

Ehm... too late...

> On 12/08/07, Jesper Juhl <jesper.juhl@gmail.com> wrote:
>> Hi,
>>
>> 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.

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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;