Re: [PATCH] cciss: Fix warnings during compilation under 32bitenvironment

Andrew Morton
2007-04-20T05:21:17Z

On Thu, 19 Apr 2007 16:27:26 -0000 "Cameron, Steve" <Steve.Cameron@hp.com> wrote:

>
> Something like
>
> if (sizeof(blah) > 4) {
> do all the assignments with shifts
> }
>
> might be slighly better since the CDB is already zeroed
> by cmd_alloc() and doesn't need to be zeroed a 2nd time.
>
> -- steve
>
> -----Original Message-----
> From: James Bottomley [mailto:James.Bottomley@SteelEye.com]
> Sent: Thu 4/19/2007 11:22 AM
> To: Miller, Mike (OS Dev)
> Cc: Hisashi Hifumi; akpm@linux-foundation.org; jens.axboe@oracle.com; linux-kernel@vger.kernel.org; linux-scsi@vger.kernel.org; Cameron, Steve
> Subject: RE: [PATCH] cciss: Fix warnings during compilation under 32bitenvironment
>
> On Thu, 2007-04-19 at 16:12 +0000, Miller, Mike (OS Dev) wrote:
> > > > Nak. You still haven't told where you saw these warnings. What
> > > > compiler are you using? I do not see these in my 32-bit environment.
> > >
> > > I think it's seen with CONFIG_LBD=n on 32 bits
> > >
> > > In that configuration, sector_t is a u32 (it's u64 even on 32
> > > bits with CONFIG_LBD=y). The proposed code change is a
> > > simple cut and paste from the sd driver.
> >
> > Isn't there a better way than testing each one?
>
> It's not such a bad option. The sizeof() test is compile time
> determinable, so the compiler simply zeros the fields in the
> CONFIG_LBD=n case and does the shift for CONFIG_LBD=y. It certainly
> never compiles to four inline condition checks.
>

Boy you guys make a mess of a nice email trail :(

--- linux-2.6.21-rc7.org/drivers/block/cciss.c 2007-04-17 16:36:02.000000000 +0900
+++ linux-2.6.21-rc7/drivers/block/cciss.c 2007-04-17 16:25:53.000000000 +0900
@@ -2552,10 +2552,10 @@ static void do_cciss_request(request_que
} 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[2]= sizeof(start_blk) > 4 ? (start_blk >> 56) & 0xff : 0; //MSB
+ c->Request.CDB[3]= sizeof(start_blk) > 4 ? (start_blk >> 48) & 0xff : 0;
+ c->Request.CDB[4]= sizeof(start_blk) > 4 ? (start_blk >> 40) & 0xff : 0;
+ c->Request.CDB[5]= sizeof(start_blk) > 4 ? (start_blk >> 32) & 0xff : 0;
c->Request.CDB[6]= (start_blk >> 24) & 0xff;
c->Request.CDB[7]= (start_blk >> 16) & 0xff;
c->Request.CDB[8]= (start_blk >> 8) & 0xff;

This is not the first time we've hit this problem and presumably it won't
be the last time.

Could we do something like

#if (BITS_PER_LONG > 32) || defined(CONFIG_LBD)
#define sector_upper_32(sector) ((sector) >> 32)
#else
#define sector_upper_32(sector) (0)
#endif

and then cciss can do

- c->Request.CDB[2]= start_blk >> 56;
+ c->Request.CDB[2]= sector_upper_32(start_blk) >> 24;

which will do the right thing.

- I think it's safer as a macro - if we make it an inline then the
compiler might still try to evaluate the argument and will still warn

- we could do something like

static inline sector_t sector_shifted_right_by(sector_t s, int distance)
{
<fancy code goes here>
}

But I think that won't be as generally useful as the very basic
sector_upper_32().

- sector_upper_32() isn't a vey nice name, but it has clarity-of-purpose..