On Mon, Dec 18, 2006 at 05:12:53PM -0500, Ming Zhang wrote: > See this code piece > > http://lxr.linux.no/source/drivers/scsi/libata-scsi.c?v=2.6.18#L1049 > > 1056 lba |= ((u64)scsicmd[2]) << 24; > 1057 lba |= ((u64)scsicmd[3]) << 16; > 1058 lba |= ((u64)scsicmd[4]) << 8; > 1059 lba |= ((u64)scsicmd[5]); > > it can also be written as > > lba = be32_to_cpu(*(u32 *)(&scsicmd[2]) > > > i wrote a simple code to test this and i found second one is several > time faster than first one. I guess the total time spend in doing the conversion is orders of magnitude smaller than the time needed to get a data block from the drive. Don't bother optimising things that don't need to be optimised. > if there any pitfall with 2nd way? AFAICS be32_to_cpu() works on native alignment 32 bit integers. Yes, it works on x86 cause x86 can do unaligned memory accesses. I don't think your second method will work on a RISC architecture (ARM, MIPS, Alpha, etc). Erik -- They're all fools. Don't worry. Darwin may be slow, but he'll eventually get them. -- Matthew Lammers in alt.sysadmin.recovery -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
