ata_piix is a common kernel module used for Intel ICH Family IDE/SATA controllers(which is very common in PCs)
In Fedora 12 and 13,the default config option is Y(CONFIG_ATA_PIIX=Y).
But in RHEL 6,Red Hat "explicitly" set it to M(CONFIG_ATA_PIIX=M).
Why i use "explicitly" here is because they put "CONFIG_ATA_PIIX=M" into "config-generic-rhel".
config-generic-rhel stores all the config options modified by redhat and will be later merged into default kernel config file.
I think redhat must have a reason to do this "YtoM" change.Can anyone give me a hint on what the reason will be.....

Most servers shipping with RHEL6 or in general any new servers are using fibre channel, SSD, eSATA, SCSI, or even SATA-3 drives but not so much the SATA-1 or IDE drives supported by ata_piix.ko.

Thanks Ehtetur, it's very helpful to me.