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Hdparm man page lists for parameter -s:

Enable/disable the power-on in standby feature, if supported by the drive. VERY DANGEROUS. Do not use unless you are abso- lutely certain that both the system BIOS (or firmware) and the operating system kernel (Linux >= 2.6.22) support probing for drives that use this feature.

Could you point to a commit or lines in source code that gives the support for "power-on in standby".

In other words what is the change in Linux kernel (2.6.22) that made PUIS/POIS possible?

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The code makes a call to ata_dev_set_feature(dev, SETFEATURES_SPINUP, 0);

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