The use of the AHCI_HFLAG_NO_PMP flag is something which we inherited from
the Allwinner android kernel sources, and I've always wanted to test if this
is really necessary.

So recently I've bought a sata port multiplexer, and I've given this a test
spin on both A10 and A20 devices, and it seems to work fine:

[    2.154456] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[    2.161092] ata1.15: Port Multiplier 1.2, 0x197b:0x0325 r0, 5 ports, feat 0x5/0xf
[    2.175511] ata1.00: hard resetting link
[    2.524929] ata1.00: SATA link up 3.0 Gbps (SStatus 123 SControl 320)
[    2.531430] ata1.01: hard resetting link
[    2.974465] ata1.01: link resume succeeded after 1 retries
[    3.094932] ata1.01: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[    3.101431] ata1.02: hard resetting link
[    4.174466] ata1.02: failed to resume link (SControl 0)
[    4.180065] ata1.02: SATA link down (SStatus 0 SControl 0)
(and the same for links 3 and 4)

Once the NO_PMP flag is removed it correctly sees the 2 disks which I've
attached, and I can mount and use them just fine.

Unfortunately when I then directly attached a disk to the sata port on the
sunxi SoC, and booted a kernel without the AHCI_HFLAG_NO_PMP flag, it would
not recognize that disk.

It turns out that the sata controller in the sunxi SoCs fails to handle
soft-resets issued to directly attached disks, and when pmp support is
enabled the kernel will always issue a soft-reset.

So add a module parameter to enable pmp usage, and default this to off, so
that directly attached disks keep working normally.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>