Revert "libata: ahci_start_engine compliant to AHCI spec"

This reverts commit 270dac35.

The commits causes command timeouts on AC plug/unplug.  It isn't yet
clear why.  As the commit was for a single rather obscure controller,
revert the change for now.

The problem was reported and bisected by Gu Rui in bug#34692.

 https://bugzilla.kernel.org/show_bug.cgi?id=34692

Also, reported by Rafael and Michael in the following thread.

 http://thread.gmane.org/gmane.linux.kernel/1138771Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Gu Rui <chaos.proton@gmail.com>
Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Michael Leun <lkml20100708@newton.leun.net>
Cc: Jian Peng <jipeng2005@gmail.com>
Cc: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/ata/libahci.c

@@ -561,27 +561,6 @@ void ahci_start_engine(struct ata_port *ap)
 {
 	void __iomem *port_mmio = ahci_port_base(ap);
 	u32 tmp;
-	u8 status;
-
-	status = readl(port_mmio + PORT_TFDATA) & 0xFF;
-
-	/*
-	 * At end of section 10.1 of AHCI spec (rev 1.3), it states
-	 * Software shall not set PxCMD.ST to 1 until it is determined
-	 * that a functoinal device is present on the port as determined by
-	 * PxTFD.STS.BSY=0, PxTFD.STS.DRQ=0 and PxSSTS.DET=3h
-	 *
-	 * Even though most AHCI host controllers work without this check,
-	 * specific controller will fail under this condition
-	 */
-	if (status & (ATA_BUSY | ATA_DRQ))
-		return;
-	else {
-		ahci_scr_read(&ap->link, SCR_STATUS, &tmp);
-
-		if ((tmp & 0xf) != 0x3)
-			return;
-	}
 
 	/* start DMA */
 	tmp = readl(port_mmio + PORT_CMD);