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ahci: Turn off DMA engines when there's no device attached

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According to section 10.3.1 of the AHCI spec, PxCMD.ST must not be set
unless there's a device attached. Following this saves us a measurable
quantity of power and does not impair hotplug support. Based on a patch
by Kristen Carlson Accardi.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
This commit is contained in:
Matthew Garrett 2010-03-28 00:37:21 -04:00 • committed by Jeff Garzik
commit 96d60303fd
1 changed files with 18 additions and 0 deletions
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18
drivers/ata/libahci.c
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@ -437,11 +437,29 @@ static int ahci_scr_write(struct ata_link *link, unsigned int sc_reg, u32 val)
return -EINVAL;
}
static int ahci_is_device_present(void __iomem *port_mmio)
{
u8 status = readl(port_mmio + PORT_TFDATA) & 0xff;
/* Make sure PxTFD.STS.BSY and PxTFD.STS.DRQ are 0 */
if (status & (ATA_BUSY | ATA_DRQ))
return 0;
/* Make sure PxSSTS.DET is 3h */
status = readl(port_mmio + PORT_SCR_STAT) & 0xf;
if (status != 3)
return 0;
return 1;
}
void ahci_start_engine(struct ata_port *ap)
{
void __iomem *port_mmio = ahci_port_base(ap);
u32 tmp;
if (!ahci_is_device_present(port_mmio))
return;
/* start DMA */
tmp = readl(port_mmio + PORT_CMD);
tmp |= PORT_CMD_START;