Re: [patch 1/2] ahci: Dont start port DMA engines unless a device is present

[Jeff Garzik <jeff@garzik.org>]

Kristen Carlson Accardi wrote:
> According to the AHCI spec, you should not set PxCMD.ST unless
> a functional device is present, as determined by PxTFD.STS.BSY=0,
> PxTFD.STS.DRQ=0, and PxSSTS.DET = 3.
> 
> Signed-off-by:  Kristen Carlson Accardi <kristen.c.accardi@intel.com>
> 
> Index: linux-2.6.25-rc3/drivers/ata/ahci.c
> ===================================================================
> --- linux-2.6.25-rc3.orig/drivers/ata/ahci.c
> +++ linux-2.6.25-rc3/drivers/ata/ahci.c
> @@ -783,11 +783,40 @@ static int ahci_scr_write(struct ata_por
>  	return -EINVAL;
>  }
>  
> +static int ahci_is_device_present(struct ata_port *ap)
> +{
> +	void __iomem *port_mmio = ahci_port_base(ap);
> +	u8 status;
> +
> +	/* Make sure PxTFD.STS.BSY and PxTFD.STS.DRQ are 0 */
> +	status = ahci_check_status(ap);
> +	if (status & (ATA_BUSY | ATA_DRQ))
> +		return 0;
> +
> +	/* Make sure PxSSTS.DET is 3h */
> +	status = readl(port_mmio + PORT_SCR_STAT);
> +	status &= 0xf;
> +	if ((status & 0xf) != 3)
> +		return 0;
> +	return 1;
> +}
> +
>  static void ahci_start_engine(struct ata_port *ap)
>  {
>  	void __iomem *port_mmio = ahci_port_base(ap);
>  	u32 tmp;
>  
> +	/*
> +	 * See AHCI specification 1.2 section 10.1.2
> +	 *
> +	 * Software shall not set PxCMD.ST to '1' until it is
> + 	 * determined that a functional device is present on
> + 	 * the port, as determined by PxTFD.STS.BSY = '0',
> + 	 * PxTFD.STS.DRQ = '0', PxSSTS.DET = 3h
> + 	 */
> +	if (!ahci_is_device_present(ap))
> +		return;
> +
>  	/* start DMA */
>  	tmp = readl(port_mmio + PORT_CMD);

This seems incomplete?

If we wish to maintain this invariant, which seems like a good idea and 
the right thing to do, then this patch fails to cover the hotplug case, 
correct?

	Jeff