Re: Why is only one command issue per time in AHCI driver?

[Jeff Garzik <jeff@garzik.org>]

On 01/13/2011 05:52 AM, 羅秉鈞 wrote:
> Hello,
>
> Inside ahci_qc_issue() function, I see that driver always write one bit
> to Px.CI register even enable the NCQ.
>
> AHCI allows to fire 32 commands at one time and AHCI driver also
> claims itself able to queue up to 31 commands.
>
> Inside ahci.c :
> static struct scsi_host_template ahci_sht = {
>         ATA_NCQ_SHT(DRV_NAME),
>         .can_queue              = AHCI_MAX_CMDS - 1,
> ............
>
> Why upper layer(Block or SCSI layer) does not fill up as many as possible
> commands into Command List before sending the commands out
> (Write to Px.CI register)?

The driver successfully executes up to 31 commands at a time, in paralle.

We submit one command at a time to hardware because the block layer 
passes commands to us one at a time.

	Jeff