From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeff Garzik <jeff@garzik.org>
Subject: Re: Why is only one command issue per time in AHCI driver?
Date: Thu, 13 Jan 2011 12:46:46 -0500
Message-ID: <4D2F3A86.3010503@garzik.org>
References: <AANLkTin5=NiZn6ZWZ=r-XS0v_Utrf6dpTb_YzCay-PJM@mail.gmail.com>
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To: =?UTF-8?B?576F56eJ6Yie?= <luobingjiun@gmail.com>
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On 01/13/2011 05:52 AM, =E7=BE=85=E7=A7=89=E9=88=9E wrote:
> Hello,
>
> Inside ahci_qc_issue() function, I see that driver always write one b=
it
> 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 =3D {
>         ATA_NCQ_SHT(DRV_NAME),
>         .can_queue              =3D AHCI_MAX_CMDS - 1,
> ............
>
> Why upper layer(Block or SCSI layer) does not fill up as many as poss=
ible
> 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 parall=
e.

We submit one command at a time to hardware because the block layer=20
passes commands to us one at a time.

	Jeff