Serial ATA (SATA) status report

From: Jeff Garzik (jgarzik_at_pobox.com)
Date: 02/26/04

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    Date:	Wed, 25 Feb 2004 21:34:37 -0500
To: Linux Kernel <linux-kernel@vger.kernel.org>

    attached.

    Serial ATA (SATA) for Linux
    status report
    Feb 25, 2004

    Hardware support
    ================

    Intel ICH5 and ICH5-R
    ---------------------
    Summary: No TCQ. Looks like a PATA controller, but with a few added,
    non-standard SATA port controls. Hardware does not support hotplug.
    "Coldplug" support is potentially feasible.

    libata driver status: Production, but see issue #2, #3, #4.

    drivers/ide driver status: Production, but see issue #1, #2.

    Issue #1: Depending on BIOS settings, IDE driver may lock up computer
    when probing drives.

    Issue #2: Excessive interrupts are seen in some configurations.

    Issue #3: "Enhanced mode" or "SATA-only mode" may need to be set in BIOS.

    Issue #4: Driver disables SATA port on unload, but does not re-enable
    SATA port on load.

    Intel ICH6 ("AHCI")
    -------------------
    Summary: Per-device queues, full SATA control including hotplug
    and PM.

    libata driver status: "looks like ICH5" support available in ata_piix
    very soon. Full support available in a week or two.

    Promise TX2/TX4/SX4
    -------------------
    Summary: Per-host queues on all controllers. Full SATA control
    including hotplug and PM on all but one controller.

    libata driver status: Beta, but see issue #5.

    Issue #5: Some boards appear to have PATA as well as SATA ports. PATA
    is not currently supported, and no plans have yet been made to rectify
    this. Ideally drivers/ide would drive PATA, but if they are the same
    PCI device, that would not be feasible.

    Silicon Image 3112/3114
    -----------------------
    Summary: No TCQ. Looks like a PATA controller, but with full SATA
    control including hotplug and PM.

    libata driver status: Alpha.

    drivers/ide driver status: Production, but see issue #4.

    Issue #4: Need to have the most recent fixes posted to lkml, for stable
    operation and full performance (where possible).

    Silicon Image 3124
    ------------------
    Soon, hopefully. User-written.

    Broadcom/ServerWorks/Apple
    --------------------------
    Summary: Huge per-device queues, full SATA control including hotplug
    and PM.

    libata driver status: Beta.

    VIA 

    ---
Summary:  No TCQ.  Looks like a PATA controller, but with full SATA
control including hotplug and PM.
libata driver status:  Beta.
SiS
---
libata driver status:  in progress (user-written)
Vitesse
-------
libata driver status:  in progress
Marvell
-------
libata driver status:  in progress
Software support
================
Basic Serial ATA support
------------------------
The "ATA host state machine", the core of the entire driver, is
considered production-stable.
The error handling is _very_ simple, but at this stage that is an
advantage.  Error handling code anywhere is inevitably both complex and
sorely under-tested.  libata error handling is intentionally simple.
Positives:  Easy to review and verify correctness.  Never data
corruption.  Negatives:  if an error occurs, libata will simply send
the error back the block layer.  There are limited retries by the block
layer, depending on the type of error, but there is never a bus reset.
Or in other words:  "it's better to stop talking to the disk than
compound existing problems with further problems."
As Serial ATA matures, and host- and device-side errata become apparent,
the error handling will be slowly refined.  I am planning to work with a
few (kind!) disk vendors, to obtain special drives/firmwares that allow
me to inject faults, and otherwise exercise error handling code.
Queueing support
----------------
Even though some SATA host controllers on the market already support
command queueing (a.k.a. "TCQ"), libata does not yet support it.
However, libata was designed from the ground-up to support queueing, so
I need only change a few lines of code, and write two functions, to
enable this behavior.
Queueing will be enabled in libata soon, but to do so requires a long
stretch of testing on a large variety of controllers and drives.  This
is very time-intensive, and is the largest part of this task.
Tangent:  Host-based queueing and Native Command Queueing
Queueing is the process of sending multiple commands to a single device,
without waiting for prior commands to finish.  This increases
performance and reduces latency.  There are three types of queueing in
the ATA world:
1) "legacy TCQ" -- some PATA devices support this.  Just ignore it,
it's going away.
2) "host-based TCQ" -- the host controller supports a queue of drive
commands, whether or not the drive supports it.
3) "Native Command Queueing" -- both host and drive cooperate in the
queueing and execution of drive commands.  This should provide the
highest performance and lowest latency of all three options.
#1 is support by drivers/ide _only_.  libata will not support this.
#2 will soon be supported by libata.
#3 will be supported by libata when hardware is available from drive
manufacturers.
Hotplug support
---------------
All SATA is hotplug.
libata does not support hotplug... yet.
Power Management support
------------------------
Over and above the power management specified in the ATA/ATAPI
specification, one can aggressively control the power consumption of
SATA hosts, the SATA bus, and the SATA device.
SMART support
-------------
Soon.  Requires the capability to directly submit ATA commands from
userspace to the low-level device, which must be added with care.
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  • Next message: Nick Piggin: "Re: 2.6.3-mm3"

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