Re: Serial ATA and SiI 3112A controller

jwendel_at_fnmoc.navy.mil

To: fedora-list@redhat.com
Date: Wed, 28 Jan 2004 09:44:15 -0800


---
Summary: No TCQ. Looks like a PATA controller, but with full SATA
control including hotplug and PM.
libata driver status: Beta.
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.
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Re: Serial ATA and SiI 3112A controller

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