Re: [PATCH 11/12] libata: use IRQ expecting

From: Jeff Garzik
Date: Sat Jun 26 2010 - 05:18:00 EST

On 06/26/2010 04:31 AM, Tejun Heo wrote:
Well, it can indicte the start of cluster of completions, which is the
necessary information anyway. From the second call on, it's a simple
flag test and return. I doubt it will affect anything even w/ high
performance SSDs but please read on.

Yes, and your patch calls unexpect_irq() at the _start_ of a cluster of completions. That is nonsensical, because it reflects the /opposite/ of the present ATA bus state, when multiple commands are in flight.

ata_qc_complete_multiple() call [un]expect_irq() only once by
introducing an internal completion function w/o irq expect handling,
say ata_qc_complete_raw() and making both ata_qc_complete() and
ata_qc_complete_multiple() simple wrapper around it w/ irq expect

Yes, this fixes problem, but it is better to create a wrapper path for the legacy PATA/SATA1 that uses irq-expecting, and a fast path for modern controllers that do not use it.

On 06/26/2010 05:45 AM, Jeff Garzik wrote:
We don't want to burden modern SATA drivers with the overhead of
dealing with silly PATA/SATA1 legacy irq nastiness, particularly the
ugliness of calling

I think we're much better off applying it to all the drivers. IRQ
expecting is very cheap and scalable and there definitely are plenty
of IRQ delivery problems with modern controllers although their
patterns tend to be different from legacy ones. Plus, it will also be
useful for power state predictions.

Modern SATA/SAS controllers, and their drivers, already have well defined methods of acknowledging interrupts, even unexpected ones, in ways that do not need this core manipulation. This is over-engineering, punishing all modern chipsets moving forward regardless of their design, by unconditionally requiring this behavior of all libata drivers.

Just like the rest of libata's layered driver architecture, it should be straightforward to apply this only to SFF/BMDMA chipsets, then tackle odd cases as needs arise.

Modern controllers acknowledge interrupts sanely, and always "expect" an interrupt when you include interrupt events like hotplug, even if the ATA bus itself is idle. There is no need to burden the millions of ahci users with irq-expecting, for example.

With regards to power state predictions, it is only useful if you are accurately reflecting the ATA bus state (idle or not) at all times. As mentioned above, this patch clearly creates a condition where unexpect_irq() is called when commands remain in flight, and libata is expecting further command completions.

IOW, patch #11 says "we are not expecting irq" when we are.

At least a halfway sane approach would be to track bus-idle status, and trigger useful code when that status changes (idle->active or active->idle). Perhaps LED, power state, and irq-expecting could all use such a triggering mechanism.


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