Re: [PATCH 1/3] acpi: Rip out EC_FLAGS_QUERY_PENDING (prevent racecondition)

From: Alan Jenkins
Date: Sun Jul 20 2008 - 10:55:18 EST

Alexey Starikovskiy wrote:
> Alan Jenkins wrote:
>> /* moved here from acpi_ec_transaction_unlocked() */
>> clear_bit(EC_FLAGS_QUERY_PENDING, &ec->flags);
>> while (acpi_ec_query(ec, &value) == 0)
>> acpi_ec_gpe_run_handler(ec, value);
>> }
>> The PENDING flag then reflects whether or not there's an item on the
>> workqueue. Does that make sense?
> Yes, this is what the flag was made for -- to guard workqueue from
> being filled up.
>>> Your second patch is redundant if you add queue entry for each
>>> interrupt, and as it does not
>>> require any investments into memory, I like it more.
>> It's not quite redundant with the first patch. We still have GPE
>> polling mode - patch #1 doesn't affect that. In polling mode, it's
>> essential to query all the pending events - otherwise, if they arrive
>> more frequently than the polling interval then you will inevitably drop
>> events.
> I meant that if we have patch #3, which drops poll mode, we only need
> one of the first two, not both. And in this case I like #2 more than #1.
>> Patch #2 is also required to fix my buggy hardware. My laptop's EC
>> buffers multiple events, but clears SCI_EVT after every query. This
>> caused problems in polling mode; with multiple events between polling
>> intervals only one gets queried - and after a while the buffer overflows
>> and it breaks completely.
> I fully agree here, it would be great to make sure that EC buffers are
> empty.
>>> Also, it is very cool to rip of things you don't care for, but that
>>> essentially reverts a fix done for 9998,
>>> so at least, you should ask these people if you broke their setups.
>> I assume you're referring to the "would benefit from wider testing"
>> patch #3. Thanks for identifying the bugzilla entry - I had difficulty
>> separating the different entries on GPEs. I'm optimistic that we can
>> fix all these crazy buffering EC's without having to disable GPE
>> interrupts.
> I would say, we should test pair of #2 and #3, or your original patch
> + my patch.
> I still like the option to not have any new interrupts until I'm done
> with current one --
> much like the PENDING bit.
>> The reason I like my proposed fix is that it makes the code simple
>> enough that I can understand it, without making any assumptions about
>> how many interrupts arrive per GPE. The EC can be broken in lots of
>> ways, so long as:
>> 1. We receive interrupts when one or more GPE's are pending.
>> 2. We don't get a constant interrupt storm.
>> I don't think I missed anything. Is there anything else I should check
>> before I try to get testing?
> Well, there is Keith Packard with concern that either high number of
> interrupts or long
> spins on polling status register could push battery life down the
> drain...
> One more argument to disable GPE until we service previous.
> Regards,
> Alex.

Ah, the powertop argument. Disabling interrupts is a nice safety
blanket, but it means you end up polling the status register during
queries. I think the interrupts are better for battery life. The
excess interrupts happen all at once, so they don't wake the CPU up more
frequently, they just keep it awake slightly longer.

I'm convinced that emptying the EC buffer will reduce the number of
interrupts, even for the reporter of #9998. I'll ask them to test this
patchset, then respin it to fix the mess I made of
EC_FLAGS_QUERY_PENDING. As I mentioned earlier I've worked out how to
avoid my theoretical race without causing increased memory usage.

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