Date:	Tue, 15 Jun 2010 15:36:41 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Greg KH <gregkh@...e.de>, mingo@...e.hu, tglx@...utronix.de,
	bphilips@...e.de, yinghai@...nel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jeff@...zik.org, linux-ide@...r.kernel.org,
	stern@...land.harvard.edu, khali@...ux-fr.org
Subject: Re: [PATCH 12/12] usb: use IRQ watching

On Tue, Jun 15, 2010 at 13:20, Tejun Heo <tj@...nel.org> wrote:
> On 06/15/2010 12:30 PM, Kay Sievers wrote:
>>> Hmm... maybe what we can do is generating an uevent when an IRQ is
>>> confirmed to be bad and then let udev notify the user.  That way we'll
>>> probably have better chance of getting bug reports and users have
>>> whiny but working system.
>>
>> Not really, uevents are not picked up by anything that could report an
>> error to userspace, they are just seen by udev. Also uevents are
>> usually not the proper passing method. They are not meant to ever
>> transport higher frequency events, or structured data. They cause to
>> run the entire udev rule matching machine, and update symlinks and
>> permissions with every event.
>
> Oh, these will be very low frequency event.  At most once per
> irq_expect or irqaction.  e.g. on a machine with four hard drives, it
> can only happen four times after boot unless the driver is unloaded
> and reloaded, so from frequency standpoint it should be okay.
>
>> We will need some better error reporting facility. On Linux you don't
>> even get notified when the kernel mounts your filesystem read-only
>> because of an error. It will only end up in 'dmesg' as a pretty much
>> undefined bunch of words. :)
>
> That one is a very low frequency too.
>
>> We will need some generic error reporting facility, with structured
>> data exported, and where userspace stuff can subscribe to.
>> Uevents/udev can not really properly provide such infrastructure.
>> Maybe that can be extended somehow, but using kobject_uevent() and
>> trigger the usual udev rule engine is not what we are looking for, for
>> sane error reporting.
>
> It's true that there are higher frequency events which will be
> horrible to report via kobject_uevent().  Hmmm... but the thing is
> that events which should annoy the user by userland notification can't
> be definition high freq.  There's only so many users can recognize and
> respond, so the frequency limitation of uevent might actually fit here
> although it would be nice to have some kind of safety mechanism.
> Still no go for uevent?

Yeah, I'm pretty sure that's not what we want. We want structured
data, and a generic channel to pass all sort of errors through, and a
userspace part to handle it in a sane way. Many error sources may also
not have a device path in /sys, and therefore no uevent to send.
Uevents/udev just seem so convinient because it's already there, but I
think, has too many limitations to provide the needed functionality.
Besides the fact that nothing listens to these events in userspace
today -- it's a lot more to think through and work on than passing
things through uevents, especially some generic classification and
structured data passing, which is needed, instead of the current
free-text 'dmsg' or the property-based stuff in uevents. I'm very sure
it's the wrong facility to use.

Kay
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