linux-kernel - Re: [PATCH] iommu: Lower severity of add/remove device messages

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Message-ID: <761b5165f6708bb707148cfdee13d9b710eef29c.camel@collabora.com>
Date:   Fri, 27 Mar 2020 19:25:27 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, kernel@...labora.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: Lower severity of add/remove device messages

On Fri, 2020-03-27 at 18:04 +0000, Robin Murphy wrote:
> On 2020-03-27 1:02 pm, Ezequiel Garcia wrote:
> > Hello Joerg,
> > 
> > Thanks for reviewing.
> > 
> > I understand this change bears some controversy
> > for IOMMU, as developers are probably used to see these
> > messages.
> > 
> > On Fri, 2020-03-27 at 10:50 +0100, Joerg Roedel wrote:
> > > On Mon, Mar 23, 2020 at 06:49:56PM -0300, Ezequiel Garcia wrote:
> > > > These user messages are not really informational,
> > > > but mostly of debug nature. Lower their severity.
> > > 
> > > Like most other messages in the kernel log, that is not a reason to
> > > lower the severity.
> > > 
> > > These messages are the first thing to look at when
> > > looking into IOMMU related issues.
> > > 
> > 
> > Sure, but the messages are still here, you can
> > always enable them when you are looking at IOMMU issues :-)
> 
> That still begs the question of who "you" is and how they know they're 
> debugging an IOMMU issue in the first place. When all the developer has 
> to go on is a third-hand bugzilla attachment from a distro user's vague 
> report of graphics corruption/poor I/O performance/boot 
> failure/whatever, being able to tell straight away from a standard dmesg 
> dump whether an IOMMU is even in the picture or not saves a lot of 
> protracted back-and-forth for everyone involved.
> > The idea is to reduce the amount of verbosity in the kernel.
> 
> Under what justification? Users with slow consoles or who just want a 
> quiet boot are already free to turn down the loglevel; a handful of 
> messages at boot-time and device hotplug seem hardly at risk of drowning 
> out all the systemd audit spam anyway. Note that the IOMMU subsystem is 
> by nature a little atypical as a lot of what it does is only visible as 
> secondary effects on other drivers and subsystems, without their 
> explicit involvement or knowledge. In that respect, hiding its activity 
> can arguably lead to more non-obvious situations than many other subsystems.
> 
> > If all subsystems would print messages that are useful
> > when looking at issues, things would be quite nasty verbose.
> 
>  From a personal standpoint, can we at least eradicate all the "Hi! I'm 
> a driver/subsystem you don't even have the hardware for!" messages 
> first, then maybe come back and reconsider the ones that convey actual 
> information later?
> 

Do we really still have those???

Thanks,
Ezequiel