linux-kernel - Re: [PATCH 2/2] thunderbolt: Initialize after IOMMUs

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Message-ID: <20180906114652.GK2283@lahna.fi.intel.com>
Date:   Thu, 6 Sep 2018 14:46:52 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     linux-kernel@...r.kernel.org,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH 2/2] thunderbolt: Initialize after IOMMUs

On Thu, Sep 06, 2018 at 01:21:01PM +0200, Lukas Wunner wrote:
> On Thu, Sep 06, 2018 at 02:07:56PM +0300, Mika Westerberg wrote:
> > On Thu, Sep 06, 2018 at 01:00:49PM +0200, Lukas Wunner wrote:
> > > On Thu, Sep 06, 2018 at 01:36:02PM +0300, Mika Westerberg wrote:
> > > > On Thu, Sep 06, 2018 at 10:13:37AM +0200, Lukas Wunner wrote:
> > > > > So with this patch, you rely on the linker ordering nhi_init() after
> > > > > ir_dev_scope_init(), however to the best of my knowledge the link
> > > > > order is not guaranteed.
> > > > 
> > > > What says that?
> > > 
> > > Within the same initcall level, the ordering is determined by the Makefile
> > > AFAIK.  Someone changes the Makefile, your dependency scheme falls apart.
> > 
> > There are other drivers doing the same so they would fail as well. It is
> > common practice AFAIK.
> 
> That doesn't make it a *good* practice.

It is good enough for our case.

> > > > > Looking at commit acb40d841257, which started this, I'm wondering
> > > > > why you did not simply export tbnet_init() and call it from the
> > > > > thunderbolt driver after the property stuff has been fully set up?
> > > > > After all, thunderbolt-net is useless without thunderbolt or am I
> > > > > missing something?  Then you could revert back to module_init().
> > > > 
> > > > The same reason you don't call PCI driver functions from PCI core. It
> > > > makes absolutely zero sense.
> > > > 
> > > > Thunderbolt is bus and provides driver API to drivers. We hopefully are
> > > > getting other service drivers (say SCSI over TBT) that are going to be
> > > > use the same interfaces.
> > > 
> > > Then add a blocking notifier chain into which these service drivers can
> > > hook.  Other buses have that as well.
> > 
> > It is really too complex to add notifier just for that. This works fine
> > and is not against any kernel principles I am aware of.
> 
> Well, there's a difference between "it works and gets the job done,
> let's move on" and "let's try to find a solution that fixes not just
> this use case but potentially benefits others as well".
> 
> FWIW, what I had in mind is a blocking notifier chain that gets called
> when a bus registers or unregisters.  TB service drivers would then check
> if it's tb_bus_type and start initialization.

Like I said, I think it is too complex.

If we ever need to change the initcall level third time (which I doubt)
we can start thinking about more complex solutions.