linux-kernel - RE: [PATCH v2 1/3] platform/x86: Add Intel Input Output Manager (IOM) driver

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Date:   Mon, 31 Aug 2020 21:40:52 +0000
From:   "Mani, Rajmohan" <rajmohan.mani@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Ayman Bagabas <ayman.bagabas@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Joseph, Jithu" <jithu.joseph@...el.com>,
        Blaž Hrastnik <blaz@...n.io>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "pmalani@...omium.org" <pmalani@...omium.org>,
        "bleung@...omium.org" <bleung@...omium.org>
Subject: RE: [PATCH v2 1/3] platform/x86: Add Intel Input Output Manager (IOM)
 driver

Hi Greg,

> Subject: Re: [PATCH v2 1/3] platform/x86: Add Intel Input Output Manager
> (IOM) driver
> 
> On Fri, Aug 28, 2020 at 03:20:22PM +0000, Mani, Rajmohan wrote:
> > Hi Greg,
> >
> > > Subject: Re: [PATCH v2 1/3] platform/x86: Add Intel Input Output
> > > Manager
> > > (IOM) driver
> > >
> > > Hi Greg,
> > >
> > > On Fri, Aug 28, 2020 at 09:43:59AM +0200, Greg Kroah-Hartman wrote:
> > > > I still find this crazy that a whole separate driver is created
> > > > just to read a single 32bit value.
> > > >
> > > > Why not put this logic in the driver that wants to read that value?
> > > > That would be much simpler, smaller, and more obvious.
> > >
> > > That would mean that we start maintaining something like DMI quirk
> > > table in those drivers. Unfortunately the IOM device is not available on
> every platform.
> > > Also, even on platforms that do have it, there is no guarantee that
> > > the device is always going to be mapped to the same address.
> > >
> > > Nevertheless, I was originally hoping that we could hide the
> > > handling of IOM somehow in ACPI without the need for an actual
> > > device object, but it now turns out that the other features of the
> > > IOM chip have created interest. At least our i915 guys probable have
> > > some use for it (I don't know exactly what they are planning to use it for).
> > >
> > > So the fact that we may later need the device for something else, on
> > > top of the clumsiness and most importantly risks involved with using
> > > ACPI to take care of extra tasks (ASL tends to have bugs - bugs that
> > > may never ever get fixed), I think the IOM device object, and the
> > > driver that binds to it, do have a valid reason for existing.
> > >
> >
> > Intel PMC USB mux device is part of the PCH, while IOM is part of the SoC.
> 
> I have no idea what a "PCH" is, what "IOM" is, and how any of this relates to a
> "SoC" :)
> 

I was just meaning to say IOM (Intel Output Manager) is a separate device, that
is not part of PCH (Platform Controller Hub) like PMC (Power Management Controller).

For the sake of completeness

PCH - Platform Controller Hub (usually that handles I/Os in Intel core platforms)
IOM - Input Output Manager (IOM) is part of the Tiger Lake SoC that handles Type-C
topology, configuration and PM functions of various Type-C devices connected
on the platform

> Don't impose arbritrary hardware "splits" to kernel code when the kernel has
> no such "partitioning" please.
> 

Ack.

> > This was another reason we had to have a separate ACPI device.
> 
> That sounds like a firmware issue you can solve in UEFI.
> 

Ack

> I think this is the most TLA-laden email I have ever written, and I used to work
> at IBM :)

I thought it was only Intel where TLAs are abundantly used.

Thanks for the reviews and the direction on this topic.

> 
> greg k-h