RE: [PATCH 0/3 v6] drivers/bus: Freescale Management Complex bus driver patch series

[Stuart Yoder]

> -----Original Message-----
> From: Alexander Graf [mailto:agraf@xxxxxxx]
> Sent: Thursday, February 26, 2015 3:38 PM
> To: Yoder Stuart-B08248; arnd@xxxxxxxx
> Cc: Rivera Jose-B46482; linux-kernel@xxxxxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH 0/3 v6] drivers/bus: Freescale Management Complex bus driver patch series
> 
> 
> 
> On 26.02.15 21:32, Stuart Yoder wrote:
> >
> >
> >> -----Original Message-----
> >> From: Alexander Graf [mailto:agraf@xxxxxxx]
> >> Sent: Thursday, February 26, 2015 8:33 AM
> >> To: Yoder Stuart-B08248; arnd@xxxxxxxx
> >> Cc: Rivera Jose-B46482; linux-kernel@xxxxxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx
> >> Subject: Re: [PATCH 0/3 v6] drivers/bus: Freescale Management Complex bus driver patch series
> >>
> >>
> >>
> >> On 27.01.15 15:35, Stuart Yoder wrote:
> >>> Hi Arnd/Alex,
> >>>
> >>> German has posted an example driver for the fsl-mc bus in his RFC
> >>> "[RFC PATCH 1/1] drivers/bus: fsl-mc object allocator driver".
> >>>
> >>> In addition I have made available the skeleton for a driver for
> >>> one of the objects/devices (crypto) that will be discovered on
> >>> the bus:
> >>>     https://github.com/stuyoder/linux
> >>>     branch: fsl-ms-bus
> >>>
> >>> ...it is not functional yet, but shows how a driver registers with
> >>> the bus, get's probed, performs initialization.
> >>
> >> Ok, so if I grasp this correctly the idea is that we have a driver
> >> attaching to an individual device on the fsl-mc bus.
> >
> > Yes.
> >
> >> That driver then
> >> goes and allocates / blocks more devices from that bus as it initializes.
> >
> > Yes, there are certain devices/objects on the bus that by themselves
> > are not standalone, functional devices.  An example is a "buffer pool".
> > Network interface drivers, crypto driver, decompression driver, etc need
> > one or more hardware buffer pools.  There is a buffer depletion interrupt
> > associated with the device.
> >
> > The buffer pools itself binds to a resource allocation driver in
> > the kernel, which then can hand out buffer pools as required by
> > other drivers.
> 
> Ok, so there are 2 things on the bus
> 
>   * devices
>   * resources

In the general sense, yes.  To be picky about terminology we call
all these things on the bus "objects".  Some are more resource-like,
in that they are handed out by an allocator to the functional drivers.

I don't want to call them 'resources' because that term actually means
something slightly different in the hardware architecture that is not 
actually visible to Linux.  

> Someone really needs to sit down and write some nice ASCII art about all
> of this and include all the abbreviations in it as well, so that  anyone
> not deeply involved in the architecture has the chance to grasp what
> this is about.

The cover letter for the patch series is a starting point, but
yes we need something for ./Documentation.

> >> Is that model always possible?
> >
> > Yes, why would it not be?
> >
> >> Which device would a NIC bind to for
> >> example?
> >
> > Network interface / Ethernet driver requires some number
> > of buffer pools, plus a management complex portal device
> > (DPMCP) used for sending commands to manage the hardware.
> 
> Ok, so there is always one object that basically "owns" a particular
> device. And then there is a cloud of resources that drivers grab as they go.
> 
> I think I got it by now and the concept makes a lot of sense. I'm not
> sure whether there's any particular benefit or downside of having
> resources be devices, but looking at the resource manager code it
> probably doesn't hurt.

They need to be real Linux devices.  The reason is that when we
bind a DPRC and the objects in it to VFIO, VFIO expects everything 
to be a device.  VFIO exposes 'devices' to user space, and so for
example a buffer pool's IRQ needs to be exposed via standard VFIO
mechanisms.

Thanks,
Stuart

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