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

[Alexander Graf <agraf@suse.de>]

On 26.02.15 23:25, Stuart Yoder wrote:
> 
> 
>> -----Original Message-----
>> From: Alexander Graf [mailto:agraf@suse.de]
>> Sent: Thursday, February 26, 2015 3:38 PM
>> To: Yoder Stuart-B08248; arnd@arndb.de
>> Cc: Rivera Jose-B46482; linux-kernel@vger.kernel.org; gregkh@linuxfoundation.org
>> 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@suse.de]
>>>> Sent: Thursday, February 26, 2015 8:33 AM
>>>> To: Yoder Stuart-B08248; arnd@arndb.de
>>>> Cc: Rivera Jose-B46482; linux-kernel@vger.kernel.org; gregkh@linuxfoundation.org
>>>> 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.

Ah, I see. Yeah, certainly works well for me :).


Alex