linux-kernel - Re: kernFS/sysfs: mmap and vm

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Date:   Wed, 27 Sep 2017 11:38:47 +0200
From:   Federico Vaga <federico.vaga@...a.pv.it>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: kernFS/sysfs: mmap and vm_operations close

On Wednesday, 27 September 2017 09:45:51 CEST Greg Kroah-Hartman wrote:
> On Wed, Sep 27, 2017 at 08:41:12AM +0200, Federico Vaga wrote:
> > On Tuesday, 26 September 2017 23:31:29 CEST Greg Kroah-Hartman wrote:
> > > On Tue, Sep 26, 2017 at 05:50:55PM +0200, Federico Vaga wrote:

[...]

> > I want to make our VME interface "as PCI as possible", so I want to
> > re-create resources that users can `mmap` to access the device memory.
> 
> Have you looked at the UIO interface?

Yes, but I do not see many differences (user prospective) in doing my mmap 
with UIO, debugfs, sysfs, or char-device. I would have used a char-device, but 
I wanted to offer a "PCI like" interface so I did as PCI does. Is the PCI 
approach obsolete and it should not be taken as example?
I chose sysfs only for this reason, not because sysfs gives me something more; 
actually it does not, and that's why we are here :D

> > This abstraction will allow us to write utilities that works on PCI and
> > VME
> > devices without dealing with the peculiarity of each interface. This is
> > particularly useful when you have FPGAs that can run the "same" code on
> > different buses, but it is true as well when you have the same hardware
> > (the same memory map) installed on VME cards or PCI cards.
> > 
> > That's why a sysfs VME resource seems to me the easiest and clean way to
> > achieve this. I do not want to create yet another layer that hides the
> > differences between the two buses when mmap is so straight forward and
> > easy to implement. Another point is that adding a new layer add
> > complexity in the architecture and for the developers; they have to learn
> > yet another non- standard interface that I invent, while the concept of
> > resource is something that everybody know.
> 
> Or if you use the VME kernel interface, everything will "just work"...

(in private explanation about why we cannot use the VME kernel)

> > > sysfs binary attributes are for dumping binary data that the kernel
> > > doesn't touch/parse, through to hardware.  Why use mmap for this?  Do
> > > you have a pointer to your code somewhere?
> > 
> > No pointer :S
> 
> Sorry, I can't help out much then.
> 
> Best of luck!

No problem, thank you for your time.

-- 
Federico Vaga
http://www.federicovaga.it