On Thu, 15 Nov 2018 18:03:47 +0000
vitor [off-list ref] wrote:

Hi Boris,


On 15/11/18 15:28, Boris Brezillon wrote:

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On Thu, 15 Nov 2018 16:01:37 +0100
Wolfram Sang [off-list ref] wrote:
 

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Hi Boris,
 

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What we could do though, is expose I3C devices that do not have a
driver in kernel space, like spidev does.  

...
 

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Mark, Wolfram, Arnd, Greg, any opinion?  

Is there a benefit for having drivers in userspace? My gut feeling is to
encourage people to write kernel drivers. If this is, for some reason,
not possible for some driver, then we have a use case at hand to test
the then-to-be-developed userspace interface against. Until then, I
personally wouldn't waste effort on designing it without a user in
sight.  

I kind of agree with that. Vitor, do you have a use case in mind for
such userspace drivers? I don't think it's worth designing an API for
something we don't need (yet).  

My use case is a tool for tests, lets say like the i2c tools.

What would you like to test exactly?

There is 
other subsystems, some of them mentioned on this thread, that have and 
ioctl system call or other method to change parameters or send data.

I don't think they added the /dev interface before having a real use
case for it.

I rise this topic because I really think it worth to define now how this 
should be design (and for me how to do the things right) to avoid future 
issues.

Actually it should be done the other way around: you should have a real
need and the /dev interface should be designed to fulfill this need.
Based on this real use case we can discuss other potential usage that
might appear in the future and try to design something more
future-proof, but clearly, this userspace interface should be driven by
a real/well-defined use case.

Also, exposing things to userspace is way more risky than adding a new
in-kernel subsystem/framework, because it then becomes part of the
stable ABI.

To make things clearer, I'm not against the idea of exposing I3C
devices (or I3C buses) to userspace, but I'd like to understand what you
plan to do with that. If this is about testing, what kind of tests
you'd like to run. If this is about developing drivers in userspace,
why can't these be done in kernel space (license issues?), and what
would those drivers be allowed to do?