Re: [PATCH v10 0/9] Add the I3C subsystem

[vitor]

Hi Boris,

On 16/11/18 13:16, Boris Brezillon wrote:
> On Fri, 16 Nov 2018 12:31:42 +0000
> vitor <vitor.soares@xxxxxxxxxxxx> wrote:
>
> > Hi Boris,
> >
> >
> > On 15/11/18 19:00, Boris Brezillon wrote:
> > > On Thu, 15 Nov 2018 18:03:47 +0000
> > > vitor <vitor.soares@xxxxxxxxxxxx> wrote:
> > >   
> > > > Hi Boris,
> > > >
> > > >
> > > > On 15/11/18 15:28, Boris Brezillon wrote:
> > > > > On Thu, 15 Nov 2018 16:01:37 +0100
> > > > > Wolfram Sang <wsa@xxxxxxxxxxxxx> wrote:
> > > > >      
> > > > > > Hi Boris,
> > > > > >      
> > > > > > > What we could do though, is expose I3C devices that do not have a
> > > > > > > driver in kernel space, like spidev does.
> > > > > > ...
> > > > > >      
> > > > > > > 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?
> >
> > Basically I need a tool that help me during the development and to avoid
> > me to write a dummy driver for each device that I test.
> But we want I3C device drivers to be upstreamed, so why not developing a
> real driver everytime you test a new device and submitting it upstream?


Usually the devices that I test aren't the final product so it isn't 
easy to do the upstream.

But when possible I plan to do that.


> > For instances do some read/write,
> Doing SDR/DDR transfers is probably acceptable, but I still think we
> should push hard to have kernel drivers when that's possible.
>
> > get/set ccc commands,
> Exposing CCC commands is definitely not a good idea, since they're not
> even exposed to kernel drivers.
>
> > if something
> > goes wrong during the bus initialization have a to debug etc...
> Can't we add such a debug infrastructure in the kernel. Maybe we can
> expose debugfs files too if that helps, though if those debugfs files
> are actually used by userspace libs/tools, it's not any better than
> ioctls or sysfs files, since they will anyway become a stable ABI.
>
> > For me this is a valid use case and I imagine when people start to
> > develop in i3c this interface will help everyone.
> How about you propose an i3cdev driver that allow users to do SDR
> transfers throuh an ioctl?

I think that was for v6 I started to something to expose the bus like in 
i2c-dev, but I liked the idea of expose only the device doesn't have a 
driver. Do you know if there is already something in the kernel doing 
the same?


Best regards,

Vitor Soares