linux-kernel - RE: [RFC 5/5] i3c: add i3cdev module to expose i3c dev in /dev

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Message-ID: <CH2PR12MB4216CE03448AF3B39C23D3BCAE5A0@CH2PR12MB4216.namprd12.prod.outlook.com>
Date:   Wed, 11 Dec 2019 15:07:36 +0000
From:   Vitor Soares <Vitor.Soares@...opsys.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Boris Brezillon <bbrezillon@...nel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        Mark Brown <broonie@...nel.org>
Subject: RE: [RFC 5/5] i3c: add i3cdev module to expose i3c dev in /dev

Hi Arnd,

From: Arnd Bergmann <arnd@...db.de>
Date: Tue, Dec 10, 2019 at 19:37:14

> On Tue, Dec 10, 2019 at 8:15 PM Vitor Soares <Vitor.Soares@...opsys.com> wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > Date: Tue, Dec 10, 2019 at 17:51:14
> >
> > > On Tue, Dec 10, 2019 at 4:37 PM Vitor Soares <Vitor.Soares@...opsys.com> wrote:
> > > >
> > > > +/* IOCTL commands */
> > > > +#define I3C_DEV_IOC_MAGIC      0x07
> > > > +
> > > > +struct i3c_ioc_priv_xfer {
> > > > +       struct i3c_priv_xfer __user *xfers;     /* pointers to i3c_priv_xfer */
> > > > +       __u32 nxfers;                           /* number of i3c_priv_xfer */
> > > > +};
> > > > +
> > > > +#define I3C_IOC_PRIV_XFER      \
> > > > +       _IOW(I3C_DEV_IOC_MAGIC, 30, struct i3c_ioc_priv_xfer)
> > > > +
> > > > +#define  I3C_IOC_PRIV_XFER_MAX_MSGS    42
> > >
> > > This is not a great data structure for UAPI, please see
> > > https://urldefense.proofpoint.com/v2/url?u=https-3A__git.kernel.org_pub_scm_linux_kernel_git_arnd_playground.git_tree_Documentation_core-2Dapi_ioctl.rst-3Fh-3Dcompat-2Dioctl-2Dendgame-26id-3D927324b7900ee9b877691a8b237e272fabb21bf5&d=DwIBaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=qVuU64u9x77Y0Kd0PhDK_lpxFgg6PK9PateHwjb_DY0&m=5Q9WjK0o93NR7DQ9NM6So6mfdgpNnZnSaP8qMpgaC7E&s=LzzjrUQAG8fx5jkVyK73dBDrahNAvk09Cxxlx3KOiXI&e=
> > >
> > > for some background. I'm planning to submit that documentation for
> > > mainline integration soon.
> > >
> > >      Arnd
> >
> > Thanks for sharing the document.
> >
> > My understanding is that I should use a data structure like the struct
> > spi_ioc_transfer, with this I may also use the same ioctl command
> > definition. Am I right?
> 
> Yes, that would be an example of a structure that follows the best
> practices from my document. It is still rather complex, so if you
> can make it any simpler, that would be ideal.

I will try to do that.

> 
> > In the documentation you also refer the compact_ioctl() and It is not
> > clear to me if the compact_ioctl() is mandatory in this case. Should I
> > implement it as well?
> 
> If the structure is defined like that, you just need to set
> ".compat_ioctl=compat_ptr_ioctl," in the file_operations structure
> and it will work, but you cannot skip that step.

Thanks, now I know that is mandatory 😊.

> 
> As your interface is basically just read/write based, I wonder
> if there is a way to completely avoid the ioctl and instead
> use io_submit() as the primary interface.
> 
>       Arnd

I confess that I wasn't familiar with io_submit() until now and went 
straightway for the ioctl() approach.
So far, my understanding is that io_submit() will call .write or .read of 
i3cdev module depending on the iocb command. if so, we won't be able to 
do a repeated start between a multiple iocb in the same list, right?

Apart from this private read/write need, another requirement that leads 
me to use the ioctl() was:
- When we support HDR command in i3c subsystem we can expand the ioctl() 
command to support it.
- For now, device API doesn't expose CCC commands but some of them are 
used for a private contract between master and device and we likely need 
that support in the future as well.

Thanks,
Best regards,
Vitor Soares