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

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Date:   Fri, 20 Dec 2019 12:39:11 +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 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

I was checking other code examples for this and found that some authors 
add extra reserved fields for future use. Should I do the same?
 
I have some doubt too if it is ok to use the same ioctl command approach 
as in spidev:

#define SPI_MSGSIZE(N) \
  	((((N)*(sizeof (struct spi_ioc_transfer))) < (1 << _IOC_SIZEBITS)) \
		? ((N)*(sizeof (struct spi_ioc_transfer))) : 0)
#define SPI_IOC_MESSAGE(N) _IOW(SPI_IOC_MAGIC, 0, char[SPI_MSGSIZE(N)])

Or the best is to use another structure to embedded N transfers like in 
this patch.

Thanks in advance for your help.
Best regards,
Vitor Soares