Date:   Fri, 21 Feb 2020 10:31:35 +0000
From:   Vitor Soares <Vitor.Soares@...opsys.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>
CC:     Joao Pinto <Joao.Pinto@...opsys.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        "bbrezillon@...nel.org" <bbrezillon@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "wsa@...-dreams.de" <wsa@...-dreams.de>,
        "arnd@...db.de" <arnd@...db.de>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "corbet@....net" <corbet@....net>
Subject: RE: [PATCH v3 5/5] add i3cdev documentation

Hi Randy,

From: Randy Dunlap <rdunlap@...radead.org>
Date: Wed, Feb 19, 2020 at 04:34:00

> On 2/18/20 4:20 PM, Vitor Soares wrote:
> > This patch add documentation for the userspace API of i3cdev module.
> > 
> > Signed-off-by: Vitor Soares <vitor.soares@...opsys.com>
> > ---
> >  Documentation/userspace-api/i3c/i3cdev.rst | 116 +++++++++++++++++++++++++++++
> >  1 file changed, 116 insertions(+)
> >  create mode 100644 Documentation/userspace-api/i3c/i3cdev.rst
> > 
> > diff --git a/Documentation/userspace-api/i3c/i3cdev.rst b/Documentation/userspace-api/i3c/i3cdev.rst
> > new file mode 100644
> > index 0000000..ada269f
> > --- /dev/null
> > +++ b/Documentation/userspace-api/i3c/i3cdev.rst
> > @@ -0,0 +1,116 @@
> > +====================
> > +I3C Device Interface
> > +====================
> > +
> > +I3C devices have the flexibility of being accessed from userspace, as well
> > +through the conventional use of kernel drivers. Userspace access, although
> > +limited to private SDR I3C transfers, provides the advantage of simplifying
> > +the implementation of straightforward communication protocols, applicable to
> > +scenarios where transfers are dedicated such for sensor bring-up scenarios
> > +(prototyping environments) or for microcontroller slave communication
> > +implementation.
> > +
> > +The major device number is dynamically attributed and it's all reserved for
> 
>                                           allocated (?)
> 
> > +the i3c devices. By default, the i3cdev module only exposes the i3c devices
> 
>        I3C                                                         I3C
> 
> > +without device driver bind and aren't of master type in sort of character
> > +device file under /dev/bus/i3c/ folder. They are identified through its
> 
> IMO:                              s/folder/directory/ or sub-directory
> 
> > +<bus id>-<Provisional ID> same way they can be found in /sys/bus/i3c/devices/.
> 
>                              in the same way
> 
> > +::
> > +
> > +# ls -l /dev/bus/i3c/
> > +total 0
> > +crw-------    1 root     root      248,   0 Jan  1 00:22 0-6072303904d2
> > +crw-------    1 root     root      248,   1 Jan  1 00:22 0-b7405ba00929
> > +
> > +The simplest way to use this interface is to not have an I3C device bound to
> > +a kernel driver, this can be achieved by not have the kernel driver loaded or
> 
>             driver. This                 by not having
> 
> > +using the Sysfs to unbind the kernel driver from the device.
> 
>          the sysfs interface to unbind
> 
> > +
> > +BASIC CHARACTER DEVICE API
> > +===============================
> > +For now, the API has only support private SDR read and write transfers.
> 
>                         only support for private
> 
> For the unfamiliar, what is this "SDR"?  (thanks)

SDR stands for Single Data Rate. In I3C we can also have High Data Rate 
(HDR) modes:
  - Double Data Rate (HDR-DDR);
  - Ternary Symbol Legacy (HDR-TSL)
  - Ternary Symbol for Pure Bus (no I2C devices present on the bus)

Should I use Single Data Rate instead SDR  for the first time?

> 
> > +Those transaction can be achieved by the following:
> > +
> > +``read(file, buffer, sizeof(buffer))``
> > +  The standard read() operation will work as a simple transaction of private
> > +  SDR read data followed a stop.
> > +  Return the number of bytes read on success, and a negative error otherwise.
> > +
> > +``write(file, buffer, sizeof(buffer))``
> > +  The standard write() operation will work as a simple transaction of private
> > +  SDR write data followed a stop.
> > +  Return the number of bytes written on success, and a negative error otherwise.
> > +
> > +``ioctl(file, I3C_IOC_PRIV_XFER(nxfers), struct i3c_ioc_priv_xfer *xfers)``
> > +  It combines read/write transactions without a stop in between.
> > +  Return 0 on success, and a negative error otherwise.
> > +
> > +NOTES:
> > +  - According to the MIPI I3C Protocol is the I3C slave that terminates the read
> 
>                                           it is the I3C slave
> 
> > +    transaction otherwise Master can abort early on ninth (T) data bit of each
> > +    SDR data word.
> > +
> > +  - Normal open() and close() operations on /dev/bus/i3c/<bus>-<provisional id>
> > +    files work as you would expect.
> > +
> > +  - As documented in cdev_del() if a device was already open during
> > +    i3cdev_detach, the read(), write() and ioctl() fops will still be callable
> > +    yet they will return -EACCES.
> > +
> > +C EXAMPLE
> > +=========
> > +Working with I3C devices is much like working with files. You will need to open
> > +a file descriptor, do some I/O operations with it, and then close it.
> > +
> > +The following header files should be included in an I3C program::
> > +
> > +#include <fcntl.h>
> > +#include <unistd.h>
> > +#include <sys/ioctl.h>
> > +#include <linux/types.h>
> > +#include <linux/i3c/i3cdev.h>
> > +
> > +To work with an I3C device, the application must open the driver, made
> > +available at the device node::
> > +
> > +  int file;
> > +
> > +  file = open("/dev/bus/i3c/0-6072303904d2", O_RDWR);
> > +  if (file < 0)
> > +  exit(1);
> 
> better indentation?
> 
> > +
> > +Now the file is opened, we can perform the operations available::
> > +
> > +  /* Write function */
> > +  uint_t8  buf[] = {0x00, 0xde, 0xad, 0xbe, 0xef}
> 
> I can't find uint_t8.  Where is it located?

Typo ☹.

> and the braces should end with a ';'.
> 
> > +  if (write(file, buf, 5) != 5) {
> > +    /* ERROR HANDLING: I3C transaction failed */
> > +  }
> > +
> > +  /*  Read function */
> > +  ret = read(file, buf, 5);
> > +  If (ret < 0) {
> > +    /* ERROR HANDLING: I3C transaction failed */
> > +  } else {
> > +    /* Iterate over buf[] to get the read data */
> > +  }
> > +
> > +  /* IOCTL function */
> > +  struct i3c_ioc_priv_xfer xfers[2];
> > +
> > +  uint8_t tx_buf[] = {0x00, 0xde, 0xad, 0xbe, 0xef};
> > +  uint8_t rx_buf[10];
> > +
> > +  xfers[0].data = (uintptr_t)tx_buf;
> > +  xfers[0].len = 5;
> > +  xfers[0].rnw = 0;
> > +  xfers[1].data = (uintptr_t)rx_buf;
> > +  xfers[1].len = 10;
> > +  xfers[1].rnw = 1;
> > +
> > +  if (ioctl(file, I3C_IOC_PRIV_XFER(2), xfers) < 0)
> > +    /* ERROR HANDLING: I3C transaction failed */
> > +
> > +The device can be closed when the open file descriptor is no longer required::
> > +
> > +  close(file);
> > \ No newline at end of file
> 
> Please fix that warning. ^^^^^
> 
> 
> -- 
> ~Randy

Thanks for your feedback. I will address them next version.


Best regards,
Vitor Soares

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