Re: [PATCH v6 20/24] iio: buffer: add ioctl() to support opening extra buffers for IIO device

[Alexandru Ardelean <ardeleanalex@gmail.com>]

On Sun, Feb 28, 2021 at 5:54 PM Lars-Peter Clausen <lars@metafoo.de> wrote:
>
> On 2/28/21 3:34 PM, Jonathan Cameron wrote:
> > On Sun, 28 Feb 2021 09:51:38 +0100
> > Lars-Peter Clausen <lars@metafoo.de> wrote:
> >
> >> On 2/15/21 11:40 AM, Alexandru Ardelean wrote:
> >>> With this change, an ioctl() call is added to open a character device for a
> >>> buffer. The ioctl() number is 'i' 0x91, which follows the
> >>> IIO_GET_EVENT_FD_IOCTL ioctl.
> >>>
> >>> The ioctl() will return an FD for the requested buffer index. The indexes
> >>> are the same from the /sys/iio/devices/iio:deviceX/bufferY (i.e. the Y
> >>> variable).
> >>>
> >>> Since there doesn't seem to be a sane way to return the FD for buffer0 to
> >>> be the same FD for the /dev/iio:deviceX, this ioctl() will return another
> >>> FD for buffer0 (or the first buffer). This duplicate FD will be able to
> >>> access the same buffer object (for buffer0) as accessing directly the
> >>> /dev/iio:deviceX chardev.
> >>>
> >>> Also, there is no IIO_BUFFER_GET_BUFFER_COUNT ioctl() implemented, as the
> >>> index for each buffer (and the count) can be deduced from the
> >>> '/sys/bus/iio/devices/iio:deviceX/bufferY' folders (i.e the number of
> >>> bufferY folders).
> >>>
> >>> Used following C code to test this:
> >>> -------------------------------------------------------------------
> >>>
> >>>    #include <stdio.h>
> >>>    #include <stdlib.h>
> >>>    #include <unistd.h>
> >>>    #include <sys/ioctl.h>
> >>>    #include <fcntl.h"
> >>>    #include <errno.h>
> >>>
> >>>    #define IIO_BUFFER_GET_FD_IOCTL      _IOWR('i', 0x91, int)
> >>>
> >>> int main(int argc, char *argv[])
> >>> {
> >>>           int fd;
> >>>           int fd1;
> >>>           int ret;
> >>>
> >>>           if ((fd = open("/dev/iio:device0", O_RDWR))<0) {
> >>>                   fprintf(stderr, "Error open() %d errno %d\n",fd, errno);
> >>>                   return -1;
> >>>           }
> >>>
> >>>           fprintf(stderr, "Using FD %d\n", fd);
> >>>
> >>>           fd1 = atoi(argv[1]);
> >>>
> >>>           ret = ioctl(fd, IIO_BUFFER_GET_FD_IOCTL, &fd1);
> >>>           if (ret < 0) {
> >>>                   fprintf(stderr, "Error for buffer %d ioctl() %d errno %d\n", fd1, ret, errno);
> >>>                   close(fd);
> >>>                   return -1;
> >>>           }
> >>>
> >>>           fprintf(stderr, "Got FD %d\n", fd1);
> >>>
> >>>           close(fd1);
> >>>           close(fd);
> >>>
> >>>           return 0;
> >>> }
> >>> -------------------------------------------------------------------
> >>>
> >>> Results are:
> >>> -------------------------------------------------------------------
> >>>    # ./test 0
> >>>    Using FD 3
> >>>    Got FD 4
> >>>
> >>>    # ./test 1
> >>>    Using FD 3
> >>>    Got FD 4
> >>>
> >>>    # ./test 2
> >>>    Using FD 3
> >>>    Got FD 4
> >>>
> >>>    # ./test 3
> >>>    Using FD 3
> >>>    Got FD 4
> >>>
> >>>    # ls /sys/bus/iio/devices/iio\:device0
> >>>    buffer  buffer0  buffer1  buffer2  buffer3  dev
> >>>    in_voltage_sampling_frequency  in_voltage_scale
> >>>    in_voltage_scale_available
> >>>    name  of_node  power  scan_elements  subsystem  uevent
> >>> -------------------------------------------------------------------
> >>>
> >>> iio:device0 has some fake kfifo buffers attached to an IIO device.
> >> For me there is one major problem with this approach. We only allow one
> >> application to open /dev/iio:deviceX at a time. This means we can't have
> >> different applications access different buffers of the same device. I
> >> believe this is a circuital feature.
> > Thats not quite true (I think - though I've not tested it).  What we don't
> > allow is for multiple processes to access them in an unaware fashion.
> > My assumption is we can rely on fork + fd passing via appropriate sockets.
> >
> >> It is possible to open the chardev, get the annonfd, close the chardev
> >> and keep the annonfd open. Then the next application can do the same and
> >> get access to a different buffer. But this has room for race conditions
> >> when two applications try this at the very same time.
> >>
> >> We need to somehow address this.
> > I'd count this as a bug :).  It could be safely done in a particular custom
> > system but in general it opens a can of worm.

I'll take a look at this.

> >
> >> I'm also not much of a fan of using ioctls to create annon fds. In part
> >> because all the standard mechanisms for access control no longer work.
> > The inability to trivially have multiple processes open the anon fds
> > without care is one of the things I like most about them.
> >
> > IIO drivers and interfaces really aren't designed for multiple unaware
> > processes to access them.  We don't have per process controls for device
> > wide sysfs attributes etc.  In general, it would be hard to
> > do due to the complexity of modeling all the interactions between the
> > different interfaces (events / buffers / sysfs access) in a generic fashion.
> >
> > As such, the model, in my head at least, is that we only want a single
> > process to ever be responsible for access control.  That process can then
> > assign access to children or via a deliberate action (I think passing the
> > anon fd over a unix socket should work for example).  The intent being
> > that it is also responsible for mediating access to infrastructure that
> > multiple child processes all want to access.
> >
> > As such, having one chrdev isn't a disadvantage because only one process
> > should ever open it at a time.  This same process also handles the
> > resource / control mediation.  Therefore we should only have one file
> > exposed for all the standard access control mechanisms.
> >
> Hm, I see your point, but I'm not convinced.
>
> Having to have explicit synchronization makes it difficult to mix and
> match. E.g. at ADI a popular use case for testing was to run some signal
> generator application on the TX buffer and some signal analyzer
> application on the RX buffer.
>
> Both can be launched independently and there can be different types of
> generator and analyzer applications. Having to have a 3rd application to
> arbitrate access makes this quite cumbersome. And I'm afraid that in
> reality people might just stick with the two devices model just to avoid
> this restriction.

I'm neutral on this part of the debate.
I feel this may be some older discussion being refreshed here (it's
just a personal feeling).

I can try to accommodate a solution if something (else) is agreed.
Though at this point it may be a little slower.
I'm no longer an ADI employee, so it may take me a little longer to
test some things.

>
> - Lars
>