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

From: Lars-Peter Clausen
Date: Sun Feb 28 2021 - 10:53:14 EST

On 2/28/21 3:34 PM, Jonathan Cameron wrote:
On Sun, 28 Feb 2021 09:51:38 +0100
Lars-Peter Clausen <lars@xxxxxxxxxx> 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'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.

- Lars