Re: [PATCH 1/1] IIO: Added write function in iio_buffer_fileops

[Aniroop Mathur]

On Wed, Aug 13, 2014 at 2:38 PM, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> On 08/13/2014 08:29 AM, a.mathur@xxxxxxxxxxx wrote:
>>
>> From: Aniroop Mathur <a.mathur@xxxxxxxxxxx>
>>
>> Earlier, user space can only read from iio device node but cannot write to
>> it.
>> This patch adds write function in iio buffer file operations,
>> which will allow user-space applications/HAL to write the data
>> to iio device node.
>> So now there will be two way communication between IIO subsystem
>> and user space. (userspace <--> kernel)
>>
>> It can be used by HAL or any user-space application which wants to
>> write data to iio device node/buffer upon receiving some data from it.
>> As an example,
>> It is useful for iio device simulator application which need to record
>> the data by reading from iio device node and replay the recorded data
>> by writing back to iio device node.
>>
>
> I'm not convinced that this is something that should be added to the kernel.
> I'm wondering why can't this be done in userspace, e.g. by having a
> simulator mode for the application or by using LD_PRELOAD. Having this in
> userspace will be much more flexible and will be easier to implement
> correctly and you'll most likely want to simulate more than just buffer
> access, for example setting/getting properties of the device or channel. For
> the libiio[1] we are planning to implement a test backend that when
> activated will allow to simulate a whole device rather than just buffer
> support.
>
> [1] https://github.com/analogdevicesinc/libiio
>
>

In normal Input Subsystem, there is two way communication between
kernel and userpace. It has both read and write functionality. :)
Why there is only one way communication in case of IIO ?

For Input devices, I completed simulation just by reading and writing at
input device node /dev/input/eventX.
But for IIO devices, I am stuck as there is no write function available
for iio device node /dev/iio:device0.

As per my understanding, if we do the simulation in userspace
then we need to create one more buffer in userpace. This will lead to
extra memory usage. With write functionality in IIO just like
in Input subsystem, we can avoid extra memory space. :)

>> Signed-off-by: Aniroop Mathur <a.mathur@xxxxxxxxxxx>
>> ---
>>   drivers/iio/iio_core.h            |    5 ++++-
>>   drivers/iio/industrialio-buffer.c |   34
>> ++++++++++++++++++++++++++++++++++
>>   drivers/iio/industrialio-core.c   |    1 +
>>   3 files changed, 39 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iio/iio_core.h b/drivers/iio/iio_core.h
>> index 5f0ea77..ba3fe53 100644
>> --- a/drivers/iio/iio_core.h
>> +++ b/drivers/iio/iio_core.h
>> @@ -47,10 +47,12 @@ unsigned int iio_buffer_poll(struct file *filp,
>>                              struct poll_table_struct *wait);
>>   ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user
>> *buf,
>>                                       size_t n, loff_t *f_ps);
>> -
>> +ssize_t iio_buffer_write_first_n_outer(struct file *filp,
>> +                            const char __user *buf, size_t n, loff_t
>> *f_ps);
>>
>>   #define iio_buffer_poll_addr (&iio_buffer_poll)
>>   #define iio_buffer_read_first_n_outer_addr
>> (&iio_buffer_read_first_n_outer)
>> +#define iio_buffer_write_first_n_outer_addr
>> (&iio_buffer_write_first_n_outer)
>>
>>   void iio_disable_all_buffers(struct iio_dev *indio_dev);
>>   void iio_buffer_wakeup_poll(struct iio_dev *indio_dev);
>> @@ -59,6 +61,7 @@ void iio_buffer_wakeup_poll(struct iio_dev *indio_dev);
>>
>>   #define iio_buffer_poll_addr NULL
>>   #define iio_buffer_read_first_n_outer_addr NULL
>> +#define iio_buffer_write_first_n_outer_addr NULL
>>
>>   static inline void iio_disable_all_buffers(struct iio_dev *indio_dev) {}
>>   static inline void iio_buffer_wakeup_poll(struct iio_dev *indio_dev) {}
>> diff --git a/drivers/iio/industrialio-buffer.c
>> b/drivers/iio/industrialio-buffer.c
>> index 9f1a140..ef889af 100644
>> --- a/drivers/iio/industrialio-buffer.c
>> +++ b/drivers/iio/industrialio-buffer.c
>> @@ -21,6 +21,7 @@
>>   #include <linux/slab.h>
>>   #include <linux/poll.h>
>>   #include <linux/sched.h>
>> +#include <asm/uaccess.h>
>
>
> linux/uaccess.h
>
>
>>
>>   #include <linux/iio/iio.h>
>>   #include "iio_core.h"
>> @@ -87,6 +88,39 @@ ssize_t iio_buffer_read_first_n_outer(struct file
>> *filp, char __user *buf,
>>   }
>>
>>   /**
>> + * iio_buffer_write_first_n_outer() - chrdev write to buffer
>> + *
>> + * This function pushes the user space data to kernel iio buffer
>> + **/
>> +ssize_t iio_buffer_write_first_n_outer(struct file *filp,
>> +                               const char __user *buf, size_t n, loff_t
>> *f_ps)
>> +{
>> +       struct iio_dev *indio_dev = filp->private_data;
>> +       struct iio_buffer *rb = indio_dev->buffer;
>> +       int ret = -1;
>> +       unsigned char *data = NULL;
>> +
>> +       if (!indio_dev->info)
>> +               return -ENODEV;
>> +
>> +       if (n != 1)
>> +               return -EINVAL;
>> +
>> +       data = kzalloc(1, GFP_KERNEL);
>> +       if (!data)
>> +               return -ENOMEM;
>> +
>> +       if (copy_from_user(data, buf, 1)) {
>> +               kfree(data);
>> +               return -EFAULT;
>> +       }
>> +
>> +       ret = iio_push_to_buffer(rb, data);
>
>
> Are you sure that this works? iio_push_to_buffer() expects a data buffer of
> size rb->bytes_per_datum bytes. On the other hand rb->bytes_per_datum is
> only valid when the buffer is enabled, so for this to work the buffer would
> need to be enabled. Which means you'd inject the fake data in the middle of
> the real data stream.
>
>

Yes, It works :)
In one patch, bytes_per_datum has been removed from kifo_in.
Patch - iio:kfifo_buf Take advantage of the fixed record size used in IIO
commit c559afbfb08c7eac215ba417251225d3a8e01062
- ret = kfifo_in(&kf->kf, data, r->bytes_per_datum);
+ ret = kfifo_in(&kf->kf, data, 1);
So, I think we can now write only one byte of data.

Initially, I wrote the code for write functionality in kernel version 3.6
using bytes_per_datum instead of fixed size of 1 byte.
It worked fine. :)
For this, we just need to replace size 1 by r->bytes_per_datum.

We are not injecting data in middle of real data stream.
When we inject the recorded data, we disabled the hardware chip,
so no new/real data is pushed to the buffer during that time.

To record, we enabled the buffer, read the real data and save it.
To replay, we disabled the hardware chip and injected saved data by
writing back to iio device node.
So, Buffer is still enabled at time of writing to iio device node. :)

Thanks !

>> +       kfree(data);
>> +       return ret;
>> +}
>
> [...]
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/