On Mon, Jan 4, 2016 at 1:34 PM, Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > On 04/01/16 04:59, Julio Cruz wrote: >> Hi Jonathan, >> >> Previously, you help me about an issue related with data loss. You suggest >> me to debug deep in the core elements. I will try to summarize the results >> below for future reference. >> >> When there is not data available in the buffer (kfifo), and the application >> try to read data (using "read" function), it return zero (0). >> >> If libiio will be used to read the data, there is a problem (detailed at >> https://github.com/analogdevicesinc/libiio/issues/23). In brief, Paul >> (pcercuei) suggest me that this issue must be manage by the driver, in this >> case, return -EAGAIN when there is not data available [Resource temporarily >> unavailable (POSIX.1)]. >> >> After review the core elements as suggested, I changed the line (in >> function iio_read_first_n_kfifo of kfifo_buf.c) as below: >> >> - return copied; >> + return copied == 0 ? -EAGAIN: copied; >> >> Do you think will be OK like this? I assume you are doing a blocking read. Because otherwise your situation should already be handled in industrialio-buffer.c: 143, In iio_buffer_read_first_n_outer function we have: » » ret = rb->access->read_first_n(rb, n, buf); » » if (ret == 0 && (filp->f_flags & O_NONBLOCK)) » » » ret = -EAGAIN; » } while (ret == 0); Now, if you are doing a blocking read and your read() returns 0 then there is a bug in IIO core and it must be fixed! :) We should again investigate iio_buffer_read_first_n function where, with a special attention to this condition: Can you paste here the parameters/ret values for relevant open and read system calls done by your application? » » ret = wait_event_interruptible(rb->pollq, » » iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size)); » » if (ret) » » » return ret; This part should block until there is some data available. > Hmm.. This is an interesting one (thanks for tracking it down) > > The man page for read indeed allows for this to occur. > > When attempting to read a file (other than a pipe or FIFO) that sup‐ > ports non-blocking reads and has no data currently available: > > * If O_NONBLOCK is set, read() shall return −1 and set errno to > [EAGAIN]. > > > However the issue here is that this is an ABI change and there may > unfortunately be code out there relying on it returning 0. > > Most of the time a read will only be made in response to poll informing the > userspace code that there is something available. Whilst obviously we > should try and fix the case of it reading when there isn't, I am a little > curious to how you hit this case. > > So what do others think? Much chance of us breaking any userspace > code by making this change? -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
