On 01/04/2016 12:34 PM, Jonathan Cameron 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? > 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. We never propagate 0 to userspace though. The referenced function is iio_read_first_n_kfifo() which is an internal function. The function that handles the userspace ABI is iio_buffer_read_first_n_outer() and here, as Daniel pointed out, there are two things that can happen. We are in non-blocking mode and iio_read_first_n_kfifo() returns 0. In that case we'll return -EAGAIN as mandated by the specification. We are in blocking mode and iio_read_first_n_kfifo() returns 0. In that case we'll go back to waiting for more data and we'll only return if either data was received or the application was interrupted by a signal. In the former case we'll return the number of received bytes in the later case -ERESTARTSYS. So either way we should never return 0, something else must be going on. Btw. letting iio_read_first_n_kfifo() return -EAGAIN will break blocking mode. - Lars -- 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
