On 04/01/16 12:46, Lars-Peter Clausen wrote: > 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. That's what I get for thinking I remembered how this code works ;) Completely forgot the outer function did anything non trivial. Thanks Daniel / Lars for picking up on this! Oops. Jonathan > > - 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
