On Thu, May 05, 2016 at 11:39:34AM +0200, Clemens Ladisch wrote:
Takashi Sakamoto wrote:
On May 4 2016 22:59, Charles Keepax wrote:
if (PCM_RUNTIME_CHECK(substream))
return -ENXIO;
return POLLIN | POLLRDNORM | POLLERR;[...] On the other hand, I think POLLOUT, POLLIN, POLLWRNORM and POLLRDNORM should not be included in the value. PCM_RUNTIME_CHECK() ensures PCM substream or PCM runtime is NULL. This means that subsequent I/O operations are failed, at least for handling PCM frames.
I think it better to return 'POLLERR | POLLHUP'.
To expand on this: POLLIN/POLLOUT imply that it is possible to read/ write data without blocking. Sockets and pipes combine POLLHUP with POLLIN because the read() (or recv()) returns 0 bytes without blocking to indicate the end of the stream.
But in this situation, snd_pcm_read*/write* will always fail, so it is, strictly speaking, indeed not appropriate to set POLLIN/OUT.
On the other hand, PCM devices do include the POLLIN/OUT bits when they are in a wrong state. (This is probably to catch programs that do not check the error bits; with POLLIN/OUT set, these programs will try to read/write, and will then get the error code.)
So for consistency, the bits should be included. (Or the other error case fixed to remove these bits.)
Thanks for the explaination guys. I definitely agree that all the return values should be consistent. I am happy to change the values if people prefer but I guess the decision really rests with Takashi and if he is happy to change the returns to POLLERR | POLLHUP, as I guess there is the potential for some user-space fall out. Perhaps I should do this as a seperate patch on top of this chain, so we can review explicitly.
I have had a look and both tinyalsa and alsalib look like they would handle the change correctly.
Thanks, Charles