On 08/14/2017 02:30 PM, Tejun Heo wrote: > Hello, Joe. > > On Thu, Aug 10, 2017 at 10:45:54AM -0400, Joe Lawrence wrote: >> In the case of my USB DVD -> laptop example, there is no media in my >> device, however I still see the DISK_EVENT_MEDIA_CHANGE event. This is >> a bit confusing, and I was wondering if there was an explanation for >> the following: >> >> drivers/scsi/sr.c :: sr_probe() >> >> disk->events = DISK_EVENT_MEDIA_CHANGE | DISK_EVENT_EJECT_REQUEST; >> ... >> cd->media_present = 1; >> >> DISK_EVENT_MEDIA_CHANGE events will pass through to userspace and >> for some reason cd->media_present defaults to 1? More on that below... > > I don't have any concrete ideas but I think the only thing it's trying > to do is to always generate at least one changed event no matter what. > > ... >> sr_check_events() compares the previous (in this case, default) >> media_present value with what the TUR returns. If it has changed, then >> turn on the DISK_EVENT_MEDIA_CHANGE event bit. >> >> In my laptop USB DVD case, !scsi_status_is_good and sshdr.asc == 0x3a, >> so last_present (1) and cd->media_present (0) mis-compare and the change >> event is set. That does not seem intuitive to me, is this a bug? > > It's not incorrect. We can try to change the behavior to avoid double > notifications but given that this has been like this for a really long > time and that it isn't technically incorrect, I'm not sure changing it > is a good idea. It might as well break other things. Without a definition of DISK_EVENT_MEDIA_CHANGE or its udev DISK_MEDIA_CHANGE counterpart it's kinda hard to say :) But I agree that changing this behavior could have inadvertent effects. >> Bringing this back to the reported BMC case, which presumably does have >> "media" present in the virtual device... is it reasonable to expect a >> DISK_EVENT_MEDIA_CHANGE even for a new device that contains media? (I >> haven't verified, but in this case GET_EVENT_STATUS_NOTIFICATION might >> be enough to set media present.) > > Yeah, I think so. > >> If there is documentation that explains DISK_EVENT_MEDIA_CHANGE conditions >> somewhere, feel free to point me there. > > AFAIK, there isn't any. The only thing it tries to do is generating > at least one event after media change. Given that media is reported > present after the last notification, I think userspace should be able > to do the right thing (and must have been doing the right thing until > recently). I have no idea if udev or other userspace has changed in this respect, or this is simply a timing window that this particular user has fallen into. This is a simulated device, so it might be fast/slower than any real hardware. Thanks for chiming in, -- Joe
