[OpenWrt-Devel] block info and DISK_MEDIA_CHANGE events

[Karl Palsson]

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John Crispin <blogic at openwrt.org> wrote:
> 
> 
> On 19/02/2016 18:00, Karl Palsson wrote:
> > Hi,
> > 
> > I'm trying to get "block mount" to behave well when a microSD
> > card is removed/inserted from a fixed USB card reader. I've
> > compiled the uevent-hotplug debugger example from
> > https://www.kernel.org/doc/pending/hotplug.txt and running that
> > on my device, I can see that "block info" is somehow triggering
> > the uevent change notifications I hope. (I can't see anywhere in
> > the fstools how the uevent is triggered, but it only happens if I
> > run "block info" manually, "blkid" doesn't trigger any events, so
> > it's something in block info
> > 
> > Example event (unfortuntely it's identical for card inserted ->
> > removed and for removed -> inserted)
> > ```
> > change@/devices/platform/ehci-platform/usb1/1-1/1-1.1/1-1.1:1.0/host0/target0:0:0/0:0:0:0/block/sda
> > ACTION=change
> > DEVPATH=/devices/platform/ehci-platform/usb1/1-1/1-1.1/1-1.1:1.0/host0/target0:0:0/0:0:0:0/block/sda
> > SUBSYSTEM=block DISK_MEDIA_CHANGE=1 MAJOR=8 MINOR=0 DEVNAME=sda
> > DEVTYPE=disk SEQNUM=637
> > ```
> > 
> 
> 
> fstools/libblkid-tiny/libblkid-tiny.c: probe_block()
> 
> does an open() on the device node which will eventually end up
> inside
> 
> drivers/scsi/sd.c: sd_open()
> 
> which will call check_disk_change(bdev);
> 
> and that triggers the uevent.

Ahh, thanks.

> 
> i am not sure how you can get it to automatically send the
> event without polling it.

I can live with that, my real concern is that I want it to be
unmounted/mounted. Should "block hotplug" be handling this to
add/remove? I can work on adding this to handle the change
events, but is that the right way, and would that be accepted, or
should something else be converting them into add/remove events
elsewhere? On desktop linux, I see the change event with
DISK_MEDIA_CHANGE=1, then a subsequent add/remove event.

As for automatically getting the event, I can enable the kernel
event polling, by setting /sys/block/sda/events_poll_msecs to
~500ms or so, and I can then see the SCSI LUN check commands with
usbmon, and I can see the responses change as soon as the card is
inserted/removed, but no events come out. I kinda feel that's
something I should take up upstream, but I'm perfectly ok with
having to poll "block info" at the moment.

Sincerely,
Karl P

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