linux-kernel - RE: [RFC 4/4] remoteproc: Add driver for STE Modem

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Date:	Tue, 14 Aug 2012 19:06:51 +0200
From:	Sjur BRENDELAND <sjur.brandeland@...ricsson.com>
To:	Ohad Ben-Cohen <ohad@...ery.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Sjur Brændeland <sjurbren@...il.com>
Subject: RE: [RFC 4/4] remoteproc: Add driver for STE Modem

Hi Ohad,

>>> It seems to me that a char device will solve these issues: we can use
>>> ioctl to control the power, if the user crashes we'll know about it
>>> via the release handler, and if the remote processor crashes we can
>>> let the user know by sending it a notification for it to read via the fd.

How about skipping definition of ioctls and simply call rproc_boot() upon
open and rproc_shutdown() upon close, this is perhaps quirky, but simpler.
I'd also like to use a char-misc device, then we get the device 
node with proper name automatically from existing udev rules.

>> In this case we need a way to make the virtio-drivers release the
>> virtio-queues in order to decrement the refcount, right?
...
>> So it seems that we need to force an unregistration of the virtio devices,
>> in order to make it release its queues.
...
> Yeah, no code is needed; something like this should do the trick:
> 
> root@...p4430-panda:/sys/bus/virtio/drivers/virtio_console# echo
> virtio1 > unbind

Looking on this once more after vacation I prefer if the ste-rproc driver
could enforce shutdown if we use the char-device.
In our case user-space will detect modem crashes and power off
the device. So if user-space uses char-device to request power off,
it feels like kernel space should do everything possible to really
trigger the shutdown, including unbinding the virtio devices.
Depending on additional "echo .../virtio > unbind" seems like an
unnecessary extra dependency. 

Alternatively, we could let user-space handle the power switch
for the modem directly. Remoteproc could simply notify user-space
(e.g. using gen-netlink) when rproc usage count has dropped to zero.
And this could trigger power-down. This would be more aligned with
our existing solution for power control.

Regards,
Sjur
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