USB storage detachment / reattachment
hungryninja101 at ...9...
Sat Jan 14 20:58:11 CET 2017
I have personally done some work related to this issue. First off, I would
suggest adding code to allow init to share child services with its parent.
I also have a service_router component that I wrote. You may not be able to
use it directly, but feel free to take some of the code to use in your
media component. Here's a link to the code:
The code is a bit messy, so any help on making it ready to merge into the
official Genode repo would be very welcome.
On Thu, Jan 12, 2017 at 7:47 AM, Boris Mulder <boris.mulder at ...434...>
> Dear Genode developers,
> Following Martijn's mail at the end of last year, which stated:
> >> As a way to support detachment / reattachment of USB storage I’m
> >> thinking about placing the rump_fs and part_blk components in a child
> >> subtree of the CLI component that is spawned on demand and cleaned-up
> >> after use. But this seems a bit like overkill.
> > That's exactly the right solution. I don't think that it's overkill
> > either. Spawning up rump_fs and part_blk dynamically is certainly quick
> > enough. Memory-wise, it does not take more resources that a static
> > scenario either. By letting your CLI component implement the protocol
> > outlined above, you have full control over chain of events. Also the
> > aborting rump_fs is nothing fatal anymore but can be gracefully handled
> > by the CLI component. As another benefit, the solution does not need us
> > to supplement the notion of hot-plugging to the file-system and block
> > session interfaces, which would otherwise inflate the complexity of
> > these interfaces (and thereby all the clients that rely on them).
> Martijn and I have been thinking of a way to implement this, and came to
> the conclusion that instead of spawning the stack as children of the CLI
> component, it might be better to use a new management component
> in-between. As Josef said earlier:
> > A custom runtime/management component could monitor the
> > usb_drv device report and spawn the whole stack if it detects a USB
> > storage device. The usb_drv's device report does not contain the device
> > class so far though but adding that to the report is easy.
> This is exactly what we're trying to do now. We want to create a custom
> component called "media" that monitors usb devices by reading the
> report. It provides a service to other components through which they can
> request a filesystem session in order to read-write from/to the usb-stick.
> this, it spawns the part_blk and rump_fs components as children if the
> usb is plugged in, and kills them once the usb is plugged out. It
> roughly looks like this:
> rump_fs part_blk
> | |
> CLI media USB_drv
> | | |
> But this raises a few questions. First, the filesystem interface needs
> to be presented to the client somehow. To avoid adding another layer of
> indirection into media, essentially duplicating rump_fs's entire API, we
> would like the client (in this case CLI) to be directly connected to
> rump_fs. The client can then ask media if the USB is connected before
> calling a function from rump_fs.
> However, this means that rump_fs provides a service, announces it to its
> parent (media), and media has to decide what to do with that announce.
> It can implement rump_fs as a slave, but that way the entire API needs
> to be copied into media so media can present it as its own service to the
> So we would like media to announce the filesystem service to its parent
> (in this case init), so any client can use this service. In the same
> way, any session request will be passed from CLI to its parent (init),
> init has to pass it to media, and media passes it to rump_fs. However,
> the current implementation and specification of genode does not allow
> services of any server to exist in any level above the server's parent.
> Services can only be provided to direct parents, and to other components
> in the parent's subtree. Therefore, copying the API from the child to
> the parent seems unavoidable.
> Another problem that pops up is that media has to spawn all these
> subcomponents as children. In order to route block session requests from
> rump-fs to part-blk, media needs to implement some routing policy and
> effectively serves the same role for these two components as init serves
> for the system. So we could:
> 1. Copy all necessary code for routing from init to media (which is
> almost all code if we want to be generic).
> 2. Let media spawn another init child component (let's call it sub-init
> for now) which in turn spawns rump-fs and part-blk and does the routing.
> To us, the second option seems much more clean as it involves no
> code-copying. However, services announced by rump-fs can not be used by
> other components that are not children of the new init, and are kind of
> useless. Their announcements can not be passed on to the parents,
> leaving us with the same problem as we had with rump_fs but with the
> additional problem that even if there would be a custom way to forward
> service announces and requests to the parent/child respectively,
> sub-init has no such policy, and this functionality has to be included
> in sub-init's code as well, adding a lot of complexity.
> Eventually both cases bottle down to the same problem: service
> announcements to a parent cannot automatically be forwarded to that
> parent's parent, and likewise, service requests need to be able to be
> delegated to children of children without a lot of hassle. The only option
> if I'm correct is implementing this functionality manually, but this does
> not work if the parent is an existing component that does not support it.
> Is there a reason this is never done? For init it is clear that it would
> never pass an announce to its parent (usually core) or receive session
> requests from it. But how about the general case?
> And how should we solve cases such as the above scenario?
> kind regards,
> Boris Mulder
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