[maemo-developers] Dealing with fcam-drivers and different kernels

[From: Eino-Ville Talvala]

Hi,

Since nobody seems to have good ideas on this, is there somewhere else I
could ask about this? 

Thanks,

Eddy Talvala
Computer Graphics Lab
Stanford University


On 8/2/2010 11:38 AM, Eino-Ville Talvala wrote:
> Hi,
>
> I'm one of the developers on the FCam project [1], and was hoping to get
> some advice on properly dealing with kernel-level dependencies in extras.
>
> All FCam applications depend on fcam-drivers, a set of camera subsystem
> kernel modules which are slightly modified from the stock kernel.
> (primarily to allow us to disable the built-in camera daemon,
> secondarily to fix a few issues in the imaging pipe statistics module
> drivers and to add a bit more control to the sensor driver)  Since these
> are kernel modules, they have to be compiled against the kernel headers
> for the kernel running on the user's device.
>
> Currently, we target just the stock N900 kernel, but given its
> widespread use, we'd also like to support kernel-power [2], which is an
> alternate N900 kernel with more customizability.  But since it's a
> different set of kernel sources, we can't insmod our fcam-drivers kernel
> modules into the kernel-power kernel (mismatched version strings, etc). 
> Of course, it's trivial to compile fcam-drivers with the kernel-power
> headers instead, to get binary modules that work with kernel-power.
> (Well, it would be, although currently the kernel-power-headers package
> appears broken - Titan, the kernel-power maintainer, says this will be
> fixed soon).
>
> So we can create the kernel modules for both kernels, but the problem is
> in packaging them.  I don't know enough about the application
> manager/apt-get to know if the following set of packaging directives is
> safe:
>
> fcam-stock-drivers:
>   Provides: fcam-drivers
>   Conflicts: kernel-power-flasher, fcam-drivers
>
> fcam-power-drivers:
>   Provides: fcam-drivers
>   Depends: kernel-power-flasher
>   Conflicts: fcam-drivers
>
> Then FCam applications would use:
>    Depends: fcam-stock-drivers | fcam-drivers
>
> The hope is that for stock kernel users, this would pull in
> fcam-stock-drivers, and for kernel-power users, it'll pull in
> fcam-power-drivers since fcam-stock-drivers will conflict.  But I was
> hoping to get confirmation on this before we try it out in the wild, and
> possibly accidentally have application manager swap user's kernels out
> from under them!
>
> Also, if there's a way to simplify the FCam application Depends:  to
> just Depends: fcam-drivers and have it work right, I'd love to know
> about it.  Since that's the part that's out of our control when others
> write applications using FCam, I'd like to keep it very simple, so
> nobody accidentally creates the installs-new-kernel scenario with their
> new camera app!  For example, would the following possibly work:
>
> fcam-drivers:
>   Conflicts: kernel-power-flasher
>
> fcam-power-drivers:
>   Provides: fcam-drivers
>   Depends: kernel-power-flasher
>   Conflicts: fcam-drivers
>
> fcam applications:
>    Depends: fcam-drivers
>
> With the idea that fcam-drivers is preferred since it's a real package,
> and fcam-power-drivers only used in case fcam-drivers conflicts?
>
>
> The other alternative we have is to just package up both sets of modules
> in fcam-drivers, and detect at runtime what kernel is in use.  But this
> doubles the space we need on the rootfs (can't optify kernel modules, as
> far as we can tell), and we'd have to update fcam-drivers for everyone
> every time kernel-power updates, instead of just for kernel-power users.
>
> Finally, we could just ask Titan to fold fcam-drivers into his kernel,
> but I'm not sure it resolves the dependency issue above - would app
> manager again correctly pull in fcam-drivers for the stock kernel when
> needed, instead of pulling down kernel-power?
>
> Any advice would be appreciated!
>
> Thanks,
>
> Eddy Talvala
> Computer Graphics Lab
> Stanford University
>
> ------
>
> [1] http://fcam.garage.maemo.org
> [2] http://wiki.maemo.org/Kernel_Power
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