[Replicant] Porting Replicant to Microsoft Surface: hurdles

[Denis 'GNUtoo' Carikli]

On Thu, 29 Oct 2015 11:37:56 +0100
"Mohamed Mediouni" <mmediouni at gmx.fr> wrote:

> There is many hurdles to port Replicant in the Tegra 3 family of SoCs,
> especially the Microsoft Surface. 
[...]
> The bootloader is of course signed, or it won't be fun.
We're desperately in need of supported devices with a free software
bootloader that users can modify.
As I understand it sunxi tablets have unsigned bootloaders and a strong
community (despite the lack of hardware vendor support).

So the idea would be to port Replicant to one of such tablets, and then
to find a way that would permit people to buy compatible tablets easily.

Paul had some ideas on how to do that.
Assuming that the device is supported by mainline Linux, it boils down
to:
-> Making the bootloader load the right dtb, the bootloader image would
   be device specific anyway so that is probably doable easily. If you
   use an upstream version, you could even build different images from
   the same source.
-> Have a common kernel with one dtb per device.
-> Have a common rootfs, if the kernel version is the same, then you
   avoid issues where BIONIC is compiled against a specific kernel
   version.
   Then, as I was reminded on IRC, some device specific libraries can
   automatically be loaded on the specific device based on their
   filename, like gralloc.<device>.so, I guess that it requires the
   right init.<device>.rc to be run, and I wonder how that's selected
   with the device tree.
   I would guess it was something like a grep to /proc/cpuinfo before,
   I didn't verify that.
   Shipping them all the libraries would of course make the rootfs
   bigger. We could delete the unneeded ones at first boot though.
-> For WiFi modules loading, we could ship the drivers
   inside the kernel image, it will consume a bit more RAM though. I
   wonder if we need to adapt userspace to not load anything and use
   the standard kernel interfaces.
-> There might be some config files stuff too, like audio config, input
   config etc... I don't remember their path nor filenames.
-> There might be some other stuff that I overlooked.

Denis.
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