Hello Damien
On Mon, Nov 4, 2024 at 2:44 AM Damien Zammit via Bug reports for the GNU
Hurd <bug-hurd@gnu.org> wrote:
> Hi,
>
> I am currently attempting to implement a drm server to provide
> a way to use libdrm with multiboot framebuffer exposed by new
> device(mbinfo).
>
> I am running into a problem that I am unable to implement a compatible
> ioctl api because of the layout of the structures.
>
> I would prefer to reuse the same api as drm ioctls rather than implement
> a modified version using traditional RPCs with many arguments.
> This is because libdrm would need to be modified substantially and I don't
> want
> to clutter the client with more parameters and conditional code.
>
> The main problem is that a few of the OUT ioctls expect the server to copy
> data
> through the RPC, and MiG is confused by nested structs, it doesn't seem to
> support something like:
>
> type drm_version_t = struct {
> int foo;
> int bar_length;
> data_t bar;
> };
>
Yes. MiG only supports fixed-sized structs and doesn't know how to deal
with members that are pointers. This would be a significant change.
>
> You _can_ specify individual parameters in the routine like so:
>
> routine drm_version (
> port: drm_t;
> foo: int;
> out bar: data_t SCP);
>
> but then the bar_length parameter comes AFTER the bar parameter,
> and has type unsigned int, (not int), and you cannot seem to pack the
> whole thing
> into a struct compatible with an ioctl like:
>
> routine drm_version (
> port: drm_t;
> out bar: drm_version_t);
>
This part is hard coded in MiG, but could be changed easily.
> How do I solve this? Can we extend MiG to be smarter about nested
> structures when
> data needs to be transferred within structs? How do we solve the ordering
> problem of
> the *_length parameter?
>
Can you provide more details on how this would be used? I assume clients
would use ioctl but I am not too familiar with how glibc maps ioctl into
RPCs.
> My attempt at coding this is currently here [1] and [2].
>
> Damien
>
>
> [1] https://git.zammit.org/hurd-sv.git/commit/?h=drm-server
> [2] https://git.zammit.org/hurd-sv.git/commit/?h=drm-server-ioctl
>
>
>
Regards
Flavio
