[RFC] drm: implement generic firmware eviction

[David Herrmann]

Hi Rob

On Fri, Aug 26, 2016 at 2:36 PM, Rob Herring <robh+dt at kernel.org> wrote:
> On Thu, Aug 25, 2016 at 7:00 PM, David Herrmann <dh.herrmann at gmail.com> wrote:
>> Provide a generic DRM helper that evicts all conflicting firmware
>> framebuffers, devices, and drivers. The new helper is called
>> drm_evict_firmware(), and takes a flagset controlling which firmware to
>> kick out.
>>
>> This is meant to be used by drivers in their ->probe() callbacks of their
>> parent bus, before calling into drm_dev_register().
>>
>> Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
>> ---
>> Hey
>>
>> This is just compile-tested for now. I just want to get some comments on the
>> design. I decided to drop the sysfb infrastructure and rather just use this
>> generic helper. It keeps things simple and should work just fine for all
>> reasonable use-cases.
>>
>> This will work with SimpleDRM out-of-the-box on x86.
>>
>> Architectures with dynamic simple-framebuffer devices are not supported yet. I
>> actually have no idea what the strategy there is? Can the DeviceTree people come
>> up with something? Am I supposed to call of_platform_depopulate()?
>
> If of_platform_populate was used to create the device, then yes call
> of_platform_depopulate. In this case, I think it wasn't. If
> of_platform_device_create was used, then platform_device_del.
>
>> Or
>> of_detach_node()? Or what?
>
> No. Only the struct device and its resources should need to be
> destroyed. The node should remain.
>
>> Looking at drivers/of/{platform,dynamic}.c, I cannot see how this is supposed to
>> work at all. Who protects platform_device_del() from being called in parallel?
>
> Not sure. In parallel to what? On most systems, nodes never go away
> and on those that do it is only a few things that get hotplugged.
> That's changing with DT overlays now, so there certainly could be some
> issues.

Two tasks calling platform_device_del() in parallel on the same device
will not work. Meaning, calling of_platform_device_destroy() in
parallel does not work either. Same for of_platform_depopulate(). Same
for of_node_detach().

Sure, all those functions are not meant to be called in parallel by
multiple tasks. They are rather meant to have a single holder which
preferably is the one instantiating and destroying the
node/device/foobar. But the framebuffer eviction code somehow needs to
trigger the removal, and thus needs some hook, that can be called in
parallel by any driver that is loaded.

I can make sure the simple-framebuffer eviction code is locked
properly. That'll work, for now. But if someone ends up providing
simple-framebuffer devices via overlays or any other dynamic
mechanism, they will race the removal. And it will be completely
non-obvious to them. I really don't want to be responsible to have
submitted that code. If anyone cares for proper device hand-over on
ARM, please come up with an idea to fix it. Otherwise, I will just
limit the simpledrm code to x86.

IOW the device handover code somehow needs to know who was responsible
for the instantiation of the simple-framebuffer device, so it can tell
them to remove it again. On x86 there is only one place where those
can be instantiated. But on OF-based systems, it can be dynamically
instantiated in many places right now.

Thanks
David