[linux-sunxi] Re: [PATCH 3/4] simplefb: disable dt node upon remove

[Stephen Warren]

On 08/13/2014 11:26 AM, jonsmirl at gmail.com wrote:
> On Wed, Aug 13, 2014 at 12:44 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 08/13/2014 02:49 AM, David Herrmann wrote:
>>>
>>> Hi
>>>
>>> On Wed, Aug 13, 2014 at 10:40 AM, Grant Likely
>>> <grant.likely at secretlab.ca> wrote:
>>>>
>>>> On Wed, Aug 13, 2014 at 8:17 AM, Luc Verhaegen <libv at skynet.be> wrote:
>>>>>
>>>>> The next commit will handle clocks correctly, so that these do not get
>>>>> automatically disabled on certain SoC simplefb implementations. As a
>>>>> result, the removal of this simplefb driver, and the release of the
>>>>> clocks, is rather final, and only a full display driver can work after
>>>>> this. So, it makes sense to also flag the dt node as disabled, even
>>>>> though it has no real value today.
>>>>>
>>>>> Signed-off-by: Luc Verhaegen <libv at skynet.be>
>>>>
>>>>
>>>> Please, no.
>>>>
>>>> Drivers should not be modifying the device tree without and
>>>> exceptionally good reason for doing so. Drivers are to treat the DT as
>>>> immutable.
>>>>
>>>> * the exception is an overlay driver which add new devices to the
>>>> kernel. Definitely not the case here.
>>>
>>>
>>> Why? I think we have exactly that case:
>>>    * DT describes the real hw properly and those parts are immutable
>>>    * Additionally, bootloaders create firmware-framebuffers and
>>>      create simple-framebuffer devices for them. Those are
>>>      valid as long as no driver reconfigured the real hw.
>>>    * Once a real hw-driver loads, it might destroy the existing
>>>      framebuffers, thus, it should also destroy the platform device.
>>>    * If the real hw-driver is unloaded, it might re-create the FB
>>>      and thus create a new (or enable the old) platform device.
>>
>>
>> My intention was always that a bootloader's addition of a simplefb node to
>> the DT would be user-configurable or driven by the original DT content. As
>> such, there shouldn't ever be both a DT node describing the "real" HW and
>> simplefb. In other words, if the DT already has the "real" DT node, the
>> bootloader should automatically (or under user command) not add the simpefb
>> node.
>
> DT is just the wrong mechanism to signal this, use an ATAG or kernel
> command line parameter.
>
> real hardware has compatible = "real-hardware-name, simplefb"
>
> simplefb is built into kernel. It will attach to the device because of
> the compatible string. Then it can look at the command line and see if
> the bootloader also supported simplefb and already set things up.
>
> Then some mechanism will have to be designed to arrange a handoff
> between simplefb and the chip specific KMS driver. But that's a Linux
> problem, not a DT one.

Having a single DT node that conforms to both the binding for 
"real-hardware-name" and "simplefb" doesn't seem like a good approach. 
What if the properties required by the two bindings conflict in some 
way? The approach you advocate certainly hasn't ever been used AFAIK.