On Wed, Aug 13, 2014 at 12:44 PM, Stephen Warren [off-list ref] wrote:

On 08/13/2014 02:49 AM, David Herrmann wrote:

quoted

Hi

On Wed, Aug 13, 2014 at 10:40 AM, Grant Likely
[off-list ref] wrote:

quoted

On Wed, Aug 13, 2014 at 8:17 AM, Luc Verhaegen [off-list ref] wrote:

quoted

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 <redacted>

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.

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Jon Smirl
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