On Wed, Oct 1, 2014 at 8:48 AM, Thierry Reding [off-list ref] wrote:

On Wed, Oct 01, 2014 at 01:20:08PM +0100, Mark Brown wrote:

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On Wed, Oct 01, 2014 at 10:14:44AM +0200, Thierry Reding wrote:

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On Tue, Sep 30, 2014 at 07:00:36PM +0100, Mark Brown wrote:

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You may well need to extend the binding in future for an actual driver
but from the point of view of what's going into the block it really
should just be a case of reading the datasheet and mechanically typing
that in.  If we can work out how to say where the framebuffer is we
really ought to be able to work this stuff out.

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I agree from a technical point of view. However given the dynamically
generated nature of the node the problem is more of a logistical nature.
As we've seen U-Boot is being enabled to add clocks to the simplefb node
but I'm fairly certain that there's a regulator somewhere that needs to
be enabled too, be it for powering the display controller, the panel or
the backlight. I wouldn't even be surprised if there were one for each
of those. If so simplefb on this board will break when the regulators
are described in the kernel's DTS.

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If we don't consider this a problem then the whole DT ABI stability
business is a farce.

I think you're setting constraints on the implementation you want to see
which make it unworkable but I don't think those constraints are needed.
You're starting from the position that the DT needs to be updated without
the bootloader

No, what I'm saying is that what the simplefb driver expects and what
the bootloader sets up may diverge as resource drivers are added to the
kernel. The DT /could/ be updated without the bootloader. You may only
be able to replace the DTB but not the bootloader on a given platform.

simplefb should be a boot console and not survive past the boot
process. Trying to get a 'generic' console driver to survive the boot
process is not a generic problem. There are about 1,000 messages in
these threads explaining why this is not a generic problem.

All of these clock and regulator issues would go away by building a
device specific framebuffer driver.  A device specific framebuffer
driver can be written in a day or two, it is far simpler than a KMS
driver. This driver would how to parse the device specific device tree
node and do the right thing with the regulators/clocks.

So simplefb is built-in and used for early boot.  During the boot
process a device specific framebuffer driver loads.  This device
specific driver takes over for simplefb and can become the user space
console.

If the device specific framebuffer does not get loaded, then simplefb
is going to stop working when the clocks and regulators get shut off.
But that is what should happen.

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               and that the DT must not contain any hint of simplefb as
shipped separately.

Well, I don't think it should because it describes the same resources
that the device tree node for the real device already describes. But
perhaps this is one of the cases where duplication isn't all that bad?

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                    That's never going to work well as far as I can see
but doesn't seem like an ABI stability issue, or at least not a
reasonable one.

It would work well under the assumption that the kernel wouldn't be
touching any of the resources that simplefb depends on. If that's not a
reasonable assumption then I think we can't make simplefb work the way
it's currently written.

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Either the bootloader needs to be updated along with the DT

I thought we had decided that this was one of the big no-nos. But
perhaps I'm misremembering.

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                                                            or the DT
needs to offer the bootloader a stable interface of its own for adding
the description of what it has set up (like a default disabled node
with the FB description, I'm sure other ideas are possible).  Obviously
the goal with the stable ABI is that the DT will be distributed along
with the platform.

So instead of pretending that this is in any way generic, maybe a better
idea would be to provide code in DRM/KMS drivers that is called early,
grabs all the resources as defined in the binding for the device and
then instantiates simplefb using the parsed information. Which is kind
of the stub driver that Grant had suggested.

Of course that means duplicating most of the resource handling from the
real driver into this stub driver. And it means that this part of the
driver would have to be built into the kernel and bloat it some more.

Thierry

-- 
Jon Smirl
jonsmirl at gmail.com