[linux-sunxi] Re: [PATCH 4/4] simplefb: add clock handling code

[Michal Suchanek]

On 1 October 2014 15:01, jonsmirl at gmail.com <jonsmirl at gmail.com> wrote:
> On Wed, Oct 1, 2014 at 8:48 AM, Thierry Reding <thierry.reding at gmail.com> wrote:
>> On Wed, Oct 01, 2014 at 01:20:08PM +0100, Mark Brown wrote:
>>> On Wed, Oct 01, 2014 at 10:14:44AM +0200, Thierry Reding wrote:
>>> > On Tue, Sep 30, 2014 at 07:00:36PM +0100, Mark Brown wrote:
>>>
>>> > > 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.
>>>
>>> > 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.
>>>
>>> > 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.

How it would know?

You need different clocks for LCD and different clocks for HDMI.

Unless it is a real driver that can drive either it can tell which is
enabled or u-boot has to tell it or you have to write a fixed entry
for the configuration you want in the DT and configure u-boot
accordingly by hand as well.

>
> 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.

Why it should be so?

It is reasonable to want working console on device which has u-boot or
other firmware graphics support but the support in kernel is still
under development.

Also the 'boot end' for kernel when it frees the clocks is way earlier
than the 'boot end' for the distribution which ends when you reach
certain init goal like multiuser environment. There is a lot between
and once the kernel hands over to init it can never tell what's going
on.

Since a modular KMS driver would load way later than the moment when
'boot end' is reached for kernel the simple function as early console
would break.

Also if you prevented resource management from happening during this
'booting' stage you could not safely load drivers during that time
which kind of defeats the purpose of this stage.

Because either the kernel can do resource management and give
resources to drivers that are loaded or it cannot do either.

Thanks

Michal