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

[Hans de Goede]

Hi,

On 10/02/2014 04:41 PM, jonsmirl at gmail.com wrote:
> On Thu, Oct 2, 2014 at 10:21 AM, Hans de Goede <hdegoede at redhat.com> wrote:
>> Hi,
>>
>> On 10/02/2014 04:16 PM, jonsmirl at gmail.com wrote:
>>> On Thu, Oct 2, 2014 at 10:08 AM, Hans de Goede <hdegoede at redhat.com> wrote:

<snip>

>>>>> So there are two ways to do this...
>>>>> 1) modify things like earlyconsole to protect device specific resource
>>>>> (I think this is a bad idea)
>>>>
>>>> Why is this a bad idea? If the bootloader tells us exactly which resources
>>>> are needed, then earlyconsole can claim them, and release them on
>>>> handover to the real display driver.
>>
>> Jon, can you please answer this ? I really really want to know why people
>> think this is such a bad idea. Understanding why people think this is a bad
>> idea is necessary to be able to come up with an alternative solution.
> 
> The list of resources should not be duplicated in the device tree -
> once in the simplefb node and again in the real device node.

It is not duplicated, the simplefb node will list the clocks used for the
mode / output as setup by the firmware, which are often not all clocks
which the display engine supports. Where as the real device node will list
all clocks the display engine may use.

> Device
> tree is a hardware description and it is being twisted to solve a
> software issue.

This is not true, various core devicetree developers have already said
that storing other info in the devicetree is fine, and being able to do so
is part of the original design.

> This problem is not limited to clocks, same problem
> exists with regulators. On SGI systems this would exist with entire
> bus controllers (but they are x86 based, console is not on the root
> bus). This is a very messy problem and will lead to a Frankenstein
> sized driver over time.

This is a "what if ..." argument, we can discuss potential hypothetical
problems all day long, what happens if the sky falls down?

> But... I think this is a red herring which is masking the real
> problem. The real problem seems to be that there is no window for
> loading device specific drivers before the resource clean up phase
> happens. That's a real problem -- multi architecture distros are going
> to have lots of loadable device specific drivers.

As Maxime pointed out to my alternative solution to fixing the clocks
problem, this is not strictly a when to do cleanup problem. If another
driver uses the same clocks, and does a clk_disable call after probing
(because the device is put in low power mode until used by userspace),
then the clk will be disabled even without any cleanup running at all.

The real problem here is simply that to work the simplefb needs certain
resources, just like any other device. And while for any other device
simply listing the needed resources is an accepted practice, for simplefb
for some reason (which I still do not understand) people all of a sudden
see listing resources as a problem.

Regards,

Hans