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

[jonsmirl at gmail.com]

On Thu, Oct 2, 2014 at 9:23 AM, Michal Suchanek <hramrach at gmail.com> wrote:
> On 2 October 2014 14:56, jonsmirl at gmail.com <jonsmirl at gmail.com> wrote:
>> On Thu, Oct 2, 2014 at 8:39 AM, Hans de Goede <hdegoede at redhat.com> wrote:
>>> Hi,
>>>
>>> On 10/02/2014 02:22 PM, jonsmirl at gmail.com wrote:
>>>> On Thu, Oct 2, 2014 at 2:42 AM, Hans de Goede <hdegoede at redhat.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On 10/01/2014 08:12 PM, Stephen Warren wrote:
>>>>>> On 10/01/2014 11:54 AM, jonsmirl at gmail.com wrote:
>>>>>>> On Wed, Oct 1, 2014 at 1:26 PM, Hans de Goede <hdegoede at redhat.com> wrote:
>>>>>> ...
>>>>>>>> We've been over all this again and again and again.
>>>>>>>>
>>>>>>>> AAAARRRRRGGGGGGGGGGGGGGGGGHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!
>>>>>>>>
>>>>>>>> All solutions provided sofar are both tons more complicated, then the
>>>>>>>> simple solution of simply having the simplefb dt node declare which
>>>>>>>> clocks it needs. And to make things worse all of them sofar have
>>>>>>>> unresolved issues (due to their complexity mostly).
>>>>>>>>
>>>>>>>> With the clocks in the simplefb node, then all a real driver has to do,
>>>>>>>> is claim those same clocks before unregistering the simplefb driver,
>>>>>>>> and everything will just work.
>>>>>>>>
>>>>>>>> Yet we've been discussing this for months, all because of some
>>>>>>>> vague worries from Thierry, and *only* from Thierry that this will
>>>>>>>> make simplefb less generic / not abstract enough, while a simple
>>>>>>>> generic clocks property is about as generic as things come.
>>>>>>
>>>>>> Note: I haven't been following this thread, and really don't have the time to get involved, but I did want to point out one thing:
>>>>>>
>>>>>> As I think I mentioned very early on in this thread, one of the big concerns when simplefb was merged was that it would slowly grow and become a monster. As such, a condition of merging it was that it would not grow features like resource management at all. That means no clock/regulator/... support. It's intended as a simple stop-gap between early platform bringup and whenever a real driver exists for the HW. If you need resource management, write a HW-specific driver. The list archives presumably have a record of the discussion, but I don't know the links off the top of my head. If nobody
>>>>>> other than Thierry is objecting, presumably the people who originally objected simply haven't noticed this patch/thread. I suppose it's possible they changed their mind.
>>>>>>
>>>>>> BTW, there's no reason that the simplefb code couldn't be refactored out into a support library that's used by both the simplefb we currently have and any new HW-specific driver. It's just that the simplefb binding and driver shouldn't grow.
>>>>>
>>>>> The whole reason why we want to use simplefb is not just to get things
>>>>> running until HW specific driver is in place, but also to have early console
>>>>> output (to help debugging boot problems on devices without a serial console),
>>>>> in a world where most video drivers are build as loadable modules, so we
>>>>> won't have video output until quite late into the boot process.
>>>>
>>>> You need both.
>>>>
>>>> 1) temporary early boot console -- this is nothing but an address in
>>>> RAM and the x/y layout. The character set from framebuffer is built
>>>> into the kernel.  The parallel to this is early-printk and how it uses
>>>> the UARTs without interrupts. This console vaporizes late in the boot
>>>> process -- the same thing happens with the early printk UART driver.
>>>> EARLYPRINTK on the command line enables this.
>>>>
>>>> 2) a device specific driver -- this sits on initrd and it loaded as
>>>> soon as possible. The same thing happens with the real UART driver for
>>>> the console. CONSOLE= on the command line causes the transition. There
>>>> is an API in the kernel to do this transition, I believe it is called
>>>> set_console() but it's been a while.
>>>
>>> Eventually we need both, yes. But 1) should stay working until 2) loads,
>>> not until some phase of the bootup is completed, but simply until 2) loads.
>>
>> No, that is where you get into trouble. The device specific driver has
>> to go onto initrd where it can be loaded as early in the boot process
>> as possible.
>>
>> Trying to indefinitely extend the life of the earlyprintk or
>> earlyframeuffer is what causes problems.  Doing that forces you to
>> basically turn them into device specific drivers which do things like
>> claiming device specific resources and gaining device specific
>> dependency knowledge, things that shouldn't be in earlyframebuffer.
>>
>
> No. When initrd is running boot has already finished as far as kernel
> is concerned.
>
> And you have to extend the life of the simplefb from the time boot has
> finished through the time kernel mounts initrd (or other root) and
> hands over to userspace found on the initrd, through the time this
> userspace searches for the kms driver and until the time it has
> finally loaded if that ever succeeds.

Does the clock and regulator cleanup happen before drivers can load
off from initrd? I didn't think it did but I might be wrong.

So maybe a solution to this is to delay that cleanup until after
initrd drivers have a chance to load. Of course it is not possible to
delay it indefinitely (like for disk based loading) but delaying over
initrd is a fixed limit.


>
> From the point of view of kernel once it has handed over to init in
> initrd the boot is finished. The init is normal userspace running off
> normal filesystem backed by a device-specific driver (initrd).
>
> That some systems do not continue to run off this filesystem
> indefinitely and in fact go out of their way to expunge the initrd
> filesystem and reclaim its resources by exercising some syscalls
> specifically devised for that use case is not relevant to the kernel.
> It cannot know when the userspace considers the boot finished enough.
> Sometimes even manual steps are required to finish booting when the
> automatic scripts fail.
>
> simplefb as early console is meant exactly for diagnosing and fixing
> such failures in absence of an uart.
>
> Thanks
>
> Michal
>
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-- 
Jon Smirl
jonsmirl at gmail.com