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

Thu Oct 2 07:16:00 PDT 2014

On Thu, Oct 2, 2014 at 10:08 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> Hi,
>
> On 10/02/2014 03:40 PM, jonsmirl at gmail.com wrote:
>> On Thu, Oct 2, 2014 at 9:33 AM, Hans de Goede <hdegoede at redhat.com> wrote:
>>> Hi,
>>>
>>> On 10/02/2014 03:27 PM, jonsmirl at gmail.com wrote:
>>>> On Thu, Oct 2, 2014 at 9:14 AM, Hans de Goede <hdegoede at redhat.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On 10/02/2014 02:56 PM, 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.
>>>>>
>>>>> This is an argument in the "you cannot do that" / "your use case is not valid"
>>>>> category, IOW this is not a technical argument. You say I cannot do that I
>>>>> say I can, deadlock.
>>>>
>>>> It is certainly possible to extend an earlyframebuffer to be able to
>>>> run as a user space console. It is just going to turn into a
>>>> Frankenmonster driver with piles of device specific, special case code
>>>> in it.
>>>
>>> There is nothing hardware specific about a framebuffer needing some
>>> clocks to not be disabled. Tons of SoC's will have this. Which clocks,
>>> that is hardware specific, but the framebuffer driver does not need to
>>> worry about that, it just sees a clocks property with some random clocks
>>> in there, and that is as generic as it gets.
>>>
>>>> I think that device specific code belongs in a device specific driver
>>>> and earlyframebuffer should handoff to it as soon as possible.
>>>>
>>>>>
>>>>> I've already explained that we not only can do that (we already have working
>>>>> code proving that), but also that this is something which we absolutely need:
>>>>>
>>>>>>> One example why this is necessary is e.g. to debug things where the problem
>>>>>>> is that the right module is not included in the initrd.
>>>>
>>>> A generic earlyframebuffer would show this error.
>>>
>>> If it reserves the clocks it needs, yes. If it does not then the clocks will
>>> be disabled before the initrd starts, and the screen will be black from then
>>
>> I thought the clock/regulator clean up happened after initrd loading,
>> but maybe that is not the case.
>>
>> A cleaner solution would then be to modify the clock/regulator clean
>> up to happen after driver loading is finished from initrd. Deferring
>> until after that completes is a fixed limit, everything is sitting
>> there in RAM. I would not propose extending it until harddisk based
>> loading happens.
>>
>> 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.
>
>> 2) delay the clock/regulator cleanup until after there is a fixed
>> window for device specific drivers to load in. Loading from initrd is
>> a fixed window.
>
> As I already explained by example in another mail, this won't work:
>
> "delaying over the initrd is not helpful. Not having the real driver
> load for whatever reasons, is not necessarily a boot blocking event,
> and if it us just missing will not lead to any error messages.
>
> So the boot will continue normally with a black screen, and things are
> still impossible to debug."
>
> We've been down the whole delay till $random point in time thing in the
> past with storage enumeration, where you need to wait for say all members
> of a raid set to show up. The lesson learned from that is that you should
> not wait $random time / event, but wait for the actual storage device to
> show up.
>
> And this is just like that, we need to wait for the actual display driver
> to have loaded and taken over before "cleaning up" the clocks used by
> the display engine.
>
> I guess we could just delay all clock cleanup until then, not really
> pretty, and I still prefer to just list the clocks in the simplefb
> dtnode, but if this version would be acceptable to all involved, I can
> live with it.
>
> Mike, would a patch adding 2 calls like these to the clock core be
> acceptable ?  :
>
> clk_block_disable_unused()
> clk_unblock_disable_unused()
>
> Where an earlyconsole driver can then call clk_block_disable_unused(),
> which will turn any calls to clk_disable_unused into a nop.

Is there a way to use the existing eprobe_defer system to do this?

>
> And when the earlyconsole gets unregistered because the real driver
> takes over, it calls clk_unblock_disable_unused(), which then checks if
> clk_disable_unused() has already been called (iow check if we're post
> late_init), and if it has calls clk_disable_unused() at that time.
>
> If this is acceptable I can whip up a patch for this.
>
> Regards,
>
> Hans
>
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-- 
Jon Smirl
jonsmirl at gmail.com