Re: [PATCH 0/3] clk: Add __clk_disable_unprepare_counts_only() and use this in simple[fb|drm]

[Hans de Goede <johannes.goede@oss.qualcomm.com>]

Hi,

On 27-May-26 14:04, Maxime Ripard wrote:
> Hi Hans,
> 
> On Wed, May 27, 2026 at 11:48:08AM +0200, Hans de Goede wrote:
>> Hi Michael, Stephen, et al.,
>>
>> The device-tree simple-framebuffer device is special in that it does not
>> describe actual hardware, but it allows firmware (e.g. u-boot) to pass a
>> firmware initialized framebuffer to the OS to use as display during boot.
>>
>> To avoid clocks used by the firmware framebuffer getting touched while in
>> use, the DT node contains a list of clocks which should not be touched
>> while the simple-framebuffer is in use.
>>
>> Conceptually this is fine, in practice the *do not touch* is implemented as
>> a clk_prepare_enable() + clk_disable_unprepare() pair, with the disabling
>> happening when the simple-framebuffer driver gets replaced by the real
>> display-engine driver.
> 
> Which is also pretty fragile because it doesn't lock the rate either.

That depends on how the clks are configured / setup by the clk driver
if the CLK_SET_RATE_GATE is set in clk_core.flags then protect_count
will get incremented and from then on the rate will be locked.

We could even have the simpledrm / simplefb code call 
clk_rate_exclusive_get() to lock the rate.

So there are multiple ways to lock the rate if necessary but so far
in the real world there has been no need for this AFAICT.

So IMHO this is something to worry about if / when we hit this
actually happening in a real world scenario.

>> The simple-framebuffer concept assumes that the order in which resources
>> are acquired does not matter since they are already on, so power-sequencing
>> is not an issue. But clk_disable_unprepare() actually turns the clock off,
>> messing with the hardware state before the real display driver loads,
>> without any knowledge of the proper power-off sequence for the display.
>>
>> The turning off of the clocks outside of a proper power-off sequence causes
>> the following error when the msm display driver tries to re-enable them:
>>
>> [    2.980181] disp_cc_mdss_dptx3_pixel0_clk_src: rcg didn't update its configuration.
>> [    2.980272] WARNING: drivers/clk/qcom/clk-rcg2.c:136 at update_config+0xdc/0x100
>>
>> Resulting in a non working display.
>>
>> To fix this, we need a way to properly implement the "do not touch" concept
>> for clocks.
> 
> I'm not sure it's something we can enforce, really. The firmware can
> modify clocks behind our back.

Generally speaking there is a contract with the firmware that either
the clock is firmware controlled and the OS can maybe make firmware calls
to request changes; or the OS is in control. Both trying to control
the same clk at the same time is a bug and IMHO unrelated to the issue
at hand which is about clks which are under OS control only.

> Other clocks might change this one as a
> side-effect of a rate change or reparenting, etc. It's just not
> something we can commit to.

See above, this is exactly what the protect_count in the clk-core
is for.

> 
>> clk_prepare_enable() is fine, the clocks are already on so no
>> ordering worries and it ensures that the clocks and their parents cannot
>> get turned off by incrementing their enable, prepare and protect counts.
>>
>> clk_disable_unprepare() is a problem though since it actually turns the
>> clocks off. Instead something is needed which only decrements those counts.
>>
>> This series introduces a new clk-core function called
>> __clk_disable_unprepare_counts_only() (1) which does just that. Prefixed
>> with '__' to indicate that normally drivers should not use this.
>>
>> Michael, Stephen sorry for needing to add a new clk-core function for this,
>> but I see no other way of solving this (2)(3). The changes are not that
>> big / not too bad.
>>
>> I've also considered making __clk_disable_unprepare_counts_only() implement
>> all the logic itself instead of adding the extra parameter to
>> clk_core_unprepare() and clk_core_disable() but that leads in duplicating
>> quite a bit of logic (4) so this seems better.
>>
>> The other 2 patches just replace the clk_disable_unprepare() calls in
>> the simple[fb|drm] driver with the new helper.
>>
>> This series fixes the display not coming up after switching to the msm
>> driver when a simple-framebuffer node with clocks listed is used.
>>
>> 1) I'm open to changing the name, this is the best I could come up with.
>>
>> 2) One option considered was detaching the simple-framebuffer driver later,
>>    after the real display driver has had a chance to claim the clocks. But
>>    this won't work in cases where the real display driver picks different
>>    parent clocks then the boot firmware did and needs to reparent clocks.
>>
>>    Basically the goal is for things to behave as if the simple-framebuffer
>>    driver was not there at all, because that leaves the hw in the state
>>    the real display driver expects.
> 
> So I know it's not really what you had in mind, but if you are only
> concerned about the transition from the bootloader to the DRM driver,
> then I think supporting the following work would help:
> 
> https://lore.kernel.org/r/20260423-drm-state-readout-v2-0-8549f87cb978@kernel.org
> 
> With that series, we can build the initial KMS state from the hardware
> state, which means that if you were to change the resolution at boot, it
> would be executed just like any mode change in KMS.

Hmm, so your suggestion would be to have the initial KMS driver probe()
only do a read back without touching anything. Then claim clks matching
the read back config and then only release the simple* driver and thus
the clocks after this?

That is certainly an interesting proposal but IMHO almost orthogonal
to this one (1). Even if it may fix the issue in the end, it seems
that that work is still quite a way from going upstream and even then
initially it only potentially fixes this for the TIDSS driver since
that solution needs a lot of per driver work.

Where as my proposed fix here is much simpler and fixes the issue
for all drivers in one go.

So I would still like to move forward with the fix proposed here.

Regards,

Hans


1) As you write yourself:

> This should help mitigate your initial problem, even though it's a
> widely different solution.