Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks

[Sowjanya Komatineni]

On 7/16/19 8:00 AM, Dmitry Osipenko wrote:
> 16.07.2019 11:06, Peter De Schrijver ÐÐÑÐÑ:
> > On Tue, Jul 16, 2019 at 03:24:26PM +0800, Joseph Lo wrote:
> > > > OK, Will add to CPUFreq driver...
> > > > > The other thing that also need attention is that T124 CPUFreq driver
> > > > > implicitly relies on DFLL driver to be probed first, which is icky.
> > > > Should I add check for successful dfll clk register explicitly in
> > > > CPUFreq driver probe and defer till dfll clk registers?
> Probably you should use the "device links". See [1][2] for the example.
>
> [1]
> https://elixir.bootlin.com/linux/v5.2.1/source/drivers/gpu/drm/tegra/dc.c#L2383
>
> [2] https://www.kernel.org/doc/html/latest/driver-api/device_link.html
>
> Return EPROBE_DEFER instead of EINVAL if device_link_add() fails. And
> use of_find_device_by_node() to get the DFLL's device, see [3].
>
> [3]
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/devfreq/tegra20-devfreq.c#n100
Will go thru and add...
> > > Sorry, I didn't follow the mail thread. Just regarding the DFLL part.
> > >
> > > As you know it, the DFLL clock is one of the CPU clock sources and
> > > integrated with DVFS control logic with the regulator. We will not switch
> > > CPU to other clock sources once we switched to DFLL. Because the CPU has
> > > been regulated by the DFLL HW with the DVFS table (CVB or OPP table you see
> > > in the driver.). We shouldn't reparent it to other sources with unknew
> > > freq/volt pair. That's not guaranteed to work. We allow switching to
> > > open-loop mode but different sources.
> Okay, then the CPUFreq driver will have to enforce DFLL freq to PLLP's
> rate before switching to PLLP in order to have a proper CPU voltage.

PLLP freq is safe to work for any CPU voltage. So no need to enforce 
DFLL freq to PLLP rate before changing CCLK_G source to PLLP during suspend

> > > And I don't exactly understand why we need to switch to PLLP in CPU idle
> > > driver. Just keep it on CL-DVFS mode all the time.
> > >
> > > In SC7 entry, the dfll suspend function moves it the open-loop mode. That's
> > > all. The sc7-entryfirmware will handle the rest of the sequence to turn off
> > > the CPU power.
> > >
> > > In SC7 resume, the warmboot code will handle the sequence to turn on
> > > regulator and power up the CPU cluster. And leave it on PLL_P. After
> > > resuming to the kernel, we re-init DFLL, restore the CPU clock policy (CPU
> > > runs on DFLL open-loop mode) and then moving to close-loop mode.
> The DFLL is re-inited after switching CCLK to DFLL parent during of the
> early clocks-state restoring by CaR driver. Hence instead of having odd
> hacks in the CaR driver, it is much nicer to have a proper
> suspend-resume sequencing of the device drivers. In this case CPUFreq
> driver is the driver that enables DFLL and switches CPU to that clock
> source, which means that this driver is also should be responsible for
> management of the DFLL's state during of suspend/resume process. If
> CPUFreq driver disables DFLL during suspend and re-enables it during
> resume, then looks like the CaR driver hacks around DFLL are not needed.
>
> > > The DFLL part looks good to me. BTW, change the patch subject to "Add
> > > suspend-resume support" seems more appropriate to me.
> > To clarify this, the sequences for DFLL use are as follows (assuming all
> > required DFLL hw configuration has been done)
> >
> > Switch to DFLL:
> > 0) Save current parent and frequency
> > 1) Program DFLL to open loop mode
> > 2) Enable DFLL
> > 3) Change cclk_g parent to DFLL
> > For OVR regulator:
> > 4) Change PWM output pin from tristate to output
> > 5) Enable DFLL PWM output
> > For I2C regulator:
> > 4) Enable DFLL I2C output
> > 6) Program DFLL to closed loop mode
> >
> > Switch away from DFLL:
> > 0) Change cclk_g parent to PLLP so the CPU frequency is ok for any vdd_cpu voltage
> > 1) Program DFLL to open loop mode
> >
> > For OVR regulator:
> > 2) Change PWM output pin from output to tristate: vdd_cpu will go back
> >     to hardwired boot voltage.
> > 3) Disable DFLL PWM output
> >
> > For I2C regulator:
> > 2) Program vdd_cpu regulator voltage to the boot voltage
> > 3) Disable DFLL I2C output
> >
> > 4) Reprogram parent saved in step 0 of 'Switch to DFLL' to the saved
> >     frequency
> > 5) Change cclk_g parent to saved parent
> > 6) Disable DFLL

This is the same sequence currently implemented. But dfll suspend/resume 
calls are thru Tegra210 clock driver.

Dmitry wants to have dfll suspend/resume along with CCLK_G restore to 
happen from CPUFreq driver pm_ops rather than tegra210 clock driver or 
tegra dfll driver.

Will move it to CPUFreq driver...

> Thanks!