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

From: Sowjanya Komatineni
Date: Tue Jul 16 2019 - 15:26:14 EST


On 7/16/19 11:43 AM, Dmitry Osipenko wrote:
16.07.2019 21:30, Sowjanya Komatineni ÐÐÑÐÑ:
On 7/16/19 11:25 AM, Dmitry Osipenko wrote:
16.07.2019 21:19, Sowjanya Komatineni ÐÐÑÐÑ:
On 7/16/19 9:50 AM, Sowjanya Komatineni wrote:
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...
Looks like I initially confused this case with getting orphaned clock.
I'm now seeing that the DFLL driver registers the clock and then
clk_get(dfll) should be returning EPROBE_DEFER until DFLL driver is
probed, hence everything should be fine as-is and there is no real need
for the 'device link'. Sorry for the confusion!

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

Sorry, please ignore my above comment. During suspend, need to change
CCLK_G source to PLLP when dfll is in closed loop mode first and then
dfll need to be set to open loop.
Okay.

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

I see during switch away from DFLL (suspend), cclk_g parent is not
changed to PLLP before changing dfll to open loop mode.

Will add this ...
The CPUFreq driver switches parent to PLLP during the probe, similar
should be done on suspend.

I'm also wondering if it's always safe to switch to PLLP in the probe.
If CPU is running on a lower freq than PLLP, then some other more
appropriate intermediate parent should be selected.

CPU parents are PLL_X, PLL_P, and dfll. PLL_X always runs at higher rate
so switching to PLL_P during CPUFreq probe prior to dfll clock enable
should be safe.
AFAIK, PLLX could run at ~200MHz. There is also a divided output of PLLP
which CCLKG supports, the PLLP_OUT4.

Probably, realistically, CPU is always running off a fast PLLX during
boot, but I'm wondering what may happen on KEXEC. I guess ideally
CPUFreq driver should also have a 'shutdown' callback to teardown DFLL
on a reboot, but likely that there are other clock-related problems as
well that may break KEXEC and thus it is not very important at the moment.

[snip]

During bootup CPUG sources from PLL_X. By PLL_P source above I meant PLL_P_OUT4.

As per clock policies, PLL_X is always used for high freq like >800Mhz and for low frequency it will be sourced from PLLP.