[PATCH 0/3] clk: bcm2835: critical clocks and parent selection

Martin Sperl kernel at martin.sperl.org
Wed May 11 01:21:27 PDT 2016

On 10.05.2016, at 21:58, Martin Sperl <kernel at martin.sperl.org> wrote:
>> On 10.05.2016, at 19:37, Eric Anholt <eric at anholt.net> wrote:
>> Martin Sperl <kernel at martin.sperl.org> writes:
>>>> On 10.05.2016 03:01, Eric Anholt wrote:
>>>> With the new patch 2 inserted between my previous pair, I think this
>>>> should cover Martin's bugs with clock disabling.
>>>> I tested patch 2 to be important on the downstream kernel: with the
>>>> DPI panel support added there, I was losing ethernet (my only I/O)
>>>> when the HDMI HSM hanging off of PLLD_PER got disabled due to
>>>> Eric Anholt (3):
>>>> clk: bcm2835: Mark the VPU clock as critical
>>>> clk: bcm2835: Mark GPIO clocks enabled at boot as critical.
>>>> clk: bcm2835: Skip PLLC clocks when deciding on a new clock parent
>>>> drivers/clk/bcm/clk-bcm2835.c | 32 ++++++++++++++++++++++++++++++--
>>>> 1 file changed, 30 insertions(+), 2 deletions(-)
>>> I gave it a try - with all 3 patches applied I get the following enabled 
>>> clocks:
>>> root at raspcm:~# grep -vE ^0 /sys/kernel/debug/clk/*/clk_enable_count
>>> /sys/kernel/debug/clk/aux_uart/clk_enable_count:1
>>> /sys/kernel/debug/clk/emmc/clk_enable_count:1
>>> /sys/kernel/debug/clk/gp1/clk_enable_count:1
>>> /sys/kernel/debug/clk/gp2/clk_enable_count:1
>>> /sys/kernel/debug/clk/osc/clk_enable_count:1
>>> /sys/kernel/debug/clk/pllc/clk_enable_count:2
>>> /sys/kernel/debug/clk/pllc_core0/clk_enable_count:1
>>> /sys/kernel/debug/clk/pllc_per/clk_enable_count:1
>>> /sys/kernel/debug/clk/vpu/clk_enable_count:2
>>> At least on my compute module gp1/gp2 is enabled, but there is no rate
>>> set - so why is it marked as critical for all devices?
>>> So why apply patch2 for all possible devices?
>> According to the CLK_IS_CRITICAL patches, the author intended critical
>> clocks not to use the included function for marking clocks as critical
>> From the DT.  I'm not sure why, but writing patches using that when they
>> say not to seemed like a waste.
>> We could check if gp1/gp2 are already on before marking them critical.
> That may seem reasonable.
>>> Loading/unloading the amba_pl011 module does not crash the system,
>>> but a simple stty -F /dev/ttyAMA0 does crash the system!
>>> Here the sequence:
>>> root at raspcm:~# dmesg -C
>>> root at raspcm:~# modprobe amba_pl011
>>> root at raspcm:~# dmesg -c
>>> [  141.708453] Serial: AMBA PL011 UART driver
>>> [  141.709158] 20201000.uart: ttyAMA0 at MMIO 0x20201000 (irq = 81, 
>>> base_baud = 0) is a PL011 rev2
>>> root at raspcm:~# rmmod  amba_pl011
>>> root at raspcm:~# dmesg -c
>>> [  150.511248] Trying to free nonexistent resource 
>>> <0000000020201000-0000000020201fff>
>>> root at raspcm:~# modprobe amba_pl011
>>> root at raspcm:~# dmesg -c
>>> [  159.385002] Serial: AMBA PL011 UART driver
>>> [  159.385714] 20201000.uart: ttyAMA0 at MMIO 0x20201000 (irq = 81, 
>>> base_baud = 0) is a PL011 rev2
>>> root at raspcm:~# stty -F /dev/ttyAMA0
>>> speed 9600 baud; line = 0;
>>> -brkint -imaxbel
>>> root at raspcm:~# Timeout, server raspcm not responding.
>>> The reason behind this is that the firmware pre-configured uart clock
>>> looks like this:
>>> root at raspcm:~# cat /sys/kernel/debug/clk/uart/regdump
>>> ctl = 0x00000296
>>> div = 0x000a6aab
>>> so it is configured to use plld_per (which itself is running, even if 
>>> not enabled
>>> in the kernel)
>>> But as plld_per is not among the enabled clocks then plld_per
>>> gets disabled as soon as the tty device is closed (by stty) and
>>> this also disables plld...
>>> Similar effect when using PCM/i2s and use speaker-test:
>>> root at raspcm:~# dmesg -C
>>> root at raspcm:~# modprobe snd-soc-bcm2835-i2s; modprobe snd-soc-pcm5102a; 
>>> modprobe snd-soc-hifiberry-dac
>>> root at raspcm:~# dmesg
>>> [   81.968591] snd-hifiberry-dac sound: pcm5102a-hifi <-> 20203000.i2s 
>>> mapping ok
>>> root at raspcm:~# speaker-test -c 2 -r 44100 -F S16_LE -f 440 -t sine&
>>> [1] 579
>>> root at raspcm:~#
>>> speaker-test 1.0.28
>>> Playback device is default
>>> Stream parameters are 44100Hz, S16_LE, 2 channels
>>> Sine wave rate is 440.0000Hz
>>> Rate set to 44100Hz (requested 44100Hz)
>>> Buffer size range from 128 to 131072
>>> Period size range from 64 to 65536
>>> Using max buffer size 131072
>>> Periods = 4
>>> was set period_size = 32768
>>> was set buffer_size = 131072
>>> 0 - Front Left
>>> 1 - Front Right
>>> root at raspcm:~#
>>> root at raspcm:~# grep -vE ^0 /sys/kernel/debug/clk/*/clk_enable_count
>>> /sys/kernel/debug/clk/aux_uart/clk_enable_count:1
>>> /sys/kernel/debug/clk/emmc/clk_enable_count:1
>>> /sys/kernel/debug/clk/gp1/clk_enable_count:1
>>> /sys/kernel/debug/clk/gp2/clk_enable_count:1
>>> /sys/kernel/debug/clk/osc/clk_enable_count:2
>>> /sys/kernel/debug/clk/pcm/clk_enable_count:1
>>> /sys/kernel/debug/clk/pllc/clk_enable_count:2
>>> /sys/kernel/debug/clk/pllc_core0/clk_enable_count:1
>>> /sys/kernel/debug/clk/pllc_per/clk_enable_count:1
>>> /sys/kernel/debug/clk/plld/clk_enable_count:1
>>> /sys/kernel/debug/clk/plld_per/clk_enable_count:1
>>> /sys/kernel/debug/clk/vpu/clk_enable_count:2
>>> root at raspcm:~# kill %1
>>> root at raspcm:~# Time per period = 106.889502
>>> Timeout, server raspcm not responding.
>>> You see that plld gets now used and when I kill speaker-test
>>> the machine crashes again.
>> Just so I can be clear here: What are you using to talk to the Pi?
>> Builtin USB ethernet?
> Compute module with USB  Ethernet adapter, but I also got an 
> enc28j60 connected via spi, but that is not enabled by default
>>> So this patchset does not really solve any of the problems that
>>> I have reported either.
>>> That is why my patchset has taken the "HAND_OFF" approach
>>> instead (which still just hides some of the issues), but at least
>>> it does not crash the system on the use of plld and it allows
>>> for custom parent and mash selection.
>> HAND_OFF sure doesn't look like it's landing in the next kernel, so
>> writing patches using it doesn't make much sense to me.
> If it is critical or hands-off does not really make a difference...
>>> In reality it would require consumers of the corresponding
>>> parent clocks in the kernel (arm, ...) and the knowledge which
>>> clocks are really needed by the firmware - i.e plld.
>>> Note that the sdram clock is using plld_core parent!
>>> root at raspcm:~#  cat /sys/kernel/debug/clk/sdram/regdump
>>> ctl = 0x00004006
>>> div = 0x00003000
>>> root at raspcm:~# cat /sys/kernel/debug/clk/sdram/clk_rate
>>> 166533331
>> However, it's not enabled, right?  Bit 4 isn't set in the CTL reg.
> You are probably right there, but still there must be something
> hidden that requires plld or plld_per or plld_core, that requires critical.
> There must be some reason why it gets set up during the
> boot process by the firmware.


Actually the SDC control register contains some more bits compared
to most other clock control registers:

See also:

If I poke that register (0x201011a8) from userspace with 0x5a000000
then the machine crashes as well - so I assume these bits control
the custom SD-Ram PLL - unfortunately it still does not say which
PLL is used.

Eric, you may have access to more data regarding this register -
maybe we need to hand this register as a special clock?
and/or expose it as an additional pll?

Note that I also tried disabling PLLD_CORE and the system crashed as well
(root at raspcm:~#  peekpoke $[0x20101000 + 0x10c] w
/dev/mem opened.
Memory mapped at address 0xb6fdd000.
Value at address 0x2010110C (0xb6fdd10c): 0x20A
root at raspcm:~# peekpoke $[0x20101000 + 0x10c] w $[0x5a000000 + 0x20a + 32]
/dev/mem opened.
Memory mapped at address 0xb6fe7000.
Value at address 0x2010110C (0xb6fe710c): 0x20A
Written 0x5A00022A; readback 0x22A
root at raspcm:~# Write failed: Broken pipe

(requires /dev/mem without restrictions)

So this means that it is plld_core related - at least as of now
and the sdram pll currently derives from PLLD_core.

@Eric: so please look at the hld (which you have hinted you have
access to) and come up with something better for sdram

As far as I can tell at the very least the sdram clock is critical
and should be marked as such…


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