Messages in this thread | | | Subject | Re: [PATCH 2/2] clk: bcm2835: Skip PLLC clocks when deciding on a new clock parent | From | Martin Sperl <> | Date | Mon, 2 May 2016 10:54:29 +0200 |
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On 30.04.2016 11:28, Martin Sperl wrote: >> On 26.04.2016, at 21:39, Eric Anholt <eric@anholt.net> wrote: >> >> If the firmware had set up a clock to source from PLLC, go along with >> it. But if we're looking for a new parent, we don't want to switch it >> to PLLC because the firmware will force PLLC (and thus the AXI bus >> clock) to different frequencies during over-temp/under-voltage, >> without notification to Linux. >> >> On my system, this moves the Linux-enabled HDMI state machine and DSI1 >> escape clock over to plld_per from pllc_per. EMMC still ends up on >> pllc_per, because the firmware had set it up to use that. >> >> Signed-off-by: Eric Anholt <eric@anholt.net> >> Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the audio domain clocks") >> — > I guess this patch looks to me as if it is a policy inside the kernel, > which is AFAIK frowned upon. > > I am looking into making "assigned-clock-parents” inside the dt > work with the driver. > > Could look something like this: > i2s: i2s@7e203000 { > assigned-clock-parents = <&cprman BCM2835_PLLD_PER>, <&clk_osc>; > assigned-clocks = <&cprman BCM2835_CLOCK_PCM>, <&cprman BCM2835_CLOCK_PCM>; > }; > (not sure if that works really - the same clock in assigned-clocks looks suspicious) > > This would move the policy out of the kernel into the device-tree, > which - i guess is a better solution.
So after some more investigation it seems that we can not really use those assigned-clock-parents properties for our purpose of filtering the parent clocks, as it: a) requires also assigned-clocks to be set (this may be OK) b) it does not allow to define a list of clocks to get used - it will just set the parent of the assigned-clock - if we take the example shown above, it would call clk_set_parent 2 times for the PCM clock - once with PLLD_PER and once with clk_osc.
So I start to wonder if it would not be better to use an approach like this: cprman: cprman@7e101000 { ... brcm,clock-flags = <flags for PCM>, <flags for PWM>; brcm,clock-index = <BCM2835_CLOCK_PCM>, <BCM2835_CLOCK_PWM>; }
the flags would be a bitfield that select the parent clocks.
So it could look like this: cprman: cprman@7e101000 { ... brcm,clock-flags = (BIT(BCM2835_PER_PARENT_OSC) | BIT(BCM2835_PER_PARENT_PLLD_PER)), ...; brcm,clock-index = <BCM2835_CLOCK_PCM>, <BCM2835_CLOCK_PWM>; }
BCM2835_PER_PARENT_PLLD_PER and BCM2835_PER_PARENT_OSC would then be defined in include/dt-bindings/clock/bcm2835.h
In addition this would also allow us to add other flags to enable higher order MASH clock dividers - we currently only allow simple fractional dividers - we could even force the use of integer dividers if there comes a need.
This would really allow us to define the parents freely and if the firmware ever changes its behavior with regards to PLLC, then we can easily change the device-tree.
Is this approach acceptable - maybe in a variation?
Thanks, Martin
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