seL4/Genode on zynq7000; TTC Timer Component

[Stefan Kalkowski]

Hi Alexander,

On Mon, Oct 28, 2019 at 06:05:19PM +0100, Alexander Weidinger wrote:
> Hello Genodians,
> 
> we adapted the zynq7000 target to run with Genode/seL4[4], which was
> done accordingly to the imx6 and imx7 targets already existing in Genode.
> 
> Since we are in need for a timer component and the seL4 doesn't provide
> a "l4_ipc_sleep()" call, like it is used for the generic timer component
> in Fiasco.OC, we started implementing a timer[1,2] based on the TTC of
> the Zynq7000[0]. By now we have it up and running but with a high
> deviation the longer it runs (already 30 seconds deviation after only 2
> minutes).
> 
> This deviation originates from a few problems we have:
> - The timers in TTC only provide a 16 Bit width -> high number of IRQs
> when trying to achieve high precision
> - The input clock is 133MHz and can only be prescaled by a division of
> 2^(n+1) with 0 < n <= 16 -> hard to work with?

As far as I understood the manual correctly, there are three different
reference clocks you can choose from.

> 
> Currently we implement it similar to the imx6 timer, by generating IRQs
> when a match in the MATCH0 register is acknowledged.
> 
> I'm not very experienced in this topic and any help is appreciated on
> how to resolve the deviation issues or how to implement it in a better
> way - e.g. by using a different hardware timer which I didn't discover
> in the TRM.
> 
> Additionally I'd like to know if there is an easy way to configure the
> maximum RAM of a platform in seL4/Genode apart from limiting it in
> 'contrib/sel4-*/src/kernel/sel4/include/plat/*/plat/machine/hardware.h'?

You hard-coded TICKS_PER_MS to "519", according to your comment in [1]
Line 88 the prescale value N calculates to: count/2^(N+1)
But you set the prescale value to "8". That means count/2^(8+1) =
count/512. And 519 * 512 * 1000 ~= 266 MHz, isn't it?

I strongly assume you are working with wrong assumptions regarding
your clocking values.

Regards
Stefan

> 
> Best regards,
> Alex
> 
> PS: As Zynq7000 platform we use the Digilent Zybo[3].
> 
> [0]:
> https://www.xilinx.com/support/documentation/user_guides/ug585-Zynq-7000-TRM.pdf
> [1]:
> https://github.com/irgendwie/genode/blob/rtcr-19.08-ttc/repos/base/src/timer/ttc/time_source.cc
> [2]:
> https://github.com/irgendwie/genode/blob/rtcr-19.08-ttc/repos/base/src/timer/ttc/time_source.h
> [3]:
> https://store.digilentinc.com/zybo-zynq-7000-arm-fpga-soc-trainer-board/
> [4]:
> https://github.com/irgendwie/genode/commit/60ffef31b52d6566f481b3de87b4720f3251f6ec
> 
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-- 
Stefan Kalkowski
Genode labs

https://github.com.skalk | https://genode.org