Re: [PATCH v4 2/2] clocksource/drivers/timer-mediatek: Implement CPUXGPT timers

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 13/06/2022 15:38, AngeloGioacchino Del Regno wrote:
> Some MediaTek platforms with a buggy TrustZone ATF firmware will not
> initialize the AArch64 System Timer correctly: in these cases, the
> System Timer address is correctly programmed, as well as the CNTFRQ_EL0
> register (reading 13MHz, as it should be), but the assigned hardware
> timers are never started before (or after) booting Linux.
> 
> In this condition, any call to function get_cycles() will be returning
> zero, as CNTVCT_EL0 will always read zero.
> 
> One common critical symptom of that is trying to use the udelay()
> function (calling __delay()), which executes the following loop:
> 
>              start = get_cycles();
>              while ((get_cycles() - start) < cycles)
>                      cpu_relax();
> 
> which, when CNTVCT_EL0 always reads zero, translates to:
> 
>              while((0 - 0) < 0)  ==> while(0 < 0)
> 
> ... generating an infinite loop, even though zero is never less
> than zero, but always equal to it (this has to be researched,
> but it's out of the scope of this commit).
> 
> To fix this issue on the affected MediaTek platforms, the solution
> is to simply start the timers that are designed to be System Timer(s).
> These timers, downstream, are called "CPUXGPT" and there is one
> timer per CPU core; luckily, it is not necessary to set a start bit
> on each CPUX General Purpose Timer, but it's conveniently enough to:
>   - Set the clock divider (input = 26MHz, divider = 2, output = 13MHz);
>   - Set the ENABLE bit on a global register (starts all CPUX timers).
> 
> The only small hurdle with this setup is that it's all done through
> the MCUSYS wrapper, where it is needed, for each read or write, to
> select a register address (by writing it to an index register) and
> then to perform any R/W on a "CON" register.
> 
> For example, writing "0x1" to the CPUXGPT register offset 0x4:
> - Write 0x4 to mcusys INDEX register
> - Write 0x1 to mcusys CON register
> 
> Reading from CPUXGPT register offset 0x4:
> - Write 0x4 to mcusys INDEX register
> - Read mcusys CON register.
> 
> Finally, starting this timer makes platforms affected by this issue
> to work correctly.

Why not fix the ATF ?




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