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

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 14/06/22 20:35, Daniel Lezcano ha scritto:
> 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 ?
> 
> 

Hello Daniel,

that's not the first time this question gets asked, and I've already
provided an explanation for that, please look at [1] for the long answer.

As for the shorter answer: OEM/ODM signatures, you can't just compile and
flash a new ATF on your own (and some ODMs don't even exist anymore).
There's no way other than this.

[1]: 
https://patchwork.kernel.org/project/linux-mediatek/patch/20220509210741.12020-3-angelogioacchino.delregno@collabora.com/#24863077

Regards,
Angelo