Date:   Fri, 28 Jan 2022 15:28:23 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        chenxiang <chenxiang66@...ilicon.com>, linux@...linux.org.uk,
        vkoul@...nel.org, linux-arm-kernel@...ts.infradead.org,
        mathieu.poirier@...aro.org
Cc:     coresight@...ts.linaro.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, Anshuman Khandual <anshuman.khandual@....com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [RFC PATCH] ACPI / amba: Skip creating amba device when
 associated cpu is not online

On 2022-01-28 11:00, Suzuki K Poulose wrote:
> Hi Xiang
> 
> On 07/01/2022 08:41, chenxiang wrote:
>> From: Xiang Chen <chenxiang66@...ilicon.com>
>>
>> If not up all the cpus with command line "maxcpus=x", system will be
>> blocked.
>> We find that some amba devices such as ETM devices, are associated with
>> special cpus, and if the cpu is not up, the register of associated device
>> is not allowed to access. BIOS reports all the ETM device nodes and a
>> amba device is created for every ETM device, so even if one cpu is not 
>> up,
>> the amba device will still be created for the associated device, and also
>> the register of device (pid and cid) will be accessed when adding amba
>> device which will cause the issue.
>> To fix it, skip creating amba device if it is associated with a cpu which
>> is not online.
> 
> I understand the issue. We do not have an issue at least on DT based 
> platforms with a similar environment (Juno). The key is the power 
> management for the components.
> 
> There are two separate issues at play here :
> 
> 1) Power management with ACPI. I believe there is a solution in progress
> to address this.
> 
> 2) The ETM is in the same power domain as that of the CPU and normal 
> device power management may not work without the CPU being online.
> 
> 3) Additionally we have other issue of supporting system instructions
> with ACPI, which do not appear on the AMBA bus.
> 
> Considering all of these, the ideal solution is to :
> 
> 1) Implement power management for ACPI, which is anyway in progress
>    (at least for SCMI based systems)
> 2) Move the ETM driver away from the AMBA framework. That would make
>     the CPU online problem and the (3) above easier to solve.
>     Anshuman is going to look into this.
> 
> In the meantime, we could have this temporary fix in and solve it
> forever by moving away from the AMBA.

Out of curiosity, what happens when you boot with "maxcpus=" and then 
manually bring up the extra cpus from userspace later? Is there a chance 
that tracing will explode if some online CPUs have ETM initialised but 
others don't?

Robin.

>>
>> Signed-off-by: Xiang Chen <chenxiang66@...ilicon.com>
>> ---
>>   drivers/acpi/acpi_amba.c | 36 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 36 insertions(+)
>>
>> diff --git a/drivers/acpi/acpi_amba.c b/drivers/acpi/acpi_amba.c
>> index ab8a4e0191b1..2369198f734b 100644
>> --- a/drivers/acpi/acpi_amba.c
>> +++ b/drivers/acpi/acpi_amba.c
>> @@ -16,6 +16,7 @@
>>   #include <linux/ioport.h>
>>   #include <linux/kernel.h>
>>   #include <linux/module.h>
>> +#include <acpi/processor.h>
>>   #include "internal.h"
>> @@ -45,6 +46,35 @@ static void amba_register_dummy_clk(void)
>>       clk_register_clkdev(amba_dummy_clk, "apb_pclk", NULL);
>>   }
>> +static int acpi_handle_to_cpuid(acpi_handle handle)
>> +{
>> +    int cpu = -1;
>> +    struct acpi_processor *pr;
>> +
>> +    for_each_possible_cpu(cpu) {
>> +        pr = per_cpu(processors, cpu);
>> +        if (pr && pr->handle == handle)
>> +            break;
>> +    }
>> +
>> +    return cpu;
>> +}
>> +
> 
> Please could we reuse the function in coresight-platform.c ?
> i.e, move it to a generic location and share it, rather than
> duplicating it ?
> 
> 
>> +static int acpi_dev_get_cpu(struct acpi_device *adev)
>> +{
>> +    acpi_handle cpu_handle;
>> +    acpi_status status;
>> +    int cpu;
>> +
>> +    status = acpi_get_parent(adev->handle, &cpu_handle);
>> +    if (ACPI_FAILURE(status))
>> +        return -1;
>> +    cpu = acpi_handle_to_cpuid(cpu_handle);
>> +    if (cpu >= nr_cpu_ids)
>> +        return -1;
>> +    return cpu;
>> +}
>> +
>>   static int amba_handler_attach(struct acpi_device *adev,
>>                   const struct acpi_device_id *id)
>>   {
>> @@ -54,11 +84,17 @@ static int amba_handler_attach(struct acpi_device 
>> *adev,
>>       bool address_found = false;
>>       int irq_no = 0;
>>       int ret;
>> +    int cpu;
>>       /* If the ACPI node already has a physical device attached, skip 
>> it. */
>>       if (adev->physical_node_count)
>>           return 0;
>> +    /* If the cpu associated with the device is not online, skip it. */
>> +    cpu = acpi_dev_get_cpu(adev);
>> +    if (cpu >= 0 && !cpu_online(cpu))
>> +        return 0;
>> +
> 
> Except for the comment above, the patch looks good to me.
> 
> Suzuki
> 
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