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Date:   Fri, 28 Jan 2022 11:00:59 +0000
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Subject: Re: [RFC PATCH] ACPI / amba: Skip creating amba device when
 associated cpu is not online
To:     chenxiang <chenxiang66@hisilicon.com>, linux@armlinux.org.uk,
        vkoul@kernel.org, linux-arm-kernel@lists.infradead.org,
        mathieu.poirier@linaro.org
Cc:     coresight@lists.linaro.org, linux-kernel@vger.kernel.org,
        linuxarm@huawei.com, Anshuman Khandual <anshuman.khandual@arm.com>,
        Sudeep Holla <sudeep.holla@arm.com>
References: <1641544906-7069-1-git-send-email-chenxiang66@hisilicon.com>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
In-Reply-To: <1641544906-7069-1-git-send-email-chenxiang66@hisilicon.com>
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Hi Xiang

On 07/01/2022 08:41, chenxiang wrote:
> From: Xiang Chen <chenxiang66@hisilicon.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.

> 
> Signed-off-by: Xiang Chen <chenxiang66@hisilicon.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