linux-kernel - Re: [PATCH 1/2] coresight: Set affinity to invalid for missing CPU phandle

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Message-ID: <f7a3592b-7ed7-b011-9ae1-dc2ca0e49ae4@arm.com>
Date:   Thu, 20 Jun 2019 14:55:20 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     saiprakash.ranjan@...eaurora.org, mathieu.poirier@...aro.org,
        leo.yan@...aro.org, alexander.shishkin@...ux.intel.com,
        andy.gross@...aro.org, david.brown@...aro.org, mark.rutland@....com
Cc:     rnayak@...eaurora.org, vivek.gautam@...eaurora.org,
        sibis@...eaurora.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 1/2] coresight: Set affinity to invalid for missing CPU
 phandle


Sai,

Thanks for the patch. Please could you change the subject to :

"coresight: Do not default to CPU0 for missing CPU phandle"

On 20/06/2019 14:45, Sai Prakash Ranjan wrote:
> Affinity defaults to CPU0 in case of missing CPU phandle
> and this leads to crashes in some cases because of such
> wrong assumption. Fix this by returning -ENODEV in

Thats not the right justification. Causing crashes is due to
bad DT/firmware. I would be happy with something like :

"Coresight platform support assumes that a missing \"cpu\" phandle
defaults to CPU0. This could be problematic and unnecessarily binds
components to CPU0, where they may not be. Let us make the DT binding
rules a bit stricter by not defaulting to CPU0 for missing "cpu"
affinity information."

Also, you must

1) update the devicetree/bindings document to reflect the same.
2) update the drivers to take appropriate action on the missing CPU
    where they are expected (e.g, CPU-debug, etm*), to prevent
    breaking a bisect.


> coresight platform for such cases and then handle it
> in the coresight drivers.
> 
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> ---
>   drivers/hwtracing/coresight/coresight-platform.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-platform.c b/drivers/hwtracing/coresight/coresight-platform.c
> index 3c5ceda8db24..b1ea60c210e1 100644
> --- a/drivers/hwtracing/coresight/coresight-platform.c
> +++ b/drivers/hwtracing/coresight/coresight-platform.c
> @@ -160,15 +160,17 @@ static int of_coresight_get_cpu(struct device *dev)
>   
>   	if (!dev->of_node)
>   		return 0;
> +
>   	dn = of_parse_phandle(dev->of_node, "cpu", 0);
> -	/* Affinity defaults to CPU0 */
> +
> +	/* Affinity defaults to invalid if no cpu nodes are found*/

The code is self explanatory here. You could drop the comment.

>   	if (!dn)
> -		return 0;
> +		return -ENODEV;
> +
>   	cpu = of_cpu_node_to_id(dn);
>   	of_node_put(dn);
>   
> -	/* Affinity to CPU0 if no cpu nodes are found */
> -	return (cpu < 0) ? 0 : cpu;
> +	return cpu;
>   }
>   

Suzuki