linux-kernel - Re: [PATCH v3 6/6] stm class: ftrace: use different channel accroding to CPU

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Date:   Fri, 18 Sep 2020 15:45:52 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Tingwei Zhang <tingwei@...eaurora.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>
Cc:     Tingwei Zhang <tingwei@...eaurora.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        tsoni@...eaurora.org,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Mao Jinlong <jinlmao@...eaurora.org>,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH v3 6/6] stm class: ftrace: use different channel accroding to CPU

Tingwei Zhang <tingwei@...eaurora.org> writes:

> @@ -63,6 +65,7 @@ static int __init stm_ftrace_init(void)
>  {
>  	int ret;
>  
> +	stm_ftrace.data.nr_chans = num_possible_cpus();

Not a problem with this patch necesarily, but this made me realize that
.nr_chans may be larger than:

 (1) what the policy permits,
 (2) what the stm device can handle.

While (1) the user can fix in the policy, they won't be able to fix (2),
in which case they won't be able to use stm_ftrace at all. I'm thinking
if a link-time callback would be good enough.

Another thing is that .nr_chans needs to be a power of 2 at the moment.

Regards,
--
Alex