linux-kernel - Re: [PATCH 1/2] include: trace: Add SCMI header with trace events

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Date:   Tue, 17 Dec 2019 10:05:28 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, rostedt@...dmis.org, sudeep.holla@....com
Subject: Re: [PATCH 1/2] include: trace: Add SCMI header with trace events

Hello Jim,

On 12/16/19 10:15 PM, Jim Quinlan wrote:
> From: Lukasz Luba <lukasz.luba@....com>
> 
> +
> +TRACE_EVENT(scmi_xfer_begin,
> +	TP_PROTO(u8 id, u8 protocol_id, u16 seq, bool poll),
> +	TP_ARGS(id, protocol_id, seq, poll),
> +
> +	TP_STRUCT__entry(
> +		__field(u8, id)
> +		__field(u8, protocol_id)
> +		__field(u16, seq)
> +		__field(bool, poll)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->id = id;
> +		__entry->protocol_id = protocol_id;
> +		__entry->seq = seq;
> +		__entry->poll = poll;
> +	),
> +
> +	TP_printk("id=%u protocol_id=%u seq=%u poll=%u", __entry->id,
> +		__entry->protocol_id, __entry->seq, __entry->poll)
> +);
> +
> +TRACE_EVENT(scmi_xfer_end,
> +	TP_PROTO(u8 id, u8 protocol_id, u16 seq, u32 status),
> +	TP_ARGS(id, protocol_id, seq, status),
> +
> +	TP_STRUCT__entry(
> +		__field(u8, id)
> +		__field(u8, protocol_id)
> +		__field(u16, seq)
> +		__field(u32, status)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->id = id;
> +		__entry->protocol_id = protocol_id;
> +		__entry->seq = seq;
> +		__entry->status = status;
> +	),
> +
> +	TP_printk("id=%u protocol_id=%u seq=%u status=%u", __entry->id,
> +		__entry->protocol_id, __entry->seq, __entry->status)
> +);
> 
> Hello,
> 
> When there are multiple messages in the mbox queue, I've found it
> a chore matching up the 'begin' event with the 'end' event for each
> SCMI msg.  The id (command) may not be unique, the proto_id may not be
> unique, and the seq may not be unique.  The combination of the three
> may not be unique.  Would it make sense to assign a monotonically
> increasing ID to each msg so that one can easily match the two events
> for each msg?  This id could be the result of an atomic increment and
> could be stored in the xfer structure.  Of course, it would be one of
> the values printed out in the events.

Hmmm, an atomic variable in this code might be too heavy, especially in
case of fast_switch from cpufreq. Let me think about it and experiment.

> 
> Also, would you consider a third event, right after the
> scmi_fetch_response() invocation in scmi_rx_callback()?  I've found
> this to be insightful in situations where we were debugging a timeout.

Yes, of course. It would be really useful. Thank you for the
suggestion.

> 
> I'm fine if you elect not to do the above; I just wanted to post
> this for your consideration.

Thant's a valuable feedback. I will definitely consider it.

Regards,
Lukasz

> 
> Thanks,
> Jim Quinlan
> Broadcom
>