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From: Felipe Balbi <felipe.balbi@linux.intel.com>
To: Pratyush Anand <panand@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>
Cc: Linux USB <linux-usb@vger.kernel.org>, linux-kernel@vger.kernel.org,
        Chunyan Zhang <zhang.chunyan@linaro.org>
Subject: Re: [PATCH] usb: gadget: functions: add ftrace export over USB
In-Reply-To: <16a6940d-d071-cd2d-5911-dad427a13cb0@redhat.com>
References: <20170609061327.17899-1-felipe.balbi@linux.intel.com> <16a6940d-d071-cd2d-5911-dad427a13cb0@redhat.com>
Date: Fri, 14 Jul 2017 09:56:12 +0300
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Hi,

Pratyush Anand <panand@redhat.com> writes:
> Hi Felipe,
>
> On Friday 09 June 2017 11:43 AM, Felipe Balbi wrote:
>> +static void notrace ftrace_write(struct trace_export *ftrace, const voi=
d *buf,
>> +				 unsigned int len)
>> +{
>> +	struct usb_ftrace		*trace =3D ftrace_to_trace(ftrace);
>> +	struct usb_request		*req =3D next_request(&trace->list);
>> +
>> +	if (!req)
>> +		return;
>> +
>> +	if (!trace->in->enabled)
>> +		return;
>> +
>> +	req->buf =3D kmemdup(buf, len, GFP_ATOMIC);
>
> Probably we can avoid the copy of trace data.

not with current setup. I have no control over data's lifetime. If I
did, then this would be zero-copy.

> We can make write() call of "struct trace_export" as posted. Can have a=20

that would require further patching in trace core which is not part of
$subject. Also, if we turn this into asynchronous calls, we could run
out of trace buffer before the first chunk of data is transferred over
to the other side.

> write_complete() callback function implemented in  struct trace_export,wh=
ich=20
> can be called from your ftrace_complete().

well, you're welcome to propose a patch :-)

=2D-=20
balbi

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