linux-kernel - Re: i.MX6: Increasing VPU frequency

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Date:	Wed, 4 Nov 2015 18:04:55 +0100
From:	Jon Nettleton <jon.nettleton@...il.com>
To:	Jean-Michel Hautbois <jean-michel.hautbois@...-labs.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-clk@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	sboyd@...eaurora.org, mturquette@...libre.com,
	Sascha Hauer <kernel@...gutronix.de>,
	Shawn Guo <shawnguo@...nel.org>,
	Fabio Estevam <fabio.estevam@...escale.com>
Subject: Re: i.MX6: Increasing VPU frequency

On Wed, Nov 4, 2015 at 5:52 PM, Jean-Michel Hautbois
<jean-michel.hautbois@...-labs.com> wrote:
> Hi !
>
> I can see in FSL kernel that VPU is configurable to 352M (it defaults
> at 264MHz in mainline I think).
> In the TRM, it is even specified at 352MHz as a default frequency,
> with a maximum of 540MHz.
>
> Would it be possible to allow this clock rating modification if, for
> instance, we select a performance governor in cpufreq, or if a coda
> encoder is started with 1080p for instance ?
> If so, then how is it doable properly ?

For some reason the FSL kernel configures the VPU to run at 352Mhz in
a very odd way that requires limiting the min cpu-frequency to 792Mhz.
It also requires clocking down a bunch of devices on pll2_pfd2_396m to
352Mhz.

The simple solution to this is to instead parent the VPU to
pll2_pfd0_352m which is unused.  I have found by default it is stable
decoding but unstable encoding at 352Mhz, most likely due to the
voltage changes needed that limiting the min cpu-freq to 792Mhz
provides.  However everything seems to work quite reliably clocking
that pfd to 327Mhz, which still gives a boost of almost 24%

In my testing the performance gain in then going from 327 to 352 is
minimal.  Generally I think you hit AXI bus limitations rather than
VPU performance.

-Jon
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