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

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Date:	Wed, 4 Nov 2015 20:33:04 +0100
From:	Jean-Michel Hautbois <jean-michel.hautbois@...-labs.com>
To:	Jon Nettleton <jon.nettleton@...il.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, Michael Turquette <mturquette@...libre.com>,
	Sascha Hauer <kernel@...gutronix.de>,
	Shawn Guo <shawnguo@...nel.org>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Philipp Zabel <p.zabel@...gutronix.de>
Subject: Re: i.MX6: Increasing VPU frequency

2015-11-04 18:04 GMT+01:00 Jon Nettleton <jon.nettleton@...il.com>:
> 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.

Interesting.
So you propose to add something like the following in
drivers/clk/imx/clk-imx6q.cdrivers/clk/imx/clk-imx6q.c :
imx_clk_set_rate(clk[IMX6QDL_CLK_PLL2_PFD0_352M], 327000000);
imx_clk_set_parent(clk[IMX6QDL_CLK_VPU_AXI_SEL],
clk[IMX6QDL_CLK_PLL2_PFD0_352M]);

This should end up with a fastest VPU (in fact ~25% boost is good).
Is it the correct way to do it ?
Should it be done in coda instead ? And only when needed ?

Thanks,
JM
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