Re: i.MX6: Increasing VPU frequency

From: Jon Nettleton
Date: Wed Nov 04 2015 - 23:23:53 EST

On Wed, Nov 4, 2015 at 8:33 PM, Jean-Michel Hautbois
<jean-michel.hautbois@xxxxxxxxxxxx> wrote:
> 2015-11-04 18:04 GMT+01:00 Jon Nettleton <jon.nettleton@xxxxxxxxx>:
>> On Wed, Nov 4, 2015 at 5:52 PM, Jean-Michel Hautbois
>> <jean-michel.hautbois@xxxxxxxxxxxx> 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 ?

That is how I have done it. I was going to implement dvfs for the vpu
but found that this change really added minimal power and thermal
overhead. Without looking it up I believe I only saw a 10-20mA rise
in power usage and virtually no thermal differences.
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