Re: i.MX6: Increasing VPU frequency

From: Jean-Michel Hautbois
Date: Wed Nov 04 2015 - 14:33:31 EST

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.

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);

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 ?

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