Hi Dave, On Fri, Oct 02, 2020 at 04:57:05PM +0100, Dave Stevenson wrote: > Hi Maxime > > On Fri, 2 Oct 2020 at 16:19, Maxime Ripard <maxime@xxxxxxxxxx> wrote: > > > > Hi Tim, > > > > On Thu, Oct 01, 2020 at 11:15:46AM +0100, Tim Gover wrote: > > > hdmi_enable_4k60=1 causes the firmware to select 3.3 GHz for the PLLC > > > VCO to support a core-frequency of 550 MHz which is the minimum > > > frequency required by the HVS at 4Kp60. The side effect is that if the > > > display clock requirements are lower than 4Kp60 then you will see > > > different core frequencies selected by DVFS. > > > > > > If enable_uart=1 and the mini-uart is selected (default unless > > > bluetooth is disabled) then the firmware will pin the core-frequency > > > to either core_freq max (500 or 550). Although, I think there is a way > > > of pinning it to a lower fixed frequency. > > > > > > The table in overclocking.md defines options for setting the maximum > > > core frequency but unless core_freq_min is specified DVFS will > > > automatically pick the lowest idle frequency required by the display > > > resolution. > > > > I'm wondering if there's some way to detect this from Linux? I guess it > > would be nice to be able to at least detect a broken config to warn / > > prevent an user that their situation is not going to be reliable / work > > really well (like if they have a 4k display without hdmi_enable_4kp60 > > set, or the issue we're discussing here) > > The main filter in the firmware is the parameter > hdmi_pixel_freq_limit. That can either be set manually from > config.txt, or defaults appropriately based on hdmi_enable_4kp60. > Under firmware_kms [1] I read back those values to use as a filter > within crtc_mode_valid[2]. > I can't think of a nice way of exposing that without the vc4 driver > gaining a DT link to the firmware, and that starts to get ugly. I had in mind something like if the clock driver can infer that somehow through some the boundaries reported by the firmware maybe? IIRC, hdmi_enable_4kp60 will already change the max frequency reported to 550MHz instead of 500MHz Maxime
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