On 10/24/18 9:41 AM, Viresh Kumar wrote: > On 22-10-18, 15:12, Dmitry Osipenko wrote: >> Because there is one Tegra20 board (tegra20-trimslice) that doesn't declare >> necessary regulators, but we want to have CPU frequency scaling. I couldn't >> find board schematics and so don't know if CPU / CORE voltages are fixed on >> Trim-Slice or it is just preferable not to have DVFS for that board, it is an >> outlet-powered device [0]. Hence tegra20-cpufreq driver will request a dummy >> regulators when appropriate. > > We have been using the regulator_get_optional() variant until now in the OPP > core to make sure that we don't do DVFS for the CPU without the mandatory > regulators being present, as that may make things unstable and cause harm to the > SoC if we try to take CPU to frequency range over the currently programmed > regulator can support. > > Now coming back to tegra-20 SoC, which actually requires a regulator normally by > design. On one of the boards (which is outlet powered), you aren't sure if there > is a programmable regulator or not, or if DVFS should really be done or not. > Isn't it worth checking the same from Tegra maintainers, or whomsoever has > information on that board ? I'll try to find out more detailed information for the next revision of the patchset. What would happen if there actually was a regulator > and its default settings aren't good enough for high end frequencies ? Usually this causes kernel/applications crashes and/or machine hang. > On the other hand, the tegra20 cpufreq driver is common across a lot of boards. > What will happen if the DT for some of the boards isn't correct and missed the > necessary regulator node ? AFAIK, there is assumption that bootloader should setup regulators in a way that kernel could work properly at max clock rates. Otherwise things won't work. And because you are moving to regulator_get() API for > the entire SoC (i.e. its cpufreq driver), people will never find the missing > regulator. Regulators core prints info message when dummy regulator is being used. > If we can do it safely for all tegra20 boards, why not migrate to using > regulator_get() instead of regulator_get_optional() in the OPP core API itself > for everyone ? > This should be a platform-specific decision. For Tegra we know that regulators should be in a good state at kernel boot time, I don't think that this applies to other platforms.
