On 26-10-18, 15:03, Dmitry Osipenko wrote: > 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. Thanks Dmitry. > What would happen if there actually was a regulator Please preserve the '>' from previous replies at the beginning of the lines. Otherwise it looks as if you have written the above line. :) > > and its default settings aren't good enough for high end frequencies ? > > Usually this causes kernel/applications crashes and/or machine hang. Sure. I also do remember from some guys (maybe TI), where they mentioned that such scenarios can harm the hardware as well sometimes. Don't remember the details though. > 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. Sure, but they are easy to miss and they are only seen by developers not regular users of a machine. > > 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. Based on the other discussion with Lucas on this thread, I don't think this is correct any more ? Don't get me wrong, I am all good for changes and another API change doesn't matter much to me. I am just wondering if it would be the right approach to fix the issue. Why not rather call dev_pm_opp_set_regulators() conditionally and avoid calling it for the specific tegra20 board. -- viresh
