On 10/29/18 9:53 AM, Viresh Kumar wrote: > 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. The right approach should be to specify device tree correctly. Probably we won't need this change at all with the correct DT, will see. Calling dev_pm_opp_set_regulators() conditionally should work, but that is extra churning in the driver that I tried to avoid.
