Hi Finley, Am Montag, 11. April 2016, 09:54:12 schrieb Finlye Xiao: > From: Finley Xiao <finley.xiao at rock-chips.com> > > Some of Rockchip's clocks should consider the priority of .set_parent > and .set_rate to prevent a too large temporary clock rate. > > For example, the gpu clock can be parented to cpll(750MHz) and > usbphy_480m(480MHz), 375MHz comes from cpll and the div is set > to 2, 480MHz comes from usbphy_480m and the div is set to 1. > > From the code, when change rate from 480MHz to 375MHz, it changes > the gpu's parent from USBPHY_480M to cpll first(.set_parent), but the > div value is still 1 and the gpu's rate will be 750MHz at the moment, > then it changes the div value from 1 to 2(.set_rate) and the gpu's > rate will be changed to 375MHz(480MHZ->750MHz->375MHz), here temporary > rate is 750MHz, the voltage which supply for 480MHz certainly can not > supply for 750MHz, so the gpu will crash. We did talk about this internally and while we refined the actual code change, it seems I forgot to look at the commit message itself. This behaviour (and the wish to not overflow a target clock rate) should be the same on all socs, so the commit message is quite a bit to Rockchip specific. I think I would go with something like: ----------- 8< ------------ When changing the clock-rate, currently a new parent is set first and a divider adapted thereafter. This may result in the clock-rate overflowing its target rate for a short time if the new parent has a higher rate than the old parent. While this often doesn't produce negative effects, it can affect components in a voltage-scaling environment, like the GPU on the rk3399 socs, where the voltage than simply is to low for the temporarily to high clock rate. For general clock hirarchies this may need more extensive adaptions to the common clock-framework, but at least for composite clocks having both parent and rate settings it is easy to create a short-term solution to make sure the clock-rate does not overflow the target. ----------- 8< ------------ But of course feel free to extend or change that as you wish ;-) . > > Signed-off-by: Finley Xiao <finley.xiao at rock-chips.com> I remember having clocks not overflow their target rate came up in some ELC talk last week (probably in Stephens Qualcomm-kernel-talk) and a general solution might need some changes closer to the core. But at least for composite clocks where we can control the rate+parent process easily, Finley's change is a nice low-hanging fruit which at least improves behaviour for those clock-types in the short term, so Reviewed-by: Heiko Stuebner <heiko at sntech.de> Heiko
