On Tue, Jul 16, 2019 at 11:13 AM Sibi Sankar <sibis@xxxxxxxxxxxxxx> wrote: > > Hey Saravana, > > On 6/18/19 2:48 AM, Saravana Kannan wrote: > > On Mon, Jun 17, 2019 at 8:44 AM Georgi Djakov <georgi.djakov@xxxxxxxxxx> wrote: > >> > >> Hi Saravana, > >> > >> On 6/14/19 07:17, Saravana Kannan wrote: > >>> Add a icc_create_devfreq() and icc_remove_devfreq() to create and remove > >>> devfreq devices for interconnect paths. A driver can create/remove devfreq > >>> devices for the interconnects needed for its device by calling these APIs. > >>> This would allow various devfreq governors to work with interconnect paths > >>> and the device driver itself doesn't have to actively manage the bandwidth > >>> votes for the interconnects. > >> > >> Thanks for the patches, but creating devfreq devices for each interconnect path > >> seems odd to me - at least for consumers that already use a governor. > > > > Each governor instance always handles one "frequency" (more like > > performance) domain at a time. So if a consumer is already using a > > governor to scale the hardware block, then using another governor to > > scale the interconnect performance points is the right way to go about > > it. In fact, that's exactly what devfreq passive governor's > > documentation even says it's meant for. That's also what cpufreq does > > for each cluster/CPU frequency domain too. > > > >> So for DDR > >> scaling for example, are you suggesting that we add a devfreq device from the > >> cpufreq driver in order to scale the interconnect between CPU<->DDR? > > > > Yes in general. Although, CPUs are a special case because CPUs don't > > go through devfreq. So passive governor as it stands today won't work. > > CPU<->DDR scaling might need a separate governor (unlikely) or some > > changes to the passive governor that I'm happy to work on once we > > settle this for general devices like GPU, etc. But the DT format for > > CPUs will be identical to GPUs or any other device. > > using icc_create_devfreq from the cpufreq-hw driver on SDM845 SoC > to scale CPU<->DDR would cause a circular dependency. (i.e) with > the addition of cpufreq notifier to the passive governor as in > https://patchwork.kernel.org/patch/11046147/ devm_devfreq_add_device > would require the cpufreq transistion notifier register and cpu > freq_cpu_get to go through. Please add your thought on addressing this. This is an old series. So not going to dive into this much. But to answer your question, I wrote the cpufreq_map governor a long time ago. So not surprised if you are finding issues with it -- it needs a rewrite anyway. -Saravana > > > >> Also if the > >> GPU is already using devfreq, should we add a devfreq per each interconnect > >> path? What would be the benefit in this case - using different governors for > >> bandwidth scaling maybe? > > > > When saying "separate/different governors" in this email, I mean both > > different instance of the same governor logic with different tunables > > AND actually different algorithms/governor logic entirely. > > > > The heuristics to use for each interconnect path might be (more like, > > will be) different based on hardware characteristics (Eg: what voltage > > domains the interconnect is sitting on) and what interconnect > > information is available (Eg: Just busy time vs bandwidth count vs no > > information etc) -- so having separate governors for each interconnect > > path makes a lot of sense. It also allows userspace to control the > > policy for scaling each of those paths based on product use cases. > > > > For example, when the GPU is just doing simple UI rendering, userspace > > can use the max_freq sysfs file for the devfreq device to disallow high > > bandwidth OPPs on the GPU<->DDR path, but those higher OPPs could be > > allowed by userspace when the GPU is used for games. Having devfreq > > device for each interconnect path also make it easy to debug > > performance issues -- you can independently change the votes for each > > path to figure out what is causing the bottleneck, etc. > > > > Adding a devfreq device for interconnect voting with a few lines gives > > all these features "for free". > > > > This doesn't mean all users of interconnect framework NEED to use > > devfreq for interconnect. They might do it simply based on > > calculations based on the use case (Eg: display driver from display > > resolution). But if they are trying to use any kind of > > algorithm/heuristics, writing it as a devfreq governor should be > > encouraged. > > > > Also want to point out that BW OPPs also work for drivers that don't > > use devfreq at all. The interconnect-opp-table just lists the > > meaningful OPP leveld for the path and the device driver can pick one > > entry from the table based on the use case. > > > > Thanks, > > Saravana > > > > > > > > -- > Qualcomm Innovation Center, Inc. > Qualcomm Innovation Center, Inc, is a member of Code Aurora Forum, > a Linux Foundation Collaborative Project > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
