Messages in this thread | | | Subject | Re: [PATCH v2 11/11] interconnect: Add devfreq support | From | Sibi Sankar <> | Date | Tue, 16 Jul 2019 23:42:58 +0530 |
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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@linaro.org> 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.
> >> 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 > > >
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