Hello Saravana, On 5/23/19 6:01 PM, Saravana Kannan wrote: ... > Having functional dependencies explicitly called out in DT and > automatically added before the devices are probed, provides the > following benefits: ... > - Supplier devices like clock providers, regulators providers, etc > need to keep the resources they provide active and at a particular > state(s) during boot up even if their current set of consumers don't > request the resource to be active. This is because the rest of the > consumers might not have probed yet and turning off the resource > before all the consumers have probed could lead to a hang or > undesired user experience. This benefit provided by the sync_state() callback function introduced in this series gives us a mechanism to solve a specific problem encountered on Qualcomm Technologies, Inc. (QTI) boards when booting with drivers compiled as modules. QTI boards have a regulator that powers the PHYs for display, camera, USB, UFS, and PCIe. When these boards boot up, the boot loader enables this regulator along with other resources in order to display a splash screen image. The regulator must remain enabled until the Linux display driver has probed and made a request with the regulator framework to keep the regulator enabled. If the regulator is disabled prematurely, then the screen image is corrupted and the display hardware enters a bad state. We have observed that when the camera driver probes before the display driver, it performs this sequence: regulator_enable(), camera register IO, regulator_disable(). Since it is the first consumer of the shared regulator, the regulator is physically disabled (even though display hardware still requires it to be enabled). We have solved this problem when compiling drivers statically with a downstream regulator proxy-consumer driver. This proxy-consumer is able to make an enable request for the shared regulator before any other consumer. It then removes its request at late_initcall_sync. Unfortunately, when drivers are compiled as modules instead of compiled statically into the kernel image, late_initcall_sync is not a meaningful marker of driver probe completion. This means that our existing proxy voting system will not work when drivers are compiled as modules. The sync_state() callback resolves this issue by providing a notification that is guaranteed to arrive only after all consumers of the shared regulator have probed. QTI boards have other cases of shared resources such as bus bandwidth which must remain at least at a level set by boot loaders in order to properly support hardware blocks that are enabled before the Linux kernel starts booting. Take care, David -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
