On Wed, Oct 28, 2020 at 7:51 AM Robin Murphy <robin.murphy@xxxxxxx> wrote: > Hmm, perhaps I'm missing something here, but even if the config options > *do* line up, what prevents arm-smmu probing before qcom-scm and > dereferencing NULL in qcom_scm_qsmmu500_wait_safe_toggle() before __scm > is initialised? Oh man, this spun me on a "wait, but how does it all work!" trip. :) So in the non-module case, the qcom_scm driver is a subsys_initcall and the arm-smmu is a module_platform_driver, so the ordering works out. In the module case, the arm-smmu code isn't loaded until the qcom_scm driver finishes probing due to the symbol dependency handling. To double check this, I added a big msleep at the top of the qcom_scm_probe to try to open the race window you described, but the arm_smmu_device_probe() doesn't run until after qcom_scm_probe completes. So at least as a built in / built in, or a module/module case its ok. And in the case where arm-smmu is a module and qcom_scm is built in that's ok too. Its just the case my patch is trying to prevent is where arm-smmu is built in, but qcom_scm is a module that it can't work (due to build errors in missing symbols, or if we tried to use function pointers to plug in the qcom_scm - the lack of initialization ordering). Hopefully that addresses your concern? Let me know if I'm still missing something. thanks -john
