On Wed, Jun 09, 2021 at 05:54:38PM +0100, Jon Hunter wrote: > > On 09/06/2021 17:30, Bjorn Helgaas wrote: > > On Wed, Jun 09, 2021 at 12:52:37AM +0530, Vidya Sagar wrote: > >> Hi, > >> I would like to know what is the use of pcie-designware-plat.c file. This > >> looks like a skeleton file and can't really work with any specific hardware > >> as such. > >> Some context for this mail thread is, if the config CONFIG_PCIE_DW_PLAT is > >> enabled in a system where a Synopsys DesignWare IP based PCIe controller is > >> present and its configuration is enabled (Ex:- Tegra194 system with > >> CONFIG_PCIE_TEGRA194_HOST enabled), then, it can so happen that the probe of > >> pcie-designware-plat.c called first (because all DWC based PCIe controller > >> nodes have "snps,dw-pcie" compatibility string) and can crash the system. > > > > What's the crash? If a device claims to be compatible with > > "snps,dw-pcie" and pcie-designware-plat.c claims to know how to > > operate "snps,dw-pcie" devices, it seems like something is wrong. > > > > "snps,dw-pcie" is a generic device type, so pcie-designware-plat.c > > might not know how to operate device-specific details of some of those > > devices, but basic functionality should work and it certainly > > shouldn't crash. > > It is not really a crash but a hang when trying to access the hardware > before it has been properly initialised. This doesn't really answer my question. If the hardware claims to be compatible with "snps,dw-pcie" and a driver knows how to operate "snps,dw-pcie" devices, it should work. If the hardware requires initialization that is not part of the "snps,dw-pcie" programming model, it should not claim to be compatible with "snps,dw-pcie". Or, if pcie-designware-plat.c is missing some init that *is* part of the programming model, maybe it needs to be enhanced? > The scenario I saw was that if the Tegra194 PCIe driver was built as a > module but the pcie-designware-plat.c was built into the kernel, then on > boot we would attempt to probe the pcie-designware-plat.c driver because > module was not available yet and this would hang. Hence, I removed the > "snps,dw-pcie" compatible string for Tegra194 to avoid this and ONLY > probe the Tegra194 PCIe driver. Maybe something like driver_override (I know this is supported via sysfs, but maybe not via a kernel parameter) or a module parameter for pcie-designware-plat.c to keep it from claiming devices? > Sagar is wondering why this hang is only seen/reported for Tegra and > could this happen to other platforms? I think that is potentially could. Maybe pcie-designware-plat.c works on other platforms, i.e., they don't require the hardware init?
