On Tue, May 01, 2018 at 04:31:14PM -0500, Rob Herring wrote:
Deferred probe will currently wait forever on dependent devices to probe, but sometimes a driver will never exist. It's also not always critical for a driver to exist. Platforms can rely on default configuration from the bootloader or reset defaults for things such as pinctrl and power domains. This is often the case with initial platform support until various drivers get enabled. There's at least 2 scenarios where deferred probe can render a platform broken. Both involve using a DT which has more devices and dependencies than the kernel supports. The 1st case is a driver may be disabled in the kernel config. The 2nd case is the kernel version may simply not have the dependent driver. This can happen if using a newer DT (provided by firmware perhaps) with a stable kernel version.
Unfortunately, this change breaks with modules as we have no way of knowing when modules are done loading. One possibility is to make this opt in or out based on compatible strings rather than at a subsystem level. Ideally this information could be extracted automatically somehow. OTOH, maybe the lists are pretty small. There's only a handful of subsystems that can be optional, and then only so many drivers in those that can be modules (at least for pinctrl, many drivers are built-in only).
Cc: Alexander Graf agraf@suse.de Signed-off-by: Rob Herring robh@kernel.org
This patch came out of a discussion on the ARM boot-architecture list[1] about DT forwards and backwards compatibility issues. There are issues with newer DTs breaking on older, stable kernels. Some of these are difficult to solve, but cases of optional devices not having kernel support should be solvable.
I tested this on a RPi3 B with the pinctrl driver forced off. With this change, the MMC/SD and UART drivers can function without the pinctrl driver.
Rob
[1] https://lists.linaro.org/pipermail/boot-architecture/2018-April/000466.html
drivers/base/dd.c | 16 ++++++++++++++++ drivers/pinctrl/devicetree.c | 2 +- include/linux/device.h | 2 ++ 3 files changed, 19 insertions(+), 1 deletion(-)
diff --git a/drivers/base/dd.c b/drivers/base/dd.c index c9f54089429b..5848808b9d7a 100644 --- a/drivers/base/dd.c +++ b/drivers/base/dd.c @@ -226,6 +226,15 @@ void device_unblock_probing(void) driver_deferred_probe_trigger(); }
+int driver_deferred_probe_optional(void) +{
- if (initcalls_done)
return -ENODEV;- return -EPROBE_DEFER;
+}
The name is ackward for this function, but I can't think of anything better at the moment, sorry. However, the overall idea for this is sane, no objection from me at all for this change.
thanks,
greg k-h