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SubjectRe: [RFC PATCH] driver core: make deferring probe forever optional
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

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