Re: [PATCH 1/5] of: base: add function to check for status = "reserved"

Greg Kroah-Hartman
2021-10-22T07:45:35Z

On Fri, Oct 22, 2021 at 12:38:40AM -0700, Zev Weiss wrote:
> On Thu, Oct 21, 2021 at 11:43:23PM PDT, Greg Kroah-Hartman wrote:
> > On Thu, Oct 21, 2021 at 07:00:28PM -0700, Zev Weiss wrote:
> > > Per v0.3 of the Devicetree Specification [0]:
> > >
> > > Indicates that the device is operational, but should not be used.
> > > Typically this is used for devices that are controlled by another
> > > software component, such as platform firmware.
> > >
> > > One use-case for this is in OpenBMC, where certain devices (such as a
> > > BIOS flash chip) may be shared by the host and the BMC, but cannot be
> > > accessed by the BMC during its usual boot-time device probing, because
> > > they require additional (potentially elaborate) coordination with the
> > > host to arbitrate which processor is controlling the device.
> > >
> > > Devices marked with this status should thus be instantiated, but not
> > > have a driver bound to them or be otherwise touched.
> > >
> > > [0] https://github.com/devicetree-org/devicetree-specification/releases/download/v0.3/devicetree-specification-v0.3.pdf
> > >
> > > Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
> > > ---
> > > drivers/of/base.c | 56 +++++++++++++++++++++++++++++++++++++++-------
> > > include/linux/of.h | 6 +++++
> > > 2 files changed, 54 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/of/base.c b/drivers/of/base.c
> > > index 0ac17256258d..3bd7c5b8a2cc 100644
> > > --- a/drivers/of/base.c
> > > +++ b/drivers/of/base.c
> > > @@ -580,14 +580,16 @@ int of_machine_is_compatible(const char *compat)
> > > EXPORT_SYMBOL(of_machine_is_compatible);
> > >
> > > /**
> > > - * __of_device_is_available - check if a device is available for use
> > > + * __of_device_check_status - check if a device's status matches a particular string
> > > *
> > > - * @device: Node to check for availability, with locks already held
> > > + * @device: Node to check status of, with locks already held
> > > + * @val: Status string to check for, or NULL for "okay"/"ok"
> > > *
> > > - * Return: True if the status property is absent or set to "okay" or "ok",
> > > - * false otherwise
> > > + * Return: True if status property exists and matches @val, or either "okay"
> > > + * or "ok" if @val is NULL, or if status property is absent and @val is
> > > + * "okay", "ok", or NULL. False otherwise.
> > > */
> > > -static bool __of_device_is_available(const struct device_node *device)
> > > +static bool __of_device_check_status(const struct device_node *device, const char *val)
> > > {
> > > const char *status;
> > > int statlen;
> > > @@ -596,17 +598,35 @@ static bool __of_device_is_available(const struct device_node *device)
> > > return false;
> > >
> > > status = __of_get_property(device, "status", &statlen);
> > > - if (status == NULL)
> > > - return true;
> > > + if (!status) {
> > > + /* a missing status property is treated as "okay" */
> > > + status = "okay";
> > > + statlen = strlen(status) + 1; /* property lengths include the NUL terminator */
> > > + }
> > >
> > > if (statlen > 0) {
> > > - if (!strcmp(status, "okay") || !strcmp(status, "ok"))
> > > + if (!val && (!strcmp(status, "okay") || !strcmp(status, "ok")))
> > > + return true;
> > > + else if (val && !strcmp(status, val))
> >
> >
> > Ick, where is this string coming from? The kernel or userspace or a
> > device tree? This feels very wrong, why is the kernel doing parsing
> > like this of different options that all mean the same thing?
> >
>
> Which string do you mean by "this string"? 'status' comes from a property
> of the device tree node; 'val' will be one of a small set of string
> constants passed by the caller. Accepting either spelling of "okay"/"ok"
> has been in place since 2008 (commit 834d97d45220, "[POWERPC] Add
> of_device_is_available function"); using NULL as a shorthand for those two
> strings was a suggestion that came up in review feedback on a previous
> incarnation of these patches (https://lore.kernel.org/openbmc/CAL_Jsq+rKGv39zHTxNx0A7=X4K48nXRLqWonecG5SobdJq3yKw@mail.gmail.com/T/#u).

I was referring to "okay". And if this really is a "we take either"
type of thing, shouldn't there be a single function to call for this
type of test, much like we have some of the sysfs helpers?

And what about using match_string() as well?

> > > return true;
> > > }
> > >
> > > return false;
> > > }
> > >
> > > +/**
> > > + * __of_device_is_available - check if a device is available for use
> > > + *
> > > + * @device: Node to check for availability, with locks already held
> > > + *
> > > + * Return: True if the status property is absent or set to "okay" or "ok",
> > > + * false otherwise
> > > + */
> > > +static bool __of_device_is_available(const struct device_node *device)
> > > +{
> > > + return __of_device_check_status(device, NULL);
> > > +}
> > > +
> > > /**
> > > * of_device_is_available - check if a device is available for use
> > > *
> > > @@ -628,6 +648,26 @@ bool of_device_is_available(const struct device_node *device)
> > > }
> > > EXPORT_SYMBOL(of_device_is_available);
> > >
> > > +/**
> > > + * of_device_is_reserved - check if a device is marked as reserved
> > > + *
> > > + * @device: Node to check for reservation
> > > + *
> > > + * Return: True if the status property is set to "reserved", false otherwise
> > > + */
> > > +bool of_device_is_reserved(const struct device_node *device)
> > > +{
> > > + unsigned long flags;
> > > + bool res;
> > > +
> > > + raw_spin_lock_irqsave(&devtree_lock, flags);
> > > + res = __of_device_check_status(device, "reserved");
> > > + raw_spin_unlock_irqrestore(&devtree_lock, flags);
> >
> > Why is this a "raw" spinlock?
> >
>
> devtree_lock being a raw_spinlock_t appears to date from commit d6d3c4e65651
> ("OF: convert devtree lock from rw_lock to raw spinlock"); "required for
> preempt-rt", according to Thomas Gleixner's commit message.
>
> > Where is this status coming from?
> >
>
> This would be specified in a DT node, e.g. via something like:
>
> &somedev {
> compatible = "foobar";
> status = "reserved";
> /* ... */
> };
>
> > > +
> > > + return res;
> > > +}
> > > +EXPORT_SYMBOL(of_device_is_reserved);
> >
> > EXPORT_SYMBOL_GPL()?
> >
>
> Its closest existing sibling, of_device_is_available(), is a plain
> EXPORT_SYMBOL(); if we want to convert things more broadly that'd be fine
> with me, but having one be GPL-only and the other not seems like it'd be a
> bit confusing and inconsistent?

Ah, ok, you are following the rest of this file for this, and the
locking stuff, sorry, I was not familiar with it.

thanks,

greg k-h