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

[Zev Weiss]

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@xxxxxxxxxxxxxxxxx>
> > ---
> >  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@xxxxxxxxxxxxxx/T/#u).

> >  			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?


Thanks,
Zev