[Skiboot] [PATCH 1/3] core/pci: Add for_each_pci_dev()

[Alistair Popple]

I reviewed this and I think I've convinced myself it's correct. That said it
isn't the most obvious code in the world although I couldn't think of a way to
simplify it further.

It does make a lot of other code far more obvious/straight forward so assuming
it's well tested (and I seem to recall Oliver tested pretty extensively) then I
think it's worth merging.

Reviewed-by: Alistair Popple <alistair at popple.id.au>

On Tue, 22 Aug 2017 06:15:41 PM Oliver O'Halloran wrote:
> Add an interation helper for PCI devices. This is functionally identical
> to the pci_walk_devs() helper function, but it allows us to write linear
> code rather than dropping us into the middle of callback hell.
> 
> Signed-off-by: Oliver O'Halloran <oohall at gmail.com>
> ---
>  core/pci.c    | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/pci.h |  9 +++++++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/core/pci.c b/core/pci.c
> index 4296180ad111..c5d2fba36508 100644
> --- a/core/pci.c
> +++ b/core/pci.c
> @@ -1798,6 +1798,90 @@ static struct pci_device *__pci_walk_dev(struct phb *phb,
>  	return NULL;
>  }
>  
> +/*
> + * pci_next_dev(phb, prev, pass) - Iteration worker function
> + *
> + * phb  - phb that we're iterating
> + * prev - the last pci_device returned by pci_next_dev
> + *
> + * pass - a cookie used to track the iteration state. See below if you
> + *        really want to know the details.
> + */
> +struct pci_device *pci_next_dev(struct phb *phb, struct pci_device *prev,
> +				int *pass)
> +{
> +	struct pci_device *next, *last;
> +	struct list_head *siblings;
> +
> +	assert(phb);
> +
> +	/* start of iteration */
> +	if (!prev) {
> +		*pass = 0;
> +		return list_top(&phb->devices, struct pci_device, link);
> +	}
> +
> +up:
> +	assert(prev);
> +
> +	if (prev->parent)
> +		siblings = &prev->parent->children;
> +	else
> +		siblings = &phb->devices;
> +
> +	last = list_tail(siblings, struct pci_device, link);
> +	assert(last);
> +
> +	/*
> +	 * You made it this far so you get an explaination!
> +	 *
> +	 * We need to ensure that the iteration order is correct, that is we are
> +	 * supposed to look at each *device* on a bus before looking at the
> +	 * *child buses* below those devices, as is done in pci_walk_devs(). In
> +	 * other words, we do two pases of every bus.
> +	 *
> +	 * The 'pass' variable is used to track which pass we are on. When it's
> +	 * even we are just visiting each device and when odd we are descending
> +	 * into child buses if we find any.
> +	 */
> +	if (prev == last) {
> +		if (*pass & 1) { /* end of the odd pass */
> +			if (!prev->parent) {
> +				assert(*pass == 1);
> +				return NULL;
> +			}
> +
> +			/* back to the odd pass across the parent's bus */
> +			(*pass) -= 2;
> +			prev = prev->parent;
> +			goto up;
> +		} else {
> +			next = list_top(siblings, struct pci_device, link);
> +			(*pass)++;
> +		}
> +	} else {
> +		next = list_entry(prev->link.next, struct pci_device, link);
> +	}
> +
> +	/* now find the device we should return */
> +	if (!(*pass & 1))
> +		return next;
> +
> +	/* for an odd pass we scan for another child bus to descend into */
> +	while (list_empty(&next->children)) {
> +		if (next == last) {
> +			prev = last;
> +			goto up;
> +		}
> +
> +		next = list_entry(next->link.next, struct pci_device, link);
> +	}
> +
> +	/* increment pass so it's even */
> +	(*pass)++;
> +	return list_top(&next->children, struct pci_device, link);
> +}
> +
>  struct pci_device *pci_walk_dev(struct phb *phb,
>  				struct pci_device *pd,
>  				int (*cb)(struct phb *,
> diff --git a/include/pci.h b/include/pci.h
> index 54a62fd19a99..81ad62f8fa19 100644
> --- a/include/pci.h
> +++ b/include/pci.h
> @@ -460,6 +460,15 @@ static inline struct phb *__pci_next_phb_idx(uint64_t *phb_id) {
>  	for (uint64_t __phb_idx = 0;				\
>  	     (phb = __pci_next_phb_idx(&__phb_idx)) ; )
>  
> +
> +struct pci_device *pci_next_dev(struct phb *phb, struct pci_device *pd,
> +				int *pass);
> +
> +/* pass should be an int *, see pci_next_dev() for details */
> +#define for_each_pci_dev(phb, pd, pass) \
> +	for (pd = pci_next_dev(phb, NULL, pass); pd; \
> +			pd = pci_next_dev(phb, pd, pass))
> +
>  /* Device tree */
>  extern void pci_std_swizzle_irq_map(struct dt_node *dt_node,
>  				    struct pci_device *pd,
>