Hi Robin,
On Tue, Mar 13, 2018 at 6:19 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> On 13/03/18 09:55, Vivek Gautam wrote:
>>
>> On Tue, Mar 13, 2018 at 3:10 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
>> wrote:
>>>
>>> On Tuesday, March 13, 2018 9:55:30 AM CET Vivek Gautam wrote:
>>>>
>>>> The lists managing the device-links can be traversed to
>>>> find the link between two devices. The device_link_add() APIs
>>>> does traverse these lists to check if there's already a link
>>>> setup between the two devices.
>>>> So, add a new APIs, device_link_find(), to find an existing
>>>> device link between two devices - suppliers and consumers.
>>>>
>>>> Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
>>>> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
>>>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>> ---
>>>>
>>>> * New patch added to this series.
>>>>
>>>> drivers/base/core.c | 30 +++++++++++++++++++++++++++---
>>>> include/linux/device.h | 2 ++
>>>> 2 files changed, 29 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/base/core.c b/drivers/base/core.c
>>>> index 5847364f25d9..e8c9774e4ba2 100644
>>>> --- a/drivers/base/core.c
>>>> +++ b/drivers/base/core.c
>>>> @@ -144,6 +144,30 @@ static int device_reorder_to_tail(struct device
>>>> *dev, void *not_used)
>>>> return 0;
>>>> }
>>>>
>>>> +/**
>>>> + * device_link_find - find any existing link between two devices.
>>>> + * @consumer: Consumer end of the link.
>>>> + * @supplier: Supplier end of the link.
>>>> + *
>>>> + * Returns pointer to the existing link between a supplier and
>>>> + * and consumer devices, or NULL if no link exists.
>>>> + */
>>>> +struct device_link *device_link_find(struct device *consumer,
>>>> + struct device *supplier)
>>>> +{
>>>> + struct device_link *link = NULL;
>>>> +
>>>> + if (!consumer || !supplier)
>>>> + return NULL;
>>>> +
>>>> + list_for_each_entry(link, &supplier->links.consumers, s_node)
>>>> + if (link->consumer == consumer)
>>>> + break;
>>>> +
>>>
>>>
>>> Any mutual exclusion?
>>>
>>> Or is the caller expected to take care of it? And if so, then how?
>>
>>
>> I think it's better that we take care of lock here in the code rather
>> than depending
>> on the caller.
>> But i can't take device_links_write_lock() since device_link_add()
>> already takes that.
>
>
> Well, the normal pattern is to break out the internal helper function as-is,
> then add a public wrapper which validates inputs, handles locking, etc.,
> equivalently to existing caller(s). See what device_link_del() and others
> do, e.g.:
>
> static struct device_link *__device_link_find(struct device *consumer,
> struct device *supplier)
> {
> list_for_each_entry(link, &supplier->links.consumers, s_node)
> if (link->consumer == consumer)
> return link;
> return NULL;
> }
>
> struct device_link *device_link_find(struct device *consumer,
> struct device *supplier)
> {
> struct device_link *link;
>
> if (!consumer || !supplier)
> return NULL;
>
> device_links_write_lock();
> link = __device_link_find(consumer, supplier);
> device_links_write_unlock();
> return link;
> }
>
> where device_link_add() would call __device_link_find() directly.
Right, I understand it now. Thanks for detailed explanation.
regards
Vivek
>
> However, as Tomasz points out (and I hadn't really considered), if the only
> reasonable thing to with a link once you've found it is to delete it, then
> in terms of the public API it may well make more sense to just implement
> something like a device_link_remove() which does both in one go.
>
> Robin.
>
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