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From:   Vivek Gautam <vivek.gautam@codeaurora.org>
Date:   Tue, 13 Mar 2018 15:25:18 +0530
Message-ID: <CAFp+6iE=0PD6fZZVd+bC1Z95Lkt=X-F4YOXz8EtdiXL=jsC1RA@mail.gmail.com>
Subject: Re: [PATCH v9 1/5] driver core: Find an existing link between two devices
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <joro@8bytes.org>,
        "robh+dt" <robh+dt@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <iommu@lists.linux-foundation.org>,
        devicetree@vger.kernel.org,
        open list <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Rob Clark <robdclark@gmail.com>,
        Tomasz Figa <tfiga@chromium.org>,
        Sricharan R <sricharan@codeaurora.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Archit Taneja <architt@codeaurora.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Tue, Mar 13, 2018 at 3:10 PM, Rafael J. Wysocki <rjw@rjwysocki.net> 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@codeaurora.org>
>> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> ---
>>
>>  * 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.

regards
Vivek

>
>> +     return link;
>> +}
>> +EXPORT_SYMBOL_GPL(device_link_find);
>> +
>>  /**
>>   * device_link_add - Create a link between two devices.
>>   * @consumer: Consumer end of the link.
>> @@ -195,9 +219,9 @@ struct device_link *device_link_add(struct device *consumer,
>>               goto out;
>>       }
>>
>> -     list_for_each_entry(link, &supplier->links.consumers, s_node)
>> -             if (link->consumer == consumer)
>> -                     goto out;
>> +     link = device_link_find(consumer, supplier);
>> +     if (link)
>> +             goto out;
>>
>>       link = kzalloc(sizeof(*link), GFP_KERNEL);
>>       if (!link)
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index b093405ed525..13bc1884c3eb 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -1278,6 +1278,8 @@ extern const char *dev_driver_string(const struct device *dev);
>>  struct device_link *device_link_add(struct device *consumer,
>>                                   struct device *supplier, u32 flags);
>>  void device_link_del(struct device_link *link);
>> +struct device_link *device_link_find(struct device *consumer,
>> +                                  struct device *supplier);
>>
>>  #ifdef CONFIG_PRINTK
>>
>>
>
>
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