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    SubjectRe: Shared i2c adapter locking (Was: linux-next: manual merge of the net tree with the i2c tree)
    On Thu, 2009-10-29 at 15:43 +0100, Jean Delvare wrote:
    > Hi Stephen,
    > On Mon, 26 Oct 2009 13:37:57 +1100, Stephen Rothwell wrote:
    > > Today's linux-next merge of the net tree got a conflict in
    > > drivers/net/sfc/sfe4001.c between commit
    > > 3f7c0648f727a6d5baf6117653e4001dc877b90b ("i2c: Prevent priority
    > > inversion on top of bus lock") from the i2c tree and commit
    > > c9597d4f89565b6562bd3026adbe6eac6c317f47 ("sfc: Merge sfe4001.c into
    > > falcon_boards.c") from the net tree.
    > >
    > > I have applied the following merge fixup patch (after removing
    > > drivers/net/sfc/sfe4001.c) and can carry it as necessary.
    > Thanks for fixing it. The core problem here IMHO is that the sfc
    > network driver touches i2c internals which it would rather leave alone.

    I'm just a little proud of having the idea that we could avoid using an
    I/O-expander on this board, but yes, the software side of this
    multiplexing is a hack.

    > This is the only driver I know of which does this.
    > I can think of 3 different ways to address the issue.
    > Method #1: add a public API to grab/release an I2C segment.
    > void i2c_adapter_lock(struct i2c_adapter *adapter)
    > {
    > rt_mutex_lock(&adapter->bus_lock);
    > }
    > void i2c_adapter_unlock(struct i2c_adapter *adapter)
    > {
    > rt_mutex_unlock(&adapter->bus_lock);
    > }
    > I'm not really sure if I have a preference yet, so please speak up if
    > you do.

    Indirect lock operations are a recipe for deadlock, and there doesn't
    seem to be any other user for this, so method 1 seems best.


    Ben Hutchings, Senior Software Engineer, Solarflare Communications
    Not speaking for my employer; that's the marketing department's job.
    They asked us to note that Solarflare product names are trademarked.

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