linux-kernel - Re: [PATCH v1] of: property: fw_devlink: Add support for remote-endpoint

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Date:   Tue, 30 Mar 2021 12:36:06 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Frank Rowand <frowand.list@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>
Cc:     kernel-team@...roid.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] of: property: fw_devlink: Add support for remote-endpoint

Quoting Saravana Kannan (2021-03-30 11:50:55)
> remote-endpoint property seems to always come in pairs where two devices
> point to each other. So, we can't really tell from DT if there is a
> functional probe order dependency between these two devices.
> 
> However, there can be other dependencies between two devices that point
> to each other with remote-endpoint. This non-remote-endpoint dependency
> combined with one of the remote-endpoint dependency can lead to a cyclic
> dependency[1].
> 
> To avoid this cyclic dependency from incorrectly blocking probes,
> fw_devlink needs to be made aware of remote-endpoint dependencies even
> though remote-endpoint dependencies by themselves won't affect probe
> ordering (because fw_devlink will see the cyclic dependency between
> remote-endpoint devices and ignore the dependencies that cause the
> cycle).
> 
> Also, if a device ever needs to know if a non-probe-blocking
> remote-endpoint has finished probing, it can now use the sync_state() to
> figure it out.
> 
> [1] - https://lore.kernel.org/lkml/CAGETcx9Snf23wrXqjDhJiTok9M3GcoVYDSyNYSMj9QnSRrA=cA@mail.gmail.com/#t
> Fixes: ea718c699055 ("Revert "Revert "driver core: Set fw_devlink=on by default""")
> Reported-by: Stephen Boyd <swboyd@...omium.org>
> Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> ---

Tested-by: Stephen Boyd <swboyd@...omium.org>

> diff --git a/drivers/of/property.c b/drivers/of/property.c
> index 5036a362f52e..2bb3158c9e43 100644
> --- a/drivers/of/property.c
> +++ b/drivers/of/property.c
> @@ -1225,6 +1230,8 @@ static struct device_node *parse_##fname(struct device_node *np,       \
>   * @parse_prop.prop_name: Name of property holding a phandle value
>   * @parse_prop.index: For properties holding a list of phandles, this is the
>   *                   index into the list
> + * @optional: The property can be an optional dependency.

This bit conflicted for me on linux-next today so I dropped it in favor
of 3915fed92365 ("of: property: Provide missing member description and
remove excess param").

> + * @node_not_dev: The consumer node containing the property is never a device.
>   *
>   * Returns:
>   * parse_prop() return values are