linux-kernel - Re: [PATCH v3 1/5] drm/dp: add connector backpointer to drm_dp

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Date:   Mon, 9 Jan 2017 11:43:41 -0500
From:   Sean Paul <seanpaul@...omium.org>
To:     Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Thierry Reding <thierry.reding@...il.com>,
        intel-gfx-trybot@...ts.freedesktop.org,
        Emil Velikov <emil.l.velikov@...il.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH v3 1/5] drm/dp: add connector backpointer to drm_dp_aux

On Mon, Jan 9, 2017 at 4:03 AM, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
> On 6 January 2017 at 16:56, Sean Paul <seanpaul@...omium.org> wrote:
>> On Fri, Jan 6, 2017 at 9:30 AM, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
>>> This backpointer allows DP helpers to access the connector it's being
>>> used for.
>>>
>>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
>>> ---
>>>
>>>  include/drm/drm_dp_helper.h | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
>>> index 55bbeb0ff594..4fa77b434594 100644
>>> --- a/include/drm/drm_dp_helper.h
>>> +++ b/include/drm/drm_dp_helper.h
>>> @@ -721,6 +721,7 @@ struct drm_dp_aux_msg {
>>>   * @name: user-visible name of this AUX channel and the I2C-over-AUX adapter
>>>   * @ddc: I2C adapter that can be used for I2C-over-AUX communication
>>>   * @dev: pointer to struct device that is the parent for this AUX channel
>>> + * @connector: backpointer to connector that uses this AUX channel
>>>   * @hw_mutex: internal mutex used for locking transfers
>>>   * @transfer: transfers a message representing a single AUX transaction
>>>   *
>>> @@ -757,6 +758,7 @@ struct drm_dp_aux {
>>>         const char *name;
>>>         struct i2c_adapter ddc;
>>>         struct device *dev;
>>> +       struct drm_connector *connector;
>>
>> Perhaps I'm misreading the series, but it seems like you could instead
>> add int crc_pipe along with the other crc fields. Then when you start
>> the crc, set the pipe number. If you have the pipe in the crc worker,
>> you can wait vblank on that pipe (no pointers needed).
>>
>> Reasonable?
>
> I think that would work fine, but is it any better? I like that the
> connector isn't going to change during the lifetime of the drm_dp_aux,

Is this actually guaranteed, though? I am having a hard time thinking
about a practical example where it's not implemented this way, but I
don't see anything actually enforcing the lifetime relationship.

> but crc_pipe could be changing between sampling sessions and any bugs
> in keeping that field updated would be quite disconcerting and
> cumbersome to debug.

Couldn't the same could be said for connector->state->crtc?

Sean

>
> crc_pipe's advantage is that we wouldn't need to update all callers of
> drm_dp_aux_register, but I think it's better to have a connector field
> that is mistakenly NULL, than a crc_pipe field with the wrong value.
>
> Regards,
>
> Tomeu
>
>> Sean
>>
>>>         struct mutex hw_mutex;
>>>         ssize_t (*transfer)(struct drm_dp_aux *aux,
>>>                             struct drm_dp_aux_msg *msg);
>>> --
>>> 2.9.3
>>>
>>
>>
>>
>> --
>> Sean Paul, Software Engineer, Google / Chromium OS



-- 
Sean Paul, Software Engineer, Google / Chromium OS