Re: [PATCH v3 06/33] drm: Minimally initialise drm_dp_aux

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Wolfram Sang <wsa@the-dreams.de>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	linux-i2c@vger.kernel.org, Dave Airlie <airlied@redhat.com>
Subject: Re: [PATCH v3 06/33] drm: Minimally initialise drm_dp_aux
Date: Fri, 3 Jun 2016 17:59:11 +0300	[thread overview]
Message-ID: <20160603145911.GZ4329@intel.com> (raw)
In-Reply-To: <1464964636-3877-7-git-send-email-chris@chris-wilson.co.uk>

On Fri, Jun 03, 2016 at 03:36:49PM +0100, Chris Wilson wrote:
> When trying to split up the initialisation phase and the registration
> phase, one immediate problem encountered is trying to use our own i2c
> devices before registration with userspace (to read EDID during device
> discovery). drm_dp_aux in particular only offers an interface for setting
> up the device *after* we have exposed the connector via sysfs. In order
> to break the chicken-and-egg problem, export drm_dp_aux_init() to
> minimally prepare the i2c device for internal use before
> drm_connector_register().
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Rafael Antognolli <rafael.antognolli@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: dri-devel@lists.freedesktop.org
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 26 +++++++++++++++++++++-----
>  include/drm/drm_dp_helper.h     |  1 +
>  2 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 4b088afa21b2..9b4ec65e1de6 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -791,15 +791,16 @@ static void unlock_bus(struct i2c_adapter *i2c, unsigned int flags)
>  }
>  
>  /**
> - * drm_dp_aux_register() - initialise and register aux channel
> + * drm_dp_aux_init() - minimally initialise an aux channel
>   * @aux: DisplayPort AUX channel
>   *
> - * Returns 0 on success or a negative error code on failure.
> + * If you need to use the drm_dp_aux's i2c adapter prior to registering it
> + * with the outside world, call drm_dp_aux_init() first. You must still
> + * call drm_dp_aux_register() once the connector has been registered to
> + * allow userspace access to the auxiliary DP channel.
>   */
> -int drm_dp_aux_register(struct drm_dp_aux *aux)
> +void drm_dp_aux_init(struct drm_dp_aux *aux)
>  {
> -	int ret;
> -
>  	mutex_init(&aux->hw_mutex);
>  
>  	aux->ddc.algo = &drm_dp_i2c_algo;
> @@ -809,6 +810,21 @@ int drm_dp_aux_register(struct drm_dp_aux *aux)
>  	aux->ddc.lock_bus = lock_bus;
>  	aux->ddc.trylock_bus = trylock_bus;
>  	aux->ddc.unlock_bus = unlock_bus;
> +}
> +EXPORT_SYMBOL(drm_dp_aux_init);

This doesn't feel very safe to me. To me it looks like the i2c core
wasn't designed to have the adapter be used before i2c_add_adapter()
is called. I guess it might work in this case since you provide your
own lock vfuncs.

I think someone should fix the i2c core to split i2c_add_adapter()
& co. into init and register phases. Cc:ing i2c folks...

> +
> +/**
> + * drm_dp_aux_register() - initialise and register aux channel
> + * @aux: DisplayPort AUX channel
> + *
> + * Returns 0 on success or a negative error code on failure.
> + */
> +int drm_dp_aux_register(struct drm_dp_aux *aux)
> +{
> +	int ret;
> +
> +	if (!aux->ddc.algo)
> +		drm_dp_aux_init(aux);
>  
>  	aux->ddc.class = I2C_CLASS_DDC;
>  	aux->ddc.owner = THIS_MODULE;
> diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> index 5a848e734422..4d85cf2874af 100644
> --- a/include/drm/drm_dp_helper.h
> +++ b/include/drm/drm_dp_helper.h
> @@ -805,6 +805,7 @@ int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
>  int drm_dp_link_power_down(struct drm_dp_aux *aux, struct drm_dp_link *link);
>  int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
>  
> +void drm_dp_aux_init(struct drm_dp_aux *aux);
>  int drm_dp_aux_register(struct drm_dp_aux *aux);
>  void drm_dp_aux_unregister(struct drm_dp_aux *aux);
>  
> -- 
> 2.8.1

-- 
Ville Syrjälä
Intel OTC
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