Re: [PATCH] Revert "drm/panel: Add device_link from panel device to DRM device"

[Sean Paul <sean@poorly.run>]

[actually Cc Eric and Andrzej]

On Thu, Sep 27, 2018 at 02:41:30PM +0200, Linus Walleij wrote:
> This reverts commit 0c08754b59da5557532d946599854e6df28edc22.
> 
> commit 0c08754b59da
> ("drm/panel: Add device_link from panel device to DRM device")
> creates a circular dependency under these circumstances:
> 
> 1. The panel depends on dsi-host because it is MIPI-DSI child
>    device.
> 2. dsi-host depends on the drm parent device (connector->dev->dev)
>    this should be allowed.
> 3. drm parent dev (connector->dev->dev) depends on the panel
>    after this patch.
> 
> This makes the dependency circular and while it appears it
> does not affect any in-tree drivers (they do not seem to have
> dsi hosts depending on the same parent device) this does not
> seem right.

Hey Linus,
I'm trying to wrap my head around this :-)

I just read through the original patch thread. It doesn't seem like this was
introduced to fix a bug? Will reverting this cause regressions on in-tree
drivers?

What's different in your in-development driver that's causing you to hit
this?

Thanks!

Sean

> 
> As noted in a response from Andrzej Hajda, the intent is
> likely to make the panel dependent on the DRM device
> (connector->dev) not its parent. But we have no way of
> doing that since the DRM device doesn't contain any
> struct device on its own (arguably it should).
> 
> Revert this until a proper approach is figured out.
> 
> Cc: Jyri Sarha <jsarha@ti.com>
> Cc: Eric Anholt <eric@anholt.net>
> Cc: Andrzej Hajda <a.hajda@samsung.com>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  drivers/gpu/drm/drm_panel.c | 10 ----------
>  include/drm/drm_panel.h     |  1 -
>  2 files changed, 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
> index b902361dee6e..1d9a9d2fe0e0 100644
> --- a/drivers/gpu/drm/drm_panel.c
> +++ b/drivers/gpu/drm/drm_panel.c
> @@ -24,7 +24,6 @@
>  #include <linux/err.h>
>  #include <linux/module.h>
>  
> -#include <drm/drm_device.h>
>  #include <drm/drm_crtc.h>
>  #include <drm/drm_panel.h>
>  
> @@ -105,13 +104,6 @@ int drm_panel_attach(struct drm_panel *panel, struct drm_connector *connector)
>  	if (panel->connector)
>  		return -EBUSY;
>  
> -	panel->link = device_link_add(connector->dev->dev, panel->dev, 0);
> -	if (!panel->link) {
> -		dev_err(panel->dev, "failed to link panel to %s\n",
> -			dev_name(connector->dev->dev));
> -		return -EINVAL;
> -	}
> -
>  	panel->connector = connector;
>  	panel->drm = connector->dev;
>  
> @@ -133,8 +125,6 @@ EXPORT_SYMBOL(drm_panel_attach);
>   */
>  int drm_panel_detach(struct drm_panel *panel)
>  {
> -	device_link_del(panel->link);
> -
>  	panel->connector = NULL;
>  	panel->drm = NULL;
>  
> diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
> index 582a0ec0aa70..777814755fa6 100644
> --- a/include/drm/drm_panel.h
> +++ b/include/drm/drm_panel.h
> @@ -89,7 +89,6 @@ struct drm_panel {
>  	struct drm_device *drm;
>  	struct drm_connector *connector;
>  	struct device *dev;
> -	struct device_link *link;
>  
>  	const struct drm_panel_funcs *funcs;
>  
> -- 
> 2.17.1
> 
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-- 
Sean Paul, Software Engineer, Google / Chromium OS
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