Re: [PATCH 4/4] drm/panel: Add helper for simple panel connector

Noralf Trønnes
2016-05-06T19:45:52Z

Den 06.05.2016 16:43, skrev Thierry Reding:
> On Fri, May 06, 2016 at 04:34:08PM +0200, Noralf Trønnes wrote:
>> Den 06.05.2016 16:15, skrev Thierry Reding:
>>> On Fri, May 06, 2016 at 04:08:16PM +0200, Daniel Vetter wrote:
>>>> On Fri, May 06, 2016 at 04:03:47PM +0200, Thierry Reding wrote:
>>>>> On Thu, May 05, 2016 at 03:24:34PM +0200, Noralf Trønnes wrote:
>>>>>> Add function to create a simple connector for a panel.
>>>>> I'm not sure I see the usefulness of this. Typically you'd attach a
>>>>> panel to an encoder/connector, in which case you already have the
>>>>> connector.
>>>>>
>>>>> Perhaps it would become more obvious why we need this if you posted
>>>>> patches that show where this is used?
>>>> The other helpers give you a simple drm pipeline with plane, crtc &
>>>> encoder all baked into on drm_simple_pipeline structure. The only thing
>>>> variable you have to hook up to that is the drm_connector. And I think for
>>>> dead-simple panels avoiding the basic boilerplate in that does indeed make
>>>> some sense.
>>> Avoiding boilerplate is good, but I have a difficult time envisioning
>>> how you might want to use this. At the same time I'm asking myself how
>>> we know that this helper is any good if we haven't seen it used anywhere
>>> and actually see the boilerplate go away.
>> I pulled out the patches from the tinydrm patchset that would go
>> into drm core and helpers. I'm not very good at juggling many patches
>> around in various version and getting it right.
>> I'm doing development in the downstream Raspberry Pi repo
>> on 4.5 to get all the pi drivers, and then apply it on linux-next...
>>
>> This is the tinydrm patch that will use it:
>> https://lists.freedesktop.org/archives/dri-devel/2016-April/104502.html
>>
>> Extract:
>> +int tinydrm_display_pipe_init(struct tinydrm_device *tdev,
>> + const uint32_t *formats, unsigned int format_count)
>> +{
>> + struct drm_device *dev = tdev->base;
>> + struct drm_connector *connector;
>> + int ret;
>> +
>> + connector = drm_simple_kms_panel_connector_create(dev, &tdev->panel,
>> + DRM_MODE_CONNECTOR_VIRTUAL);
>> + if (IS_ERR(connector))
>> + return PTR_ERR(connector);
>> +
>> + ret = drm_simple_display_pipe_init(dev, &tdev->pipe,
>> + &tinydrm_display_pipe_funcs,
>> + formats, format_count,
>> + connector);
>> +
>> + return ret;
>> +}
>> +EXPORT_SYMBOL(tinydrm_display_pipe_init);
>>
>> drm_simple_kms_panel_connector_create() changed name when Daniel
>> suggested it be moved to drm_panel.c or drm_panel_helper.c.
> Okay, that gives me a better understanding of where things are headed.
> That said, why would these devices even need to deal with drm_panel in
> the first place? Sounds to me like they are devices on some sort of
> control bus that you talk to directly. And they will represent
> essentially the panel with its built-in display controller.
>
> drm_panel on the other hand was designed as an interface between display
> controllers and panels, with the goal to let any display controller talk
> to any panel.
>
> While I'm sure you can support these types of simple panels using the
> drm_panel infrastructure I'm not sure it's necessary, since your driver
> will always talk to the panel directly, and hence the code to deal with
> the panel specifics could be part of the display pipe functions.

In the discussion following the "no more fbdev drivers" call, someone
pointed me to drm_panel. It had function hooks that I needed, so I built
tinydrm around it. If this is misusing drm_panel, it shouldn't be too
difficult to drop it.

Actually I could just do this:

struct tinydrm_funcs {
int (*prepare)(struct tinydrm_device *tdev);
int (*unprepare)(struct tinydrm_device *tdev);
int (*enable)(struct tinydrm_device *tdev);
int (*disable)(struct tinydrm_device *tdev);
int (*dirty)(struct drm_framebuffer *fb, void *vmem, unsigned
flags,
unsigned color, struct drm_clip_rect *clips,
unsigned num_clips);
};

When it comes to the simple connector, one solution could be to extend
drm_simple_display_pipe to embed a connector with (optional) modes:

struct drm_simple_display_pipe {
- struct drm_connector *connector;
+ struct drm_connector connector;
+ const struct drm_display_mode *modes;
+ unsigned int num_modes;
};

And maybe optional detect and get_modes hooks:

struct drm_simple_display_pipe_funcs {
+ enum drm_connector_status detect(struct drm_connector *connector,
+ bool force);
+ int (*get_modes)(struct drm_connector *connector);
};

int drm_simple_display_pipe_init(struct drm_device *dev,
struct drm_simple_display_pipe *pipe,
struct drm_simple_display_pipe_funcs
*funcs,
const uint32_t *formats,
unsigned int format_count,
- struct drm_connector *connector);
+ int connector_type);

Another solution is to create a simple connector with modes:

struct drm_simple_connector {
struct drm_connector connector;
const struct drm_display_mode *modes;
unsigned int num_modes;
};

struct drm_connector *drm_simple_connector_create(struct drm_device *dev,
const struct drm_display_mode *modes, unsigned int num_modes,
int connector_type);

Noralf.