Date:	Wed, 4 Jan 2012 11:43:48 +0530
From:	Thomas Abraham <thomas.abraham@...aro.org>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	rpurdie@...ys.net, linux-samsung-soc@...r.kernel.org,
	grant.likely@...retlab.ca, rob.herring@...xeda.com,
	kgene.kim@...sung.com, jg1.han@...sung.com,
	broonie@...nsource.wolfsonmicro.com, kyungmin.park@...sung.com,
	cbou@...l.ru, kwangwoo.lee@...il.com, augulis.darius@...il.com,
	ben-linux@...ff.org, patches@...aro.org
Subject: Re: [RFC][PATCH 0/4] lcd: platform-lcd: Add lcd panel and device tree support

Hi Lars,

On 4 January 2012 00:06, Lars-Peter Clausen <lars@...afoo.de> wrote:
> On 01/03/2012 06:07 PM, Thomas Abraham wrote:
>> On 3 January 2012 17:56, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>> On 01/03/2012 12:54 PM, Thomas Abraham wrote:
>>>> Hi Lars,
>>>>
>>>> On 3 January 2012 14:36, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>>>> On 01/02/2012 06:54 AM, Thomas Abraham wrote:
>>>>>> The platform-lcd driver depends on platform-specific callbacks to setup the
>>>>>> lcd panel. These callbacks are supplied using driver's platform data. But
>>>>>> for adding device tree support for platform-lcd driver, providing such
>>>>>> callbacks is not possible (without using auxdata).
>>>>>>
>>>>>> Since the callbacks are usually lcd panel specific, it is possible to include
>>>>>> the lcd panel specific setup and control functionality in the platform-lcd
>>>>>> driver itself, thereby eliminating the need for supplying platform specific
>>>>>> callbacks to the driver. The platform-lcd driver can include support for
>>>>>> multiple lcd panels.
>>>>>>
>>>>>> This patchset removes the need for platform data for platform-lcd driver and
>>>>>> adds support which can be used to implement lcd panel specific functionality
>>>>>> in the driver. As an example, the support for Hydis hv070wsa lcd panel is added
>>>>>> to the platform-lcd driver which is then used on the Exynos4 based Origen board.
>>>>>> This currently breaks build for other users of platform-lcd driver. Those can be
>>>>>> fixed if this approach is acceptable.
>>>>>
>>>>> The whole approach looks rather backwards to me. The exact purpose of the
>>>>> platform_lcd driver is to redirect the lcd driver callbacks to board code.
>>>>> So by removing this support you not only break all the existing driver but
>>>>> also create a driver which does nothing. Then you add another layer of
>>>>> abstraction to implement custom drivers in this driver. A better approach in
>>>>> my opinion is to simply implement these drivers as first level LCD drivers.
>>>>> So leave the platform-lcd driver as it is and just add a gpio (or maybe
>>>>> regulator) lcd driver instead.
>>>>
>>>> The existing platform-lcd driver mostly depends on the platform
>>>> callbacks for lcd panel power controls. Looking at the current users
>>>> of platform-lcd driver, a majority of the users of platform-lcd driver
>>>> use a gpio to enable/disable the display/power. The gpio is controlled
>>>> by the platform callbacks which the platform-lcd driver calls.
>>>>
>>>> Hence, it is possible to extend the platform-lcd driver to include the
>>>> functionality of managing the gpio for lcd control. The platform code
>>>> is only expected to provide a gpio number to the platform-lcd driver.
>>>> This allows consolidation of the all the different platform callbacks
>>>> that use a gpio for simple enable/disable of the lcd display.
>>>>
>>>> But there are other users of platform-lcd that do lot more than just
>>>> control a single gpio. For such cases, it is possible to reuse the
>>>> existing infrastructure of platform-lcd driver and extend it to handle
>>>> such lcd panel specific functionality.
>>>>
>>>> The main advantages that I see here is the consolidation of platform
>>>> specific callbacks into the driver which inturn allows adding device
>>>> tree support without depending on platform data which have pointers to
>>>> platform specific functions. In the next version of this patchset, it
>>>> will be ensured that no platform breaks due to this change.
>>>
>>> Consolidation of the different implementations which use a GPIO to control
>>> the LCD state is a good idea. But as I wrote above in this series you added
>>> more or less another layer of abstraction. Namely introducing the
>>> platform-lcd driver as a intermediate layer between the actual driver and
>>> the LCD framework. But the layer is so thin that all it does is to call the
>>> plat_lcd_driver_data callback from the lcd_ops callback. There is really no
>>> point in doing this since you can setup the lcd_ops callbacks directly. Also
>>> following your approach we would end up with a bunch of unrelated LCD
>>> drivers in the platform-lcd driver module. The platform-lcd driver is so
>>> generic that basically any LCD driver could be implemented on-top of it, but
>>> this does not mean it has to.
>>
>> Yes, this will lead to including support for multiple lcd panels in
>> the platform-lcd driver. But, we get to reuse most of the
>> infrastruture in the platform-lcd driver such as module init/cleanup,
>> suspend/resume, probe and lcd driver registration. There is a plan to
>> include regulator support in the platform-lcd driver as well. If we go
>> for independent drivers for all existing users of platform-lcd driver,
>> then this common code in platform-lcd driver will have to be
>> duplicated in multiple new drivers.
>
> Most of the infrastructure code is driver boiler-plate code. And you'll add
> about the same amount of code due to your additional layer redirection. You'll
> still have a probe and remove function per driver. You'll have to define a set
> of plat_lcd_driver_data per driver. You'll have all these ugly ifdefs.
>
> An exception is the suspend/resume handling. But this does not look like it is
> something which is specific to simple displays, more "complex" ones or
> non-platform driver lcd drivers might want to use a similar scheme. A good idea
> would be to add support for this to the LCD framework. Similar to the backlight
> frameworks BL_CORE_SUSPENDRESUME.
>
>>
>> What would be your suggestion considering this. Actually, I don't
>> clearly understand the technical/maintainability reasons which do not
>> favour including support for multiple types of simple lcd panels in
>> the platform-lcd driver.
>
> To me it seems very unidiomatic to put multiple drivers into the same driver.
> E.g. where will we draw the line, which kind of device is simple enough so
> support for this device should be embedded in the plaform-lcd driver. Once you
> have support for multiple types of simple lcd panels in one driver things will
> start to get messy. Image how the driver will look like if it has support for
> say 5 or 10 different types of simple lcd panels.

Ok, I understand your point now. A new lcd driver will be written
mostly derived from platform-lcd driver (minus the platform specific
callbacks in platform data) that can manage power control for lcd
panels which can be controlled by a single gpio. Also, regulator and
device tree support will be added to this driver. Thanks for your
comments.

Regards,
Thomas.

>
> - Lars
>
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