[PATCH v5 1/3] ARM: SAMSUNG: Add keypad device support

[Eric Miao]

On Tue, Jun 22, 2010 at 11:27 AM, Joonyoung Shim
<jy0922.shim at samsung.com> wrote:
> On 6/22/2010 12:02 PM, Eric Miao wrote:
>> On Tue, Jun 22, 2010 at 8:48 AM, Joonyoung Shim <jy0922.shim at samsung.com> wrote:
>>> On 6/21/2010 8:16 PM, Russell King - ARM Linux wrote:
>>>> On Mon, Jun 21, 2010 at 06:39:10PM +0800, Eric Miao wrote:
>>>>> On Mon, Jun 21, 2010 at 5:19 PM, Russell King - ARM Linux
>>>>> <linux at arm.linux.org.uk> wrote:
>>>>>> On Mon, Jun 21, 2010 at 05:05:34PM +0800, Eric Miao wrote:
>>>>>>> On Mon, Jun 21, 2010 at 2:26 PM, Joonyoung Shim <jy0922.shim at samsung.com> wrote:
>>>>>>>> +void __init samsung_keypad_set_platdata(struct samsung_keypad_platdata *pd)
>>>>>>>> +{
>>>>>>>> + � � � struct samsung_keypad_platdata *npd;
>>>>>>>> +
>>>>>>>> + � � � if (!pd) {
>>>>>>>> + � � � � � � � printk(KERN_ERR "%s: no platform data\n", __func__);
>>>>>>>> + � � � � � � � return;
>>>>>>>> + � � � }
>>>>>>>> +
>>>>>>>> + � � � npd = kmemdup(pd, sizeof(struct samsung_keypad_platdata), GFP_KERNEL);
>>>>>>>> + � � � if (!npd)
>>>>>>>> + � � � � � � � printk(KERN_ERR "%s: no memory for platform data\n", __func__);
>>>>>>> This part of the code is actually duplicated again and again and again
>>>>>>> for each device, PXA and other legacy platforms are bad references for
>>>>>>> this. In arch/arm/mach-mmp/, it might be a bit cleaner, there are three
>>>>>>> major points:
>>>>>>>
>>>>>>> �1. A minimum 'struct pxa_device_desc' for a simple description of a
>>>>>>> � � device (more than 90% of the devices can be described that way),
>>>>>>> � � and avoid using a comparatively heavier weight platform_device,
>>>>>>> � � which can be generated at run-time
>>>>>>>
>>>>>>> �2. pxa_register_device() to allocate and register the platform_device
>>>>>>> � � at run-time, along with the platform data
>>>>>> It's a bad idea to make platform data be run-time discardable like this:
>>>>>>
>>>>>>>> +struct samsung_keypad_platdata {
>>>>>>>> + � � � const struct matrix_keymap_data *keymap_data;
>>>>>> What you end up with is some platform data structures which must be kept
>>>>>> (those which have pointers to them from the platform data), and others
>>>>>> (the platform data itself) which can be discarded at runtime.
>>>>>>
>>>>>> We know that the __initdata attributations cause lots of problems -
>>>>>> they're frequently wrong. �Just see the constant hastle with __devinit
>>>>>> et.al. �The same issue happens with __initdata as well.
>>>>>>
>>>>>> So why make things more complicated by allowing some platform data
>>>>>> structures to be discardable and others not to be? �Is their small
>>>>>> size (maybe 6 words for this one) really worth the hastle of getting
>>>>>> __initdata attributations wrong (eg, on the keymap data?)
>>>>>>
>>>>> Russell,
>>>>>
>>>>> The benefit I see is when multiple boards are compiled in, those
>>>>> data not used can be automatically discarded.
>>>> Yes, but only some of the data can be discarded.  Continuing with the
>>>> example in hand, while you can discard the six words which represent
>>>> samsung_keypad_platdata, but the keymap_data can't be because that won't
>>>> be re-allocated, which is probably a much larger data structure.
>>>>
>>> No. the keymap_data is possible too. The keypad driver allocates other
>>> keymap area of input device and it is assigned from datas based on this
>>> keymap_data.
>>>
>>
>> This is a generic issue. Even if in your example, you can avoid this by
>> re-allocation and re-assignment (ignore the performance issue for such
>> behavior), the real question is the difficult to track all these down. Since
>
> Right, it can occur difficulty of maintain. I wanted just to inform the
> current fact.
>
>> matrix_keypad_data is something out of your control (it was actually
>> drafted by me and Dmitry if you are interested), and think about one day
>> I changed it's definition, now you have to sync your driver and code every
>> time to make sure the discarded data is not referenced.
>>
>
> if matrix_keypad_data is changed, i think the patchset should included
> change of related other parts using it.
>

That's reasonable but difficult in practice, every keypad driver using
matrix_keypad_data could be doing things differently. That's what I'm
concerned about. Things will be much easier for driver writers if he
knows the data passed in will always be reference-able.