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

[Eric Miao]

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
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.