Joe's Blog----TECH: 2012年3月22日

2012年3月22日星期四

Android Conditional Compilation( JAVA)

我們都知道JAVA其實是不支援條件式編譯的例如在C/C++

#ifdef CONFIG_XXX_YYY
        ...
#ENDIF

$ gcc xxx.c -DCONFIG_XXX_YYY

所以我的方法是，在JAVA的地方產生一個JAVA檔，內容為

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
public class ConditionFlag {
    public static final Set  ConditionConfig;
    static {
        HashSet  config =new HashSet();
        config.put("CONFIG_XXX_YYY");
        ConditionConfig = Collections.unmodifiableSet(config);
    }
}

在其他的程式碼裡就可以:

if(ConditionFlag.ConditionConfig.Contains("CONFIG_XXX_YYY")){
     ...
}

這樣應該可以達到類似的目的了

repo 常用指令

取得source
```
$ repo init -u [address] -b [branch]
```
更換remote branch
```
$ repo init -b [branch]
```

建立/跳轉local branch

$ repo start [branch] [dir]    or $ repo start --all [branch]

清除未進track的檔案
```
$ repo forall -c git clean -fdD
```
清除未commit 檔案
```
$ repo forall -c git reset --hard HEAD
```
取得 repository status
```
$ repo status
```
清除未commit 檔案
```
$ repo forall -c git reset --hard HEAD
```

completion

在Linux kernel 中我們可以使用completion來等待某樣事情完成
ex.

#include linux/completion.h
struct completion comp;
void module_init{

         //初始化my_completion, set comp.done=0
         init_completion(&comp);
}


ssize_t complete_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
    //等待所有的comp 都被complete. 
    wait_for_completion(&comp);
    printk(KERN_DEBUG "awoken %i (%s)\n", current->pid, current->comm);
    return 0; /* EOF */
}

ssize_t complete_write (struct file *filp, const char __user *buf, size_t count, loff_t *pos)
{
 
    complete(&comp);
    return count; /* succeed, to avoid retrial */
}

kzalloc()

static inline void *kzalloc(size_t size, gfp_t flags): allocate memory. The memory is set to zero.
Flag:

size: 欲申請大小
flags: same as kmalloc, see kmalloc

static inline void *kzalloc(size_t size, gfp_t flags)
{
return kmalloc(size, flags | __GFP_ZERO);
}

Reference:

include/linux/slab.h

kmalloc()

void * kmalloc (size_t size, int flags) : 在kernel配置記憶體位置
INCLUDE :include/linux/slab.h
FLAG:

size:欲分配大小
flags:

Action Modifiers( 通常使用後面提到的type modifier)

`__GFP_WAIT`	The allocator can sleep.
`__GFP_HIGH`	The allocator can access emergency pools.
`__GFP_IO`	The allocator can start physical I/O.
`__GFP_FS`	The allocator can start filesystem I/O.
`__GFP_COLD`	The allocator should use cache cold pages.
`__GFP_NOWARN`	The allocator will not print failure warnings.
`__GFP_REPEAT`	The allocator will repeat the allocation if it fails, but the allocation can potentially fail. This depends upon the particular VM implementation.
`__GFP_NOFAIL`	The allocator will indefinitely repeat the allocation. The allocation cannot fail.
`__GFP_NORETRY`	The allocator will never retry if the allocation fails.
`__GFP_COMP`	Add compound page metadata. Used internally by the`hugetlb` code.
`__GFP_ZERO`	Add compound page metadata. Used internally by the`hugetlb` code.
`__GFP_NOMEMALLOC`	Don't use emergency reserves.
`__GFP_HARDWALL`	Enforce hardwall cpuset memory allocs.
`__GFP_THISNODE`	No fallback, no policies.
`__GFP_RECLAIMABLE`	Page is reclaimable.
`__GFP_NOTRACK`	Don't track with kmemcheck.
`__GFP_NO_KSWAPD`
`__GFP_OTHER_NODE`	On behalf of other node.

Example:

ptr = kmalloc(size, __GFP_WAIT | __GFP_IO | __GFP_FS);

Zone Modifiers ( 選擇要從哪分配空間預設是從ZONE_NORMAL)

`__GFP_DMA`	Allocate only from `ZONE_DMA`
`__GFP_HIGHMEM`	Allocate from `ZONE_HIGHMEM` or `ZONE_NORMAL`
`__GFP_DMA32`
`__GFP_MOVABLE`

Type Flags( 定義了action 及 zone modifier)

`GFP_ATOMIC`	The allocation is high priority and must not sleep. This is the flag to use in interrupt handlers, in bottom halves, while holding a spinlock, and in other situations where you cannot sleep.
`GFP_NOIO`	This allocation can block, but must not initiate disk I/O. This is the flag to use in block I/O code when you cannot cause more disk I/O, which might lead to some unpleasant recursion.
`GFP_NOFS`	This allocation can block and can initiate disk I/O, if it must, but will not initiate a filesystem operation. This is the flag to use in filesystem code when you cannot start another filesystem operation.
`GFP_KERNEL`	This is a normal allocation and might block. This is the flag to use in process context code when it is safe to sleep. The kernel will do whatever it has to in order to obtain the memory requested by the caller. This flag should be your first choice.
`GFP_USER`	This is a normal allocation and might block. This flag is used to allocate memory for user-space processes.
`GFP_HIGHUSER`	This is an allocation from `ZONE_HIGHMEM` and might block. This flag is used to allocate memory for user-space processes.
`GFP_DMA`	This is an allocation from `ZONE_DMA`. Device drivers that need DMA-able memory use this flag, usually in combination with one of the above.

以下列出各Type modifier的組合:

`GFP_ATOMIC`	`__GFP_HIGH`
`GFP_NOIO`	`__GFP_WAIT`
`GFP_NOFS`	`(__GFP_WAIT \| __GFP_IO)`
`GFP_KERNEL`	`(__GFP_WAIT \| __GFP_IO \| __GFP_FS)`
`GFP_USER`	`(__GFP_WAIT \| __GFP_IO \| __GFP_FS)`
`GFP_HIGHUSER`	`(__GFP_WAIT \| __GFP_IO \| __GFP_FS \| __GFP_HIGHMEM)`
`GFP_DMA`	`__GFP_DMA`

建議的用法:

Situation	Solution
Process context, can sleep	Use `GFP_KERNEL`
Process context, cannot sleep	Use `GFP_ATOMIC`, or perform your allocations with `GFP_KERNEL` at an earlier or later point when you can sleep
Interrupt handler	Use `GFP_ATOMIC`
Softirq	Use `GFP_ATOMIC`
Tasklet	Use `GFP_ATOMIC`
Need DMA-able memory, can sleep	Use `(GFP_DMA \| GFP_KERNEL)`
Need DMA-able memory, cannot sleep	Use `(GFP_DMA \| GFP_ATOMIC)`, or perform your allocation at an earlier point when you can sleep