第一：linux系統(tǒng)中內(nèi)存分配關(guān)注問題

? ???在編寫Linux驅(qū)動(dòng)過程中，不可避免涉及外設(shè)操作，而外設(shè)地址空間與DDR地址空間一般不連續(xù)，在linux上電時(shí)，并不會(huì)為外設(shè)地址空間建立頁表，又因?yàn)閘inux訪問內(nèi)存使用的都是虛擬地址，因此如果想訪問外設(shè)的寄存器（一般包括數(shù)據(jù)寄存器、控制寄存器與狀態(tài)寄存器），需要在驅(qū)動(dòng)初始化中將外設(shè)所處的物理地址映射為虛擬地址，linux為應(yīng)對(duì)該問題提供了較多接口，包括：ioremap/ioremap_wc/devm_ioremap/devm_ioremap_resource等，以應(yīng)對(duì)不同的場景需求，本文即闡述這些接口的使用，以及需要注意的區(qū)別。

第二：場景的應(yīng)用背景

? ?在系統(tǒng)運(yùn)行時(shí)，外設(shè)IO資源的物理地址是已知的，由硬件的設(shè)計(jì)決定（參考SOC的datesheet，一般會(huì)有memorymap）。驅(qū)動(dòng)程序不能通過物理地址訪問IO資源，必須將其映射到內(nèi)核態(tài)的虛擬地址空間（通過頁表）[1]，然后根據(jù)映射所得到的內(nèi)核虛擬地址范圍，通過線性偏移（virt_addr = virt_base + phy_addr - phy_base）訪問這些IO內(nèi)存資源。

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代碼路徑:arch/arm/include/asm/io.h
/*                                                                                                  
349  * ioremap() and friends.                                                                           
350  *                                                                                                  
351  * ioremap() takes a resource address, and size.  Due to the ARM memory                             
352  * types, it is important to use the correct ioremap() function as each                             
353  * mapping has specific properties.                                                                 
354  *                                                                                                  
355  * Function             Memory type     Cacheability    Cache hint                                  
356  * ioremap()            Device          n/a             n/a                                         
357  * ioremap_nocache()    Device          n/a             n/a                                         
358  * ioremap_cache()      Normal          Writeback       Read allocate                               
359  * ioremap_wc()         Normal          Non-cacheable   n/a                                         
360  * ioremap_wt()         Normal          Non-cacheable   n/a                                         
361  *                                                                                                  
362  * All device mappings have the following properties:                                               
363  * - no access speculation                                                                          
364  * - no repetition (eg, on return from an exception)                                                
365  * - number, order and size of accesses are maintained                                              
366  * - unaligned accesses are "unpredictable"                                                         
367  * - writes may be delayed before they hit the endpoint device                                      
368  *                                                                                                  
369  * ioremap_nocache() is the same as ioremap() as there are too many device                          
370  * drivers using this for device registers, and documentation which tells                           
371  * people to use it for such for this to be any different.  This is not a  
    
372  * safe fallback for memory-like mappings, or memory regions where the                              
373  * compiler may generate unaligned accesses - eg, via inlining its own                              
374  * memcpy.                                                                                          
375  *                                                                                                  
376  * All normal memory mappings have the following properties:                                        
377  * - reads can be repeated with no side effects                                                     
378  * - repeated reads return the last value written                                                   
379  * - reads can fetch additional locations without side effects                                      
380  * - writes can be repeated (in certain cases) with no side effects                                 
381  * - writes can be merged before accessing the target                                               
382  * - unaligned accesses can be supported                                                            
383  * - ordering is not guaranteed without explicit dependencies or barrier                            
384  *   instructions                                                                                   
385  * - writes may be delayed before they hit the endpoint memory                                      
386  *                                                                                                  
387  * The cache hint is only a performance hint: CPUs may alias these hints.                           
388  * Eg, a CPU not implementing read allocate but implementing write allocate                         
389  * will provide a write allocate mapping instead.                                                   
390??*/

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ioremap函數(shù)組共有五個(gè)接口，根據(jù)函數(shù)實(shí)現(xiàn)可以分為三類:

ioremap & ioremap_nocache實(shí)現(xiàn)相同，使用場景為映射device memory類型內(nèi)存；

ioremap_cached，使用場景為映射normal memory類型內(nèi)存，且映射后的虛擬內(nèi)存支持cache；

ioremap_wc & ioremap_wt實(shí)現(xiàn)相同，使用場景為映射normal memory類型內(nèi)訓(xùn)，且映射后的虛擬內(nèi)存不支持cache。

第三：何為memory type？

? ? ?內(nèi)存類型（memory type）分為設(shè)備（device）類型與一般（normal）類型。

第四：ioremap & ioremap_nocache

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代碼路徑:arch/arm/include/asm/io.h
void __iomem *ioremap(resource_size_t res_cookie, size_t size);
#define ioremap ioremap                                                                     #define ioremap_nocache ioremap

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? ? ioremap用來映射memory type為device memory的設(shè)備，同時(shí)不使用cache(device memory本身就沒有cacheable這個(gè)屬性)，即CPU的讀寫操作直接操作設(shè)備內(nèi)存。ioremap_nocache的實(shí)現(xiàn)與ioremap完全相同，保留該符號(hào)是因?yàn)橄蚝蠹嫒菔褂胕oremap_nocache接口的驅(qū)動(dòng)程序。

? ? API接口中的res_cookie參數(shù)是需要映射的物理地址，size參數(shù)是需要映射的內(nèi)存大小，單位是Byte。不需要考慮物理地址的頁對(duì)齊問題，底層通過PAGE_ALIGN接口完成了頁對(duì)齊。

第五：ioremap_cached

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/*                                                                                                  
* Do not use ioremap_cached in new code. Provided for the benefit of                               
* the pxa2xx-flash MTD driver only.                                                                
 */                                                                                                 
void __iomem *ioremap_cached(resource_size_t res_cookie, size_t size);

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ioremap_cached用來映射memory type為normal memory的設(shè)備，同時(shí)使用cache，這會(huì)提高內(nèi)存的訪問速度，提高系統(tǒng)的性能。

第六：ioremap_wc & ioremap_wt

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void __iomem *ioremap_wc(resource_size_t res_cookie, size_t size);                                  
#define ioremap_wc ioremap_wc                                                                       
#define ioremap_wt ioremap_wc

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ioremap_wc用來映射memory type為normal memory的設(shè)備，同時(shí)不使用cache。

第七：解映射API

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代碼路徑:arch/arm/include/asm/io.h
void?iounmap(volatile?void?__iomem?*iomem_cookie);

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不論采用以上哪種映射方式，解映射為統(tǒng)一接口，其中iomem_cookie參數(shù)為映射后的虛擬地址。

第八：IO資源讀寫過程

我們已經(jīng)知道如何映射和解映射虛擬內(nèi)存，內(nèi)存映射是為了訪問，理論上這時(shí)候可以像讀寫RAM那樣直接通過虛擬地址指針讀寫IO內(nèi)存資源，但是為了保證驅(qū)動(dòng)程序跨平臺(tái)的可移植性【這一點(diǎn)還不太理解，直接指針操作怎么影響的移植性，望大家指教】，我們應(yīng)該采用linux中特定的接口函數(shù)來訪問IO內(nèi)存[1]。常用接口如下：

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代碼路徑:arch/arm/include/asm/io.h
299 #define readb(c)                ({ u8  __v = readb_relaxed(c); __iormb(); __v; })                   
300 #define readw(c)                ({ u16 __v = readw_relaxed(c); __iormb(); __v; })                   
301 #define readl(c)                ({ u32 __v = readl_relaxed(c); __iormb(); __v; })                   
                                                                                                     
303 #define writeb(v,c)             ({ __iowmb(); writeb_relaxed(v,c); })                               
304 #define writew(v,c)             ({ __iowmb(); writew_relaxed(v,c); })                               
305 #define writel(v,c)             ({ __iowmb(); writel_relaxed(v,c); })   
    
  
316 static inline void memset_io(volatile void __iomem *dst, unsigned c,                                size_t count);
324 static inline void memcpy_fromio(void *to, const volatile void __iomem *from,                         size_t count);
332?static?inline?void?memcpy_toio(volatile?void?__iomem?*to,?const?void?*from,???????????????????????????size_t?count);

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　　審核編輯：湯梓紅