Ch9.硬體的操作

I/O port and I/O memory

通常一個device會有許多registers，並且可以在一段連續範圍的address存取到這些registers。這些address會有兩種address space:

• Memory address space
• I/O address space

Linux虛構了一組I/O port的存取機制，即使target plantform的CPU只有單一address space。實際程序是依照CPU跟BUS的Chipset決定的。

I/O register and 傳統memory

雖然register跟memory很相似，但在program存取I/O port時，不能將I/O port當成normal memory。這是因為compiler在最佳化過程中會影響實際的存取順序，這樣會導致預期的I/O行為被改變。
這是因為讀取I/O registers時，並不一定是要讀取register的內容值，而是藉由read/write的動作來改變device的state。當我們將normal memory的最佳化技術(cache, reschedule instruction, etc.)套用在I/O registers上時就會發生無法預期的結果，這是因為原本安排的I/O行為可能不被正確的執行。

writel(dev->registers.addr, io_destination_address);
writel(dev->registers.size, io_size);
writel(dev->registers.operation, DEV_READ);
wmb();
writel(dev->registers.control, DEV_GO);

#include <linux/kernel.h>
void barrier(void);

#include <asm/system.h> / kernel被compile為SMP時，使用smp_*
void rmb(void); //void smp_rmb(void);
void wmb(void); //void smp_void wmb(void);
void mb(void);  //void smp_mb(void);
void read_barrier_depends(void); //弱化版rmb();
//void smp_read_barrier_depends(void);

順帶一提，spin_lock, atomic_t 等同步處理也有memory barrier作用。並有些平台容許一個動作就給予一個barrier，可使用以下Macro：

#define set_mb(var, value) do {var = value; mb();} while 0
//ps. 只有少數平台有set_rmb

do ... while是讓Marco展開後可以在不同環境下正常運作的慣例。有時候展開Marco會跟前後文不小心結合或讓if else判斷跟預期不同。

I/O port 用法

對於device driver而言，存取I/O port為重要的議題。

配置I/O port

#include <linux/ioport.h>
struct resource *request_region(unsigned long first, unsigned long n, const char *name);

#include <linux/ioport.h>
void release_region(unsigned long start, unsigned long n);

操作I/O port

對I/O進行read/write時，會涉及到bus的寬度問題，因此對於不同的寬度需要不同的function來存取。
對於只支援MMIO(memory-mapped I/O)的平台，可將I/O register mapping到memory address，藉此模擬出I/O port。

當我們看到只有unsigned卻沒有明確型別，表示確切型別隨平台而定。為的是提高移植性。

並沒有64-bit port I/O，就算是64-bit的系統上，I/O port的address space頂多只有32-bit。

#include <asm/io.h>
unsigned inb(unsigned port);
void outb(unsigned char byte, unsigned port);

unsigned inw(unsigned port);
void outw(unsigned short word, unsigned port);

unsigned inl(unsigned port);
void outl(unsigned longword, unsigned port);

從user-space存取I/O port

GNU libc將可存取I/O port的fuction定義在<sys/io.h>。在user-space使用inb()系列fuction時，須遵守：

• 使用-O option編譯，強制展開inlin fuction
• 必須使用ioperm() or iopl() system call 來取得I/O port的權限。只能在intel x86上使用這輛個function。
• program要使用root權限來執行。或危險的設定setuid權限為元。
• target plantform上沒有ioperm() or iopl()，可透過/dev/port device file來存取I/O port。通常不太有用

String操作

#include <asm/io.h>
void insb(unsigned port, void *addr, unsigned long count);
void outsb(unsigned port, void *addr, unsigned long count);
void insw(unsigned port, void *addr, unsigned long count);
void outsw(unsigned port, void *addr, unsigned long count);
void insl(unsigned port, void *addr, unsigned long count);
void outsl(unsigned port, void *addr, unsigned long count);

I/O暫停

避免I/O速度較慢而流失data，可插入delay，或是*_p(inb_p...)這種會暫停的function。

平台相依性

除了x86外，其他都沒有區分I/O-space跟memory-space。

IA-32 (x86)
-x86_64
The architecture supports all the functions described in this chapter. Port num- bers are of type unsigned short.
-IA-64 (Itanium)
All functions are supported; ports are unsigned long (and memory-mapped). String functions are implemented in C.
-Alpha
All the functions are supported, and ports are memory-mapped. The implemen- tation of port I/O is different in different Alpha platforms, according to the chipset they use. String functions are implemented in C and defined in arch/ alpha/lib/io.c. Ports are unsigned long.
-ARM
Ports are memory-mapped, and all functions are supported; string functions are implemented in C. Ports are of type unsigned int.
Cris
M68k
-M68k-nommu
Ports are memory-mapped. String functions are supported, and the port type is unsigned char *.
MIPS
-MIPS64
The MIPS port supports all the functions. String operations are implemented with tight assembly loops, because the processor lacks machine-level string I/O. Ports are memory-mapped; they are unsigned long.
-PA-RISC
All of the functions are supported; ports are int on PCI-based systems and unsigned short on EISA systems, except for string operations, which use unsigned long port numbers.
PowerPC
-PowerPC64
All the functions are supported; ports have type unsigned char * on 32-bit sys- tems and unsigned long on 64-bit systems.
-S390
Similar to the M68k, the header for this platform supports only byte-wide port I/O with no string operations. Ports are char pointers and are memory-mapped.
-Super-H
Ports are unsigned int (memory-mapped), and all the functions are supported.
SPARC
-SPARC64
Once again, I/O space is memory-mapped. Versions of the port functions are defined to work with unsigned long ports.

I/O port的例子

Parallel Port的介紹

每台PC都會有兩個parallel port，第一個從0x378開始，第二個是從0x278開始。基本是由三個8-bits port register構成：

• data port
  • 第一個是雙向data register，連結到pin2 ~ pin9
• status port
  • 第二個是read-only，可得到device的狀態
• control port
  • 最後一個是write-only，決定是否要發出hardware interrupt等控制

Figure 9-1是parallel port的規格。有12個ouput跟5個input。並且1,4,11,17的path上有inverter。並且control port的0x10用來決定interrupt。

自行去看ldd3提供的short driver，並可在板子上插入LED來觀察資料在pin的傳遞。

使用I/O memory

與device互動的主要機制是將device的registers跟memory mapping到CPU的memory-space。
I/O memory不一定受到page table的控管，主要是看plantform跟bus來決定。必須要使用page table的I/O memory的情況下，driver必須要告知kernel將physical address搬入到driver的可見範圍(這表示需要先使用ioremap())。
不管要不要用page table(ioremap)，都要避免直接使用一般的pointer來存取I/O memory。

I/O Memory Allocation and Mapping

#include <linux/ioport.h>
struct resource *request_mem_region(unsigned long start, unsigned long len, char *name);
void release_mem_region(unsigned long start, unsigned long len);
int check_mem_region(unsigned long start, unsigned long len);

#include <asm/io.h>
void *ioremap(unsigned long phys_addr, unsigned long size);
void *iounmap(unsigned long phys_addr, unsigned long size);
void *ioremap_nocache(unsigned long phys_addr, unsigned long size);
void iounmap(void * addr);

unsigned int ioread8(void *addr);
unsigned int ioread16(void *addr);
unsigned int ioread32(void *addr);
void iowrite8(u8 value, void *addr);
void iowrite16(u16 value, void *addr);
void iowrite32(u32 value, void *addr);

void ioread8_rep(void *addr, void *buf, unsigned long count);
void ioread16_rep(void *addr, void *buf, unsigned long count);
void ioread32_rep(void *addr, void *buf, unsigned long count);
void iowrite8_rep(void *addr, const void *buf, unsigned long count);
void iowrite16_rep(void *addr, const void *buf, unsigned long count);
void iowrite32_rep(void *addr, const void *buf, unsigned long count);

void memset_io(void *addr, u8 value, unsigned int count);
void memcpy_fromio(void *dest, void *source, unsigned int count);
void memcpy_toio(void *dest, void *source, unsigned int count);

unsigned readb(address); //8-bit
unsigned readw(address); //16-bit
unsigned readl(address); //32-bit
void writeb(unsigned value, address);
void writew(unsigned value, address);
void writel(unsigned value, address);
/*
*某些64-bits平台提供readq()&writeq()，可用來處理PCI的quad-word(8-bytes)
*/

Port as I/O memory

void *ioport_map(unsigned long port, unsigned int count);
void ioport_unmap(unsigned long port, unsigned int count);

把count個I/O ports mapping到I/O memory。並回傳這段address的pointer。使用ioport_unmap來解除。

ISA

自己去看ldd3的範例程式，這邊不贅述。