使用Linux中的“直接渲染管理器”在dumbbuffer上调用mmap在使用C

有一段时间,我一直在使用 Linux‘ Direct Rendering Manager,这允许我们进行一些非常低级别的图形管理.这通常在C中完成,在 libdrm的帮助下,或直接使用 DRM headers.

我正在尝试在Rust中创建一个与libdrm等效的东西,它不仅仅是对C库的绑定,而是直接使用系统调用.这不是一项容易的任务,因为那里几乎没有DRM的文档,但我正在关注this example in C以获取从哪里开始的提示.

我现在到了我应该创建一个哑缓冲区并将其映射到内存中的点,所以我可以修改屏幕上显示的每像素像素数.为此,我必须使用mmap,但我得到一个非常奇怪的错误.

这是C中的最小工作代码：

#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <drm/drm.h>
#include <drm/drm_mode.h>
#include <sys/ioctl.h>
#include <sys/mman.h>

int main()  {

    // STEP 1: GET ACCESS TO?DRM

    int fd = open("/dev/dri/card0",O_RDWR | O_CLOEXEC);
    if (fd < 0) {
        printf("Error in function open(): %sn",strerror(errno));
        return 1;
    }

    // STEP 2: CREATE DUMBBUFFER

    struct drm_mode_create_dumb dreq;
    dreq.height = 1080,dreq.width  = 1920,dreq.bpp    = 32,dreq.flags  = 0,dreq.handle = 0,dreq.pitch  = 0,dreq.size   = 0;
    int ret = ioctl(fd,DRM_IOCTL_MODE_CREATE_DUMB,&dreq);
    if (ret == -1)  {
        printf("Call to DRM_IOCTL_MODE_CREATE_DUMB failed: %sn",strerror(errno));
        return 1;
    }

    // STEP 3: ADD FRAMEBUFFER

    struct drm_mode_fb_cmd creq;
    creq.fb_id  = 0;
    creq.width  = dreq.width;
    creq.height = dreq.height;
    creq.pitch  = dreq.pitch;
    creq.bpp    = dreq.bpp;
    creq.depth  = 24;
    creq.handle = dreq.handle;
    ret = ioctl(fd,DRM_IOCTL_MODE_ADDFB,&creq);
    if (ret == -1)  {
        printf("Call to DRM_IOCTL_MODE_ADDFB failed: %sn",strerror(errno));
        return 1;
    }

    // STEP 4: PREPARE FOR?MAPPING

    struct drm_mode_map_dumb mreq;
    mreq.handle = dreq.handle;
    mreq.pad    = 0;
    mreq.offset = 0;
    ret = ioctl(fd,DRM_IOCTL_MODE_MAP_DUMB,&mreq);
    if (ret == -1)  {
        printf("Call to DRM_IOCTL_MODE_MAP_DUMB failed: %sn",strerror(errno));
        return 1;
    }

    // STEP 5: MAPPING?PROPER

    void *map = mmap(0,dreq.size,PROT_READ | PROT_WRITE,MAP_SHARED,fd,mreq.offset);
    if (map == MAP_FAILED)  {
        printf("Error in function mmap(): %sn",strerror(errno));
        return 1;
    } else {
        printf("Address of mapped data: 0x%xn",map);
    }

    return 0;
}

这是Rust中完全相同的代码.当然,我的真实代码中有更多的东西,但这个最小的代码足以得到错误：

#![feature(libc)]

extern crate libc;
use self::libc::{c_char,c_int,c_ulong,c_void,off_t,size_t};

extern {
    pub fn ioctl(fd : c_int,request : c_ulong,arg : *mut c_void) -> c_int;
}

fn errno() -> c_int {
    unsafe { *libc::__errno_location() }
}

fn get_c_error() -> String  {
    unsafe {
        let strerr = libc::strerror(errno()) as *mut u8;
        let length = libc::strlen(strerr as *const c_char) as usize;

        let mut string = String::with_capacity(length);

        for i in 0..length   {
            let car = *strerr.offset(i as isize) as char;
            if car == (0 as char)   { break; }
            string.push(car);
        }

        string
    }
}

#[repr(C)]
struct CCreateDumb  {
    height : u32,width  : u32,bpp    : u32,_flags : u32,handle : u32,pitch  : u32,size   : u64,}

#[repr(C)]
struct CFrameBuffer {
    _fb_id  : u32,_width  : u32,_height : u32,_pitch  : u32,_bpp    : u32,_depth  : u32,_handle : u32,}

#[repr(C)]
struct CMapDumb {
    _handle : u32,_pad    : u32,offset  : u32,}

fn main()   {

    // STEP 1: GET ACCESS TO?DRM

    let pathname = "/dev/dri/card0".to_string();
    let fd : c_int = unsafe {
        libc::open(pathname.as_ptr() as *const c_char,libc::O_RDWR | libc::O_CLOEXEC)
    };
    if fd < 0 {
       panic!("Error in call of C function open(): {}",get_c_error());
    }

    // STEP 2: CREATE DUMBBUFFER

    let mut dreq = CCreateDumb {
        height : 1080,width  : 1920,bpp    : 32,_flags : 0,handle : 0,pitch  : 0,size   : 0,};
    // NB?: 0xc02064b2 = DRM_IOCTL_MODE_CREATE_DUMB
    let mut ret = unsafe { 
        ioctl(fd,0xc02064b2 as c_ulong,&mut dreq as *mut _ as *mut c_void)
    };
    if ret == -1    {
        panic!("Call to DRM_IOCTL_MODE_CREATE_DUMB failed: {}",get_c_error());
    }

    // STEP 3: ADD FRAMEBUFFER

    let mut creq = CFrameBuffer {
        _fb_id  : 0,_width  : dreq.width,_height : dreq.height,_pitch  : dreq.pitch,_bpp    : dreq.bpp,_depth  : 24,_handle : dreq.handle,};
    // NB?: 0xc01c64ae = DRM_IOCTL_MODE_ADDFB
    ret = unsafe { 
        ioctl(fd,0xc01c64ae as c_ulong,&mut creq as *mut _ as *mut c_void)
    };
    if ret == -1    {
        panic!("Call to DRM_IOCTL_MODE_ADDFB failed: {}",get_c_error());
    }

    // STEP 4: PREPARE FOR?MAPPING

    let mut mreq = CMapDumb {
        _handle : dreq.handle,_pad    : 0,offset  : 0,};
    // NB?: 0xc01064b3 = DRM_IOCTL_MODE_MAP_DUMB
    ret = unsafe { 
        ioctl(fd,0xc01064b3 as c_ulong,&mut mreq as *mut _ as *mut c_void)
    };
    if ret == -1    {
        panic!("Call to DRM_IOCTL_MODE_MAP_DUMB failed: {}",get_c_error());
    }

    // STEP 5: MAPPING?PROPER

    let map = unsafe { 
        libc::mmap(
            0 as *mut c_void,dreq.size as size_t,libc::PROT_READ | libc::PROT_WRITE,libc::MAP_SHARED,mreq.offset as off_t
        )
    };
    if map == libc::MAP_FAILED  {
        panic!("Error in call of C function mmap(): {}",get_c_error());
    } else {
        println!("Address of mapped data: 0x{:p}",map);
    }
}

它编译得很好,但是当我执行它时,我得到了这个错误.

thread ” panicked at ‘Error in call of C function mmap(): Invalid argument’,memmapping.rs:139
note: Run with RUST_BACKTRACE=1 for a backtrace.

使用extern块直接链接到原始的C mmap函数而不是Rust的crate libc并不会改变任何东西.

我看了this project如何调用mmap,并尝试做同样的事情,以确保大小和偏移量是页面对齐的,但它没有改变任何东西,因为它们已经是页面对齐的.

This SO question使用名为std :: os :: MemoryMap的stdlib工具,但它不再存在.