接前一篇文章：DRM遇到的实际问题及领悟（1）

三、进一步定位

上回说到使用网上例程有同样会遇到mmap错误的问题，对于这个错误，perror只给出了“Invalid argument”的错误原因，具体错在哪里，并不知道。

1. 源码准备

没有办法，只能再找一个能够成功的例程。功夫不负有心人，在网上不断搜索，最终找到了以下链接中的例程：

https://github.com/tiagovignatti/intel-gpu-tools/blob/master/tools/intel_framebuffer_dump.c

代码如下：

/*
 * Copyright © 2013 Intel Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 *
 */

/*
 * Read back all the KMS framebuffers attached to the CRTC and record as PNG.
 */

#define _GNU_SOURCE
#include <stdint.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <errno.h>
#include <xf86drmMode.h>
#include <i915_drm.h>
#include <cairo.h>

#include "intel_io.h"
#include "drmtest.h"

int main(int argc, char **argv)
{
	drmModeResPtr res;
	int fd, n;

	fd = drmOpen("i915", NULL);
	if (fd < 0)
		return ENOENT;

	res = drmModeGetResources(fd);
	if (res == NULL)
		return ENOMEM;

	for (n = 0; n < res->count_crtcs; n++) {
		struct drm_gem_open open_arg;
		struct drm_gem_flink flink;
		drmModeCrtcPtr crtc;
		drmModeFBPtr fb;

		crtc = drmModeGetCrtc(fd, res->crtcs[n]);
		if (crtc == NULL)
			continue;

		fb = drmModeGetFB(fd, crtc->buffer_id);
		drmModeFreeCrtc(crtc);
		if (fb == NULL)
			continue;

		flink.handle = fb->handle;
		if (drmIoctl(fd, DRM_IOCTL_GEM_FLINK, &flink)) {
			drmModeFreeFB(fb);
			continue;
		}

		open_arg.name = flink.name;
		if (drmIoctl(fd, DRM_IOCTL_GEM_OPEN, &open_arg) == 0) {
			struct drm_i915_gem_mmap_gtt mmap_arg;
			void *ptr;

						mmap_arg.handle = open_arg.handle;
			if (drmIoctl(fd, DRM_IOCTL_I915_GEM_MMAP_GTT, &mmap_arg) == 0 &&
			    (ptr = mmap(0, open_arg.size, PROT_READ, MAP_SHARED, fd, mmap_arg.offset)) != (void *)-1) {
				cairo_surface_t *surface;
				cairo_format_t format;
				char name[80];

				snprintf(name, sizeof(name), "fb-%d.png",  fb->fb_id);

				switch (fb->depth) {
				case 16: format = CAIRO_FORMAT_RGB16_565; break;
				case 24: format = CAIRO_FORMAT_RGB24; break;
				case 30: format = CAIRO_FORMAT_RGB30; break;
				case 32: format = CAIRO_FORMAT_ARGB32; break;
				default: format = CAIRO_FORMAT_INVALID; break;
				}

				surface = cairo_image_surface_create_for_data(ptr, format,
									      fb->width, fb->height, fb->pitch);
				cairo_surface_write_to_png(surface, name);
				cairo_surface_destroy(surface);

				munmap(ptr, open_arg.size);
			}
			drmIoctl(fd, DRM_IOCTL_GEM_CLOSE, &open_arg.handle);
		}

		drmModeFreeFB(fb);
	}

	return 0;
}

但是此代码直接编译执行在笔者电脑上会有问题，于是笔者边摸索边对其进行改进。最终经过笔者改造后的代码如下：

/*
 * Read back all the KMS framebuffers attached to the CRTC and record as PNG.
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <stdint.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <errno.h>
#include <xf86drmMode.h>
#include <xf86drm.h>
#include <i915_drm.h>
#include <cairo.h>

#include "intel_io.h"
//#include "drmtest.h"


int main(int argc, char **argv)
{
	drmModeResPtr res;
	int fd, n;

	drmModeConnector *connector = NULL;
	drmModeRes *resources = NULL;
	drmModeEncoder *encoder = NULL;
	drmModeCrtc *crtc = NULL;
	struct drm_gem_open open_arg;
	struct drm_gem_flink flink;
	uint32_t conn_id;
	uint32_t crtc_id;

	fd = drmOpen("i915", NULL);
	//fd = open("/dev/dri/card0", O_RDWR); //with same effect
	if (fd < 0)
	{
		perror("drmOpen failed!");
		return ENOENT;
	}

	res = drmModeGetResources(fd); //获取drmModeRes资源,包含fb、crtc、encoder、connector等
	if (res == NULL)
	{
		perror("drmModeGetResources failed!");
		return ENOMEM;
	}

	for (int i = 0; i < res->count_connectors; ++i)
	{
		connector = drmModeGetConnector(fd, res->connectors[i]); //根据connector_id获取connector资源
		if (connector == NULL || connector->connection != DRM_MODE_CONNECTED)
			continue;
		for (int j = 0; j < res->count_encoders; ++j)
		{
			encoder = drmModeGetEncoder(fd, res->encoders[j]);
			if (encoder == NULL || encoder->encoder_id != connector->encoder_id)
				continue;
			break;
		}
		break;
	}

	crtc = drmModeGetCrtc(fd, encoder->crtc_id);
	uint32_t framebuffer_id = crtc->buffer_id;

	drmModeFBPtr fb = drmModeGetFB(fd, crtc->buffer_id);
	drmModeFreeCrtc(crtc);
	if (fb == NULL)
	{
		printf("daozhelilema2?\n");
		//continue;
	}

	flink.handle = fb->handle;
	if (drmIoctl(fd, DRM_IOCTL_GEM_FLINK, &flink))
	{
		perror("DRM_IOCTL_GEM_OPEN failed!");
		//drmModeFreeFB(fb);
		printf("daozhelilema3?\n");
	}

	open_arg.name = flink.name;
	if (drmIoctl(fd, DRM_IOCTL_GEM_OPEN, &open_arg))
	{
		perror("DRM_IOCTL_GEM_OPEN failed!");
		//drmModeFreeFB(fb);
		printf("daozhelilema4?\n");
	}

	struct drm_i915_gem_mmap_gtt mmap_arg;
	void *ptr;

	mmap_arg.handle = open_arg.handle;
	if (drmIoctl(fd, DRM_IOCTL_I915_GEM_MMAP_GTT, &mmap_arg))
	{
		perror("DRM_IOCTL_I915_GEM_MMAP_GTT failed!");
		//drmModeFreeFB(fb);
		printf("daozhelilema5?\n");
	}

	ptr = mmap(NULL, open_arg.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mmap_arg.offset);
	if (ptr == (void *)MAP_FAILED)
	{
		perror("mmap failed!");
		//drmModeFreeFB(fb);
		printf("daozhelilema6?\n");
	}

	unsigned char tmp;
	void *dup_ptr = malloc(open_arg.size);
#if 0
	for (int i=0; i<open_arg.size; i++)
	{
		tmp = *((unsigned char *) ptr +i);
		//tmp = !tmp;
		tmp += 64;
		//*((unsigned char *)ptr + i) = tmp;
		*((unsigned char *)dup_ptr + i) = tmp;
	}
#endif
	cairo_surface_t *surface;
	cairo_format_t format;

	char name[80] = {0};
	snprintf(name, sizeof(name), "fb-%d.png",  fb->fb_id);

	printf("fb->depth is: %d\n", fb->depth);
	switch (fb->depth)
	{
		case 16:
			format = CAIRO_FORMAT_RGB16_565;
			break;
		case 24:
			format = CAIRO_FORMAT_RGB24;
			break;
		case 30:
			format = CAIRO_FORMAT_RGB30;
			break;
		case 32:
			format = CAIRO_FORMAT_ARGB32;
			break;
		default:
			format = CAIRO_FORMAT_INVALID;
			break;
	}

	surface = cairo_image_surface_create_for_data(ptr, format, fb->width, fb->height, fb->pitch);
	//surface = cairo_image_surface_create_for_data(dup_ptr, format, fb->width, fb->height, fb->pitch);
	cairo_surface_write_to_png(surface, name);
	cairo_surface_destroy(surface);

	FILE *output_file = fopen("./framebuffer.bin", "wb");
	if (output_file == NULL)
	{
		fprintf(stderr, "Unable to open output file\n");
		return EXIT_FAILURE;
	}
	printf("open_arg.size is %d\n", open_arg.size);
	//fwrite(framebuffer_data, sizeof(uint8_t), framebuffer_size, output_file);
	fwrite(ptr, sizeof(uint8_t), open_arg.size, output_file);
	fclose(output_file);

	drmIoctl(fd, DRM_IOCTL_GEM_CLOSE, &open_arg.handle);
	drmModeFreeFB(fb);

	return 0;
}

2. 源码编译

通过以下命令进行源码编译：

gcc intel_framebuffer_dump.c -o intel_framebuffer_dump -I/usr/include/drm -I/usr/include/cairo -ldrm -lcairo

最终生成可执行文件intel_framebuffer_dump。

3. 源码执行及结果

执行此可执行文件，得到以下结果：

$ ./intel_framebuffer_dump DRM_IOCTL_GEM_OPEN failed!: Permission denied
daozhelilema3?
DRM_IOCTL_GEM_OPEN failed!: Permission denied
daozhelilema4?
DRM_IOCTL_I915_GEM_MMAP_GTT failed!: No such file or directory
daozhelilema5?
mmap failed!: Invalid argument
daozhelilema6?
fb->depth is: 30
段错误（核心已转储）

需要使用root权限或者sudo执行，运行结果如下：

$ sudo ./intel_framebuffer_dump 
fb->depth is: 30

同时，会生成以下图片：

说明截屏功能正常实现。

下一步又该如何走，如何更加深入地定位问题？请看下回。