Android HardwareComposer中的fence机制
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最近在写HardwareComposer,这东西的主要功能简单来说,就是把SurfaceFlinger准备好的各个layer data送到需要的地方去,以aosp中hardware/samsung_slsi/exynos5/libhwc 这个hwc为例,就是把layer data送到/dev/graphics/fb0去显示.
不过所谓的"sf准备好的layer",其实未必真的准备好了.
举个例子来说,gralloc分配的一块buffer在apk和sf之间共享,apk作为buffer的producer,sf作为buffer的consumer.apk执行完必要的gl绘图操作后,会把buffer的控制权交给sf,sf再把buffer对应的handle交给hwc做compose,这个时候apk中gl的func虽然都已经return了,但是真正的操作可能还在gpu的queue里面,buffer的内容还没有更新.如果hwc把buffer信息送到fb driver后,fb driver马上就把buffer内容显示到画面上的话,看到的就是错误的画面.反过来也是一样,当画面的内容还在显示状态,apk又在同一块buffer上重新绘制,也会出现类似问题.所以,这就需要一个producer和consumer之间的同步机制,也就是本篇要讨论的fence.
我们来看下fence在hwc是如何运用的,当sf有内容需要hwc去显示的时候,sf会call hwc的set函数(其实在这之前还会call prepare,不过prepare不会处理fence).set函数return之前不必等待每个layer真正的准备好,只要正确处理了fence,buffer在ready的时候会signal相关的fence,通知真正消费buffer data的人.
hardware/libhardware/include/hardware/hwcomposer.h
int (*set)(struct hwc_composer_device_1 *dev, size_t numDisplays, hwc_display_contents_1_t** displays);其中hwc_display_contents_1_t是这样定义的(忽略和fence无关的部分):
typedef struct hwc_display_contents_1 { int retireFenceFd; size_t numHwLayers; hwc_layer_1_t hwLayers[0];} hwc_display_contents_1_t;
numHwLayers表示这次需要显示的layer个数,hwLyaers[0]是一个大小为0的数组,指向numHwLayers的下一个地址,sf在构建hwc_display_contents_1_t的时候会根据numHwLayers来分配大小为sizeof(hwc_display_contents_1_t)+numHwLayers*sizeof(hwc_layer_1_t)的memory,这样hwLyaers就指向了真正保存了numHwLayers个hwc_layer_1_t的地址.
retireFenceFd需要在这次composition retire后被singal,对于physical display来说,retire的概念是:当下个画面替换掉本次画面的时候.对于virtual display来说,retire的概念是:当内容被完整的写进outbuffer,并可以被read的时候.总的来说就是本次composition的结果不再被需要的时候,就可以把hwc_display_contents_1_t标记为retire了.
retireFenceFd控制的是整个composition,也就是整组的layer.
再来看hwc_layer_1_t
typedef struct hwc_layer_1 { ... buffer_handle_t handle; int acquireFenceFd; int releaseFenceFd; ...} hwc_layer_1_t;handle就是本layer对应的buffer.set函数被call到的时候并不知道buffer是否真正ready,acquireFenceFd会在buffer内容真正ready的时候被signal,hwc要负责close acquireFenceFd.set函数return之前并不能保证buffer内容已经使用完毕,不再需要,所以hwc需要负责为这个buffer构建一个fence,releaseFenceFd在set函数return之前被set起来,sf会等待releaseFenceFd被signal之后才会把buffer拿去重用,sf要负责close releaseFenceFd.
acquireFenceFd和releaseFenceFd控制的是单个layer.
仍然以hardware/samsung_slsi/exynos5/libhwc/为例,(简化code只保留和fence相关的部分,完整code请参考aosp):
static int exynos5_set(struct hwc_composer_device_1 *dev, size_t numDisplays, hwc_display_contents_1_t** displays){ if (!numDisplays || !displays) return 0; exynos5_hwc_composer_device_1_t *pdev = (exynos5_hwc_composer_device_1_t *)dev; hwc_display_contents_1_t *fimd_contents = displays[HWC_DISPLAY_PRIMARY]; int fimd_err = 0; if (fimd_contents) fimd_err = exynos5_set_fimd(pdev, fimd_contents); return fimd_err;}set函数拿到编号为HWC_DISPLAY_PRIMARY的hwc_display_contents_1_t,也就是andorid内建主显示屏需要显示的内容,call到exynos5_set_fimd.
static int exynos5_set_fimd(exynos5_hwc_composer_device_1_t *pdev, hwc_display_contents_1_t* contents){ hwc_layer_1_t *fb_layer = NULL; int err = 0; int fence; if (!err) { fence = exynos5_post_fimd(pdev, contents); if (fence < 0) err = fence; } for (size_t i = 0; i < NUM_HW_WINDOWS; i++) {int dup_fd = dup(fence); layer.releaseFenceFd = dup_fd; } contents->retireFenceFd = fence; return err;}exynos5_post_fimd return了int fence,fence其实是个fd,hwc把这个fd dup了n次,赋值给每个layer的releaseFeceFd,最后在把fence本身赋值给整个layer组的retireFenceFd.也就是说,retireFenceFd和所有的releaseFenceFd对应到kernel中都是同一个file.截至目前为止,acqireFenceFd还没有出场,肯定还在exynos5_post_fimd 中.
dev->fd = open("/dev/graphics/fb0", O_RDWR);
static int exynos5_post_fimd(exynos5_hwc_composer_device_1_t *pdev, hwc_display_contents_1_t* contents){ exynos5_hwc_post_data_t *pdata = &pdev->bufs; struct s3c_fb_win_config_data win_data; struct s3c_fb_win_config *config = win_data.config; memset(config, 0, sizeof(win_data.config)); for (size_t i = 0; i < NUM_HW_WINDOWS; i++) config[i].fence_fd = -1; for (size_t i = 0; i < NUM_HW_WINDOWS; i++) { int layer_idx = pdata->overlay_map[i]; if (layer_idx != -1) { hwc_layer_1_t &layer = contents->hwLayers[layer_idx]; private_handle_t *handle = private_handle_t::dynamicCast(layer.handle); exynos5_config_overlay(&layer, config[i], pdev); } } int ret = ioctl(pdev->fd, S3CFB_WIN_CONFIG, &win_data); for (size_t i = 0; i < NUM_HW_WINDOWS; i++) if (config[i].fence_fd != -1) close(config[i].fence_fd); return win_data.fence;}
每个layer对应一个config,所有的config组成win_data,exynos5_config_overlay会从每个layer中抽取config的需要的信息填写进cofnig,最后整个win_data作为参数,通过S3CFB_WIN_CONFIG ioctl送去fb0了,exynos5_post_fimd return的fence其实是经由fb driver处理后的win_data.fence.
acqireFenceFd还是没有出现,但是我们注意到,在ioctl之前,每个config的fence_fd被设成-1,ioctl之后,又再次判断fence_fd,如果不是-1,就close它.config.fence_fd一定和acqireFenceFd有关!不过在这里就close fence_fd会不会有点太早了?这个fence不再需要了吗?不是说set函数返回之前不需要等待acquireFenceFd被signal吗?我们再进去exynos5_config_overlay看看.
static void exynos5_config_overlay(hwc_layer_1_t *layer, s3c_fb_win_config &cfg, exynos5_hwc_composer_device_1_t *pdev){ private_handle_t *handle = private_handle_t::dynamicCast(layer->handle); exynos5_config_handle(handle, layer->sourceCrop, layer->displayFrame, layer->blending, layer->acquireFenceFd, cfg, pdev);}
static void exynos5_config_handle(private_handle_t *handle, hwc_rect_t &sourceCrop, hwc_rect_t &displayFrame, int32_t blending, int fence_fd, s3c_fb_win_config &cfg, exynos5_hwc_composer_device_1_t *pdev){ uint32_t x, y; uint32_t w = WIDTH(displayFrame); uint32_t h = HEIGHT(displayFrame); uint8_t bpp = exynos5_format_to_bpp(handle->format); uint32_t offset = (sourceCrop.top * handle->stride + sourceCrop.left) * bpp / 8; cfg.state = cfg.S3C_FB_WIN_STATE_BUFFER; cfg.fd = handle->fd; cfg.x = x; cfg.y = y; cfg.w = w; cfg.h = h; cfg.format = exynos5_format_to_s3c_format(handle->format); cfg.offset = offset; cfg.stride = handle->stride * bpp / 8; cfg.blending = exynos5_blending_to_s3c_blending(blending); cfg.fence_fd = fence_fd;}acquireFenceFd终于出现了,它作为每个layer对应的config的cfg.fence_fd,经由S3CFB_WIN_CONFIG ioctrl送去了fb driver.既然送去了fb driver,那么fence的生死就由driver负责了,hwc不用再理会.所以上面exynos5_post_fimd 才把每个layer的config.fence_fd,也就是layer->acquireFenceFd给close掉了.
至此,hwc中对于fence的处理流程就走完了,总结一下:
1. hwc把所有的layer->acquireFenceFd一起送到fb driver后,就把layer->acquireFenceFd close,把fence的管理权交给fb driver.
2. ioctl rentrun时,win_data.fence被driver填写为一个新的fd,这个fd被dup n次并赋值给每个layer的layer->releaseFenceFd.
3. win_data.fence赋值给整个hwc_display_contents_1_t,也就是所有layer组合的retireFenceFd.
4.之后releaseFenceFd和retaireFenceFd的生死就交给sf来负责了.
接下来的讨论进入fd driver,请参考Android fb driver中的fence机制
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