0 前言

    Linux桌面通常包含多个窗口（含系统窗口和应用窗口，应用只管渲染各自的窗口，系统负责将它们和进行合成并送显。

1 合成

    图像合成工作可由窗口管理器完成，也可由显示服务器完成。

1.1 窗管合成

    窗口管理器主要有平铺式窗口管理器、堆叠式窗口管理器、动态窗口管理器以及复合窗口管理器四大类型，其中复合窗口管理器支持图像合成功能：应用程序窗口先画到离屏缓冲区，然后由复合器合成显示到屏幕上[2]：

图1.1 窗口合成[2]

    常用复合窗口管理器主要有Mutter和KWin，其中后者可关闭合成功能，相关命令如下[2]：

# UOS/Kubuntu
# 查看合成状态以及类型
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.active          # 合成状态[2]
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.compositingType # 合成类型

# 关闭/开启 合成
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.suspend         # 关闭合成[2]
qdbus org.kde.KWin /Compositor org.kde.kwin.Compositing.resume          # 开启合成[2]

1.2 显服合成

    对于窗口管理器不支持合成（或关闭合成）的情况，需要显示服务器来完成图像合成工作，xorg-server代码流程如下：

（1）opengl copy/swap buffer最终调用XShmPutImage()

glXCopySubBufferMESA(Display * dpy, GLXDrawable drawable, int x, int y, int width, int height)@mesa-21.2.6/src/glx/glxcmds.c
|-->...
	|
|<--|
|
|   glXSwapBuffers(Display * dpy, GLXDrawable drawable)@src/glx/glxcmds.c
|	|-->...
|<-----|
|
|-->XShmPutImage(Display *dpy, Drawable d, GC gc, XImage *image, int src_x, int src_y, int dst_x, int dst_y, unsigned int src_width, unsigned int src_height, Bool send_event)@libXext-1.3.5/src/XShm.c
    |-->XExtDisplayInfo *info = find_display (dpy) 
	|-->XShmSegmentInfo *shminfo = (XShmSegmentInfo *)image->obdata
	|-->xShmPutImageReq *req // xShmPutImageReq @ xorgproto-2022.2/include/X11/extensions/shmproto.h
	|-->ShmCheckExtension (dpy, info, 0)
	|-->LockDisplay(dpy)
	|-->FlushGC(dpy, gc)
	|-->GetReq(ShmPutImage, req)
	|-->req->reqType = info->codes->major_opcode
	|-->req->shmReqType = X_ShmPutImage
	|-->req->drawable = d
	|-->req->gc = gc->gid
	|-->req->srcX = src_x
	|-->req->srcY = src_y
	|-->req->srcWidth = src_width
	|-->req->srcHeight = src_height
	|-->req->dstX = dst_x
	|-->req->dstY = dst_y
	|-->req->totalWidth = image->width
	|-->req->totalHeight = image->height
	|-->req->depth = image->depth
	|-->req->format = image->format
	|-->req->sendEvent = send_event
	|-->req->shmseg = shminfo->shmseg
	|-->req->offset = image->data - shminfo->shmaddr
	|-->UnlockDisplay(dpy)
	|-->SyncHandle()

（2）XShmPutImage()在xorg-server中更新damage并发出通知：

dix_main()@dix/main.c
|-->...
    |-->ProcShmPutImage(ClientPtr client)@xorg-server-1.20.13/Xext/shm.c
    	|-->if ((((stuff->format == ZPixmap) && (stuff->srcX == 0)) || ((stuff->format != ZPixmap) && (stuff->srcX < screenInfo.bitmapScanlinePad) && ((stuff->format == XYBitmap) || ((stuff->srcY == 0) && (stuff->srcHeight == stuff->totalHeight))))) && ((stuff->srcX + stuff->srcWidth) == stuff->totalWidth))
        |	|-->(*pGC->ops->PutImage) (pDraw, pGC, stuff->depth, stuff->dstX, stuff->dstY, stuff->totalWidth, stuff->srcHeight, stuff->srcX, stuff->format, shmdesc->addr + stuff->offset + (stuff->srcY * length)/* Q：加上最后一个，地址不是超出了图像范围吗 */)
        |		|-->damagePutImage(DrawablePtr pDrawable, GCPtr pGC, int depth, int x, int y, int w, int h, int leftPad, int format, char *pImage)@xorg-server-1.20.13/miext/damage/damage.c
    	|			|-->damageRegionProcessPending(pDrawable) 
		|				|-->for (; pDamage != NULL; pDamage = pDamage->pNext)
		|					|-->if (pDamage->reportAfter)
		|						|-->if (pDamage->damageReport)
		|							|-->DamageReportDamage(pDamage, &pDamage->pendingDamage)
    	|								|-->DamageReportDamage(DamagePtr pDamage, RegionPtr pDamageRegion)@xorg-server-1.20.13/miext/damage/damage.c
		|									|-->switch (pDamage->damageLevel)
        |										|-->case DamageReportNonEmpty
		|											|-->was_empty = !RegionNotEmpty(&pDamage->damage)
		|											|-->RegionUnion(&pDamage->damage, &pDamage->damage, pDamageRegion)
		|											|-->if (was_empty && RegionNotEmpty(&pDamage->damage))
		|												|-->(*pDamage->damageReport)(pDamage, &pDamage->damage, pDamage->closure)
        |													| // KWin打开合成，报给应用层
        |													|-->DamageExtReport()
        |													|	|-->... // 同上，此处略去
        |													or // KWin关闭合成，Xorg内部消化掉！
        |													|-->compReportDamage(DamagePtr pDamage, RegionPtr pRegion, void *closure)@xorg-server-1.20.4/composite/compalloc.c
		|														|-->WindowPtr pWin = (WindowPtr) closure
		|														|-->ScreenPtr pScreen = pWin->drawable.pScreen
		|														|-->CompScreenPtr cs = GetCompScreen(pScreen)
		|														|-->CompWindowPtr cw = GetCompWindow(pWin)
		|														|-->if (!cs->pendingScreenUpdate)
		|														|	|-->QueueWorkProc(compScreenUpdate, serverClient, pScreen) // compScreenUpdate()调用流程详见《X扩展 —— XComposite》
		|														|	|	|-->QueueWorkProc(Bool (*function) (ClientPtr pClient, void *closure), ClientPtr client, void *closure)@xorg-server-1.20.4/dix/dixutils.c        
		|														|	|		|-->WorkQueuePtr q = malloc(sizeof *q)
		|														|	|		|-->q->function = function // 瞧这里！
		|														|	|		|-->q->client = client
		|														|	|		|-->q->closure = closure
		|														|	|		|-->q->next = NULL
		|														|	|		|-->*workQueueLast = q // 下面workQueue指向的就是该列表
		|														|	|		|-->workQueueLast = &q->next
		|														|	|		|-->return TRUE
		|														|	|-->cs->pendingScreenUpdate = TRUE
		|														|-->cw->damaged = TRUE
		|														|-->compMarkAncestors(pWin)
        |-->else 
    		|-->doShmPutImage(DrawablePtr dst, GCPtr pGC, int depth, unsigned int format, int w, int h, int sx, int sy, int sw, int sh, int dx, int dy, char *data)@xorg-server-1.20.13/Xext/shm.c
				|-->if (format == ZPixmap || (format == XYPixmap && depth == 1))
					|-->pPixmap = GetScratchPixmapHeader(dst->pScreen, w, h, depth, BitsPerPixel(depth), PixmapBytePad(w, depth), data)
					|-->pGC->ops->CopyArea((DrawablePtr) pPixmap, dst, pGC, sx, sy, sw, sh, dx, dy)
            			|-->damageCopyArea(DrawablePtr pSrc, DrawablePtr pDst, ...)@xorg-server-1.15.1/miext/damage/damage.c
        					|-->damageRegionProcessPending(pDst)
        						|-->... // 前面有，此处略去

（3）xorg-server处理上述damage并触发内核更新dirty区域

dix_main()@dix/main.c
|-->Dispatch()@xorg-server-1.20.13/dix/dispatch.c
    |-->WaitForSomething(Bool are_ready)@xorg-server-1.20.4/os/WaitFor.c
    	|-->while(1)
            | // 合成（处理damage）
        	|-->if (workQueue) // workQueue @ xorg-server-1.20.4/dix/dixutils.c， = workQueueLast
            	|-->ProcessWorkQueue()@xorg-server/1.20.4/dix/dixutils.c
            		|-->WorkQueuePtr q, *p = &workQueue
					|-->while ((q = *p))
					|	|-->if ((*q->function) (q->client, q->closure))
            		|	|	|	|-->compScreenUpdate(ClientPtr pClient, void *closure)@xorg-server/1.20.4/composite/compalloc.c
					|	|	|		|-->compPaintChildrenToWindow(pScreen->root) // pScreen->root类型为WindowPtr
            		|	|	|		|	|-->compPaintChildrenToWindow(WindowPtr pWin)@xorg-server-1.20.4/composite/compwindow.c
					|	|	|		|		|-->for (pChild = pWin->lastChild; pChild; pChild = pChild->prevSib)
					|	|	|		|		|	|-->compPaintWindowToParent(pChild)
            		|	|	|		|		|		|-->compPaintWindowToParent(WindowPtr pWin)@
					|	|	|		|		|			|-->compPaintChildrenToWindow(pWin)
					|	|	|		|		|			|-->if (pWin->redirectDraw != RedirectDrawNone)
					|	|	|		|		|				|-->CompWindowPtr cw = GetCompWindow(pWin)
					|	|	|		|		|				|-->if (cw->damaged)
					|	|	|		|		|					|-->compWindowUpdateAutomatic(pWin)
            		|	|	|		|		|					|	|-->compWindowUpdateAutomatic(WindowPtr pWin)@xorg-server/1.20.4/composite/compwindow.c
					|	|	|		|		|					|		|-->CompositePicture(PictOpSrc, pSrcPicture, 0, pDstPicture, 0, 0, 0, 0, pSrcPixmap->screen_x - pParent->drawable.x, pSrcPixmap->screen_y - pParent->drawable.y, pSrcPixmap->drawable.width, pSrcPixmap->drawable.height)
            		|	|	|		|		|					|		|	|-->CompositePicture(CARD8 op, PicturePtr pSrc, PicturePtr pMask, PicturePtr pDst, INT16 xSrc, INT16 ySrc, INT16 xMask, INT16 yMask, INT16 xDst, INT16 yDst, CARD16 width, CARD16 height)
					|	|	|		|		|					|		|		|-->(*ps->Composite) (op, pSrc, pMask, pDst, xSrc, ySrc, xMask, yMask, xDst, yDst, width, height)
            		|	|	|		|		|					|		|			|-->damageComposite(CARD8 op, PicturePtr pSrc, PicturePtr pMask, PicturePtr pDst, INT16 xSrc, INT16 ySrc, INT16 xMask, INT16 yMask, INT16 xDst, INT16 yDst, CARD16 width, CARD16 height)
            		|	|	|		|		|					|		|				|-->ScreenPtr pScreen = pDst->pDrawable->pScreen
					|	|	|		|		|					|		|				|-->PictureScreenPtr ps = GetPictureScreen(pScreen)
					|	|	|		|		|					|		|				|-->damageScrPriv(pScreen)
            		|	|	|		|		|					|		|				|	|-->DamageScrPrivPtr pScrPriv = damageGetScrPriv(pScreen)
					|	|	|		|		|					|		|				|-->if (checkPictureDamage(pDst))
					|	|	|		|		|					|		|				|	|-->BoxRec box
					|	|	|		|		|					|		|				|	|-->box.x1 = xDst + pDst->pDrawable->x
					|	|	|		|		|					|		|				|	|-->box.y1 = yDst + pDst->pDrawable->y
					|	|	|		|		|					|		|				|	|-->box.x2 = box.x1 + width
					|	|	|		|		|					|		|				|	|-->box.y2 = box.y1 + height
					|	|	|		|		|					|		|				|	|-->TRIM_PICTURE_BOX(box, pDst)
					|	|	|		|		|					|		|				|	|-->if (BOX_NOT_EMPTY(box))
					|	|	|		|		|					|		|				|		|-->damageDamageBox(pDst->pDrawable, &box, pDst->subWindowMode)
            		|	|	|		|		|					|		|				|			|-->... // 前面有，此处略去...
					|	|	|		|		|					|		|				|-->if (pSrc->pDrawable && WindowDrawable(pSrc->pDrawable->type))
					|	|	|		|		|					|		|				|	|-->miCompositeSourceValidate(pSrc)
					|	|	|		|		|					|		|				|-->if (pMask && pMask->pDrawable && WindowDrawable(pMask->pDrawable->type))
					|	|	|		|		|					|		|				|	|-->miCompositeSourceValidate(pMask);
					|	|	|		|		|					|		|				|-->unwrap(pScrPriv, ps, Composite)
            		|	|	|		|		|					|		|				|	|-->ps->Composite = pScrPriv->Composite
					|	|	|		|		|					|		|				|-->(*ps->Composite) (op, pSrc, pMask, pDst, xSrc, ySrc, xMask, yMask, xDst, yDst, width, height)
					|	|	|		|		|					|		|				|-->damageRegionProcessPending(pDst->pDrawable)
            		|	|	|		|		|					|		|				|	|-->... // 前面有，此处略去...
					|	|	|		|		|					|		|				|-->wrap(pScrPriv, ps, Composite, damageComposite)
         			|	|	|		|		|					|		|					|-->pScrPriv->Composite = ps->Composite
            		|	|	|		|		|					|		|					|-->ps->Composite = damageComposite
					|	|	|		|		|					|		|-->FreePicture(pSrcPicture, 0)
					|	|	|		|		|					|		|-->FreePicture(pDstPicture, 0)
					|	|	|		|		|					|		|-->DamageEmpty(cw->damage)
					|	|	|		|		|					|-->cw->damaged = FALSE
					|	|	|		|		|-->pWin->damagedDescendants = FALSE
					|	|	|		|-->cs->pendingScreenUpdate = FALSE
					|	|	|-->*p = q->next
					|	|	|-->free(q)
					|	|-->else
					|		|-->p = &q->next      /* don't fetch until after func called */    
					|-->workQueueLast = p

2 送显

    触发内核更新dirty区域即可达到送显的目的：

dix_main()@dix/main.c
|-->Dispatch()@xorg-server-1.20.13/dix/dispatch.c
    |-->WaitForSomething(Bool are_ready)@xorg-server-1.20.4/os/WaitFor.c
    	|-->while(1)
			| // 送显（内核更新dirty）
        	|-->BlockHandler(void *pTimeout)@xorg-server-1.20.13/dix/dixutils.c
	 			|-->++inHandler
        	    |-->for (int i = 0; i < screenInfo.numScreens; i++)
        	        |-->(*screenInfo.screens[i]->BlockHandler)(screenInfo.screens[i], pTimeout)
						|-->msBlockHandler(ScreenPtr pScreen, void *timeout)
							|-->if (pScreen->isGPU && !ms->drmmode.reverse_prime_offload_mode)
							|	|-->dispatch_slave_dirty(pScreen)
							|-->else if (ms->dirty_enabled)
								|-->dispatch_dirty(pScreen)
									|-->ScrnInfoPtr scrn = xf86ScreenToScrn(pScreen)
									|-->modesettingPtr ms = modesettingPTR(scrn)
									|-->PixmapPtr pixmap = pScreen->GetScreenPixmap(pScreen)
									|-->int fb_id = ms->drmmode.fb_id
									|-->dispatch_dirty_region(scrn, pixmap, ms->damage, fb_id)
										|-->dispatch_dirty_region(ScrnInfoPtr scrn, PixmapPtr pixmap, DamagePtr damage, int fb_id)@xorg-server-1.20.13/hw/xfree86/drivers/modesetting/driver.c
											|-->modesettingPtr ms = modesettingPTR(scrn)
											|-->RegionPtr dirty = DamageRegion(damage)
											|-->unsigned num_cliprects = REGION_NUM_RECTS(dirty)
											|-->if (num_cliprects)
												|-->drmModeClip *clip = xallocarray(num_cliprects, sizeof(drmModeClip))
												|-->BoxPtr rect = REGION_RECTS(dirty)
												|-->for (int i = 0; i < num_cliprects; i++, rect++)
												|   |-->clip[i].x1 = rect->x1
												|   |-->clip[i].y1 = rect->y1
												|   |-->clip[i].x2 = rect->x2
												|   |-->clip[i].y2 = rect->y2
												|-->drmModeDirtyFB(ms->fd, fb_id, clip, num_cliprects) // 触发内核更新dirty区域
												|-->free(clip)
												|-->DamageEmpty(damage)

3 帧率

    采用窗管合成时，合成和送显的帧率通常和屏幕刷新率一致；采用显示服务器合成时，以Xorg为例，合成和送显的帧率将会变得不可控，主要由各应用的渲染帧率决定。

参考资料

[1]Linux桌面环境

[2]Linux窗口管理器

[3]KWin窗口管理器