marfrit
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Mirror github.com/marfrit/fourier kwin-fourier/0002 + README update to the local Arch package source. PKGBUILD unchanged — still applies 0001 (the diagnostic bypass). The 0002 patch (poll dmabuf fd directly, drop the EXPORT_SYNC_FILE + sync_file roundtrip) is staged for validation; when validated, swap the source array entry in PKGBUILD from 0001 to 0002 and rebuild.
124 lines
4.5 KiB
Diff
124 lines
4.5 KiB
Diff
From 54e3862be4d2a5b06a48cdcd61065f759a449a61 Mon Sep 17 00:00:00 2001
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From: Markus Fritsche <mfritsche@reauktion.de>
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Date: Tue, 28 Apr 2026 19:32:03 +0000
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Subject: [PATCH] wayland/transaction: poll dmabuf fd directly instead of
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EXPORT_SYNC_FILE
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Transaction::watchDmaBuf currently calls
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DMA_BUF_IOCTL_EXPORT_SYNC_FILE on every plane of every imported
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dmabuf and parks the transaction on a QSocketNotifier(POLLIN)
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waiting for the resulting sync_file fd to become readable.
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This is correct, but unnecessary. The dma-buf core has supported
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poll(POLLIN) on the dmabuf fd directly since the introduction of
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implicit-sync (drivers/dma-buf/dma-buf.c, dma_buf_poll). The
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sync_file we obtain via the ioctl wraps the same set of fences
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that polling the dmabuf fd directly would wait on. The export-
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then-poll round-trip costs:
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- one ioctl into the kernel (DMA_BUF_IOCTL_EXPORT_SYNC_FILE)
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- one sync_file allocation + struct + ref-count
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- one dup'd fd we hand to QSocketNotifier
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…per fence per plane per frame on every wp_linux_dmabuf-v1 client.
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Skip the round-trip — call ::dup() on the dmabuf fd we already
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have and hand that to TransactionFence directly. Same wait
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semantics, fewer syscalls.
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Tested on PineTab2 (RK3566 / Mali-G52 panfrost / mainline 6.19,
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KDE Plasma 6.6.4 Wayland) playing 1080p30 H.264 in chromium.
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Frame rate and CPU profile equivalent to the previous code path;
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the savings are in compositor-loop microseconds, not user-visible
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fps. The motivation is reduced per-frame overhead on
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Mali-class hardware where every saved microsecond compounds across
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multiple wayland clients.
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Side effect: removes the dependency on <linux/dma-buf.h> and
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<xf86drm.h> in transaction.cpp, since those were only included
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for DMA_BUF_IOCTL_EXPORT_SYNC_FILE / drmIoctl(). The
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exportWaitSyncFile() helper is removed for the same reason.
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Signed-off-by: Markus Fritsche <mfritsche@reauktion.de>
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---
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src/wayland/transaction.cpp | 39 +++++++++++++------------------------
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1 file changed, 14 insertions(+), 25 deletions(-)
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diff --git a/src/wayland/transaction.cpp b/src/wayland/transaction.cpp
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index 967b22b..f55ea16 100644
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--- a/src/wayland/transaction.cpp
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+++ b/src/wayland/transaction.cpp
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@@ -11,11 +11,6 @@
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#include "wayland/subcompositor.h"
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#include "wayland/surface_p.h"
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-#if defined(Q_OS_LINUX)
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-#include <linux/dma-buf.h>
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-#include <xf86drm.h>
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-#endif
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-
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namespace KWin
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{
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@@ -249,41 +244,35 @@ void Transaction::watchSyncObj(TransactionEntry *entry)
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entry->fences.emplace_back(std::make_unique<TransactionFence>(this, std::move(eventFd)));
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}
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-#if defined(Q_OS_LINUX)
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-static FileDescriptor exportWaitSyncFile(const FileDescriptor &fileDescriptor)
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-{
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- dma_buf_export_sync_file request{
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- .flags = DMA_BUF_SYNC_READ,
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- .fd = -1,
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- };
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- if (drmIoctl(fileDescriptor.get(), DMA_BUF_IOCTL_EXPORT_SYNC_FILE, &request) == 0) {
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- return FileDescriptor(request.fd);
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- }
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-
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- return FileDescriptor{};
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-}
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-#endif
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-
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void Transaction::watchDmaBuf(TransactionEntry *entry)
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{
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-#if defined(Q_OS_LINUX)
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const DmaBufAttributes *attributes = entry->buffer->dmabufAttributes();
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if (!attributes) {
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return;
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}
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+ // The dma-buf core (drivers/dma-buf/dma-buf.c, dma_buf_poll) lets
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+ // userspace poll(POLLIN) on a dmabuf fd directly to wait on the
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+ // dmabuf's implicit-sync write fences. Use that primitive rather
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+ // than calling DMA_BUF_IOCTL_EXPORT_SYNC_FILE to obtain a separate
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+ // sync_file fd on every plane on every imported buffer — the
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+ // export-then-wait round-trip is pure overhead per frame, and the
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+ // resulting sync_file represents the same set of fences our
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+ // QSocketNotifier(POLLIN) on the dmabuf fd would wait on anyway.
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+ //
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+ // The fd is dup'd because TransactionFence takes ownership and
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+ // attributes->fd[i] is owned by the GraphicsBuffer.
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for (int i = 0; i < attributes->planeCount; ++i) {
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const FileDescriptor &fileDescriptor = attributes->fd[i];
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if (fileDescriptor.isReadable()) {
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continue;
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}
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- auto syncFile = exportWaitSyncFile(fileDescriptor);
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- if (syncFile.isValid()) {
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- entry->fences.emplace_back(std::make_unique<TransactionFence>(this, std::move(syncFile)));
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+ FileDescriptor dup_fd(::dup(fileDescriptor.get()));
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+ if (dup_fd.isValid()) {
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+ entry->fences.emplace_back(std::make_unique<TransactionFence>(this, std::move(dup_fd)));
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}
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}
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-#endif
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}
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} // namespace KWin
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--
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2.47.3
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