On 2023/5/18 12:08, Jakub Kicinski wrote:

On Tue, 16 May 2023 18:18:36 +0200 Alexander Lobakin wrote:

quoted

+		/* Try to avoid calling no-op syncs */
+		pool->p.flags |= PP_FLAG_DMA_MAYBE_SYNC;
+		pool->p.flags &= ~PP_FLAG_DMA_SYNC_DEV;
 	}
 
 	if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&

@@ -323,6 +327,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
 
 	page_pool_set_dma_addr(page, dma);
 
+	if ((pool->p.flags & PP_FLAG_DMA_MAYBE_SYNC) &&
+	    dma_need_sync(pool->p.dev, dma)) {
+		pool->p.flags |= PP_FLAG_DMA_SYNC_DEV;
+		pool->p.flags &= ~PP_FLAG_DMA_MAYBE_SYNC;
+	}

is it just me or does it feel cleaner to allocate a page at init,
and throw it into the cache, rather than adding a condition to a
fast(ish) path?

Is dma_need_sync() not reliable until a dma map is called?
Is there any reason why not just clear PP_FLAG_DMA_SYNC_DEV if
dma_need_sync() is false without introducing the PP_FLAG_DMA_MAYBE_SYNC
flag?

.