From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B86ECC43387 for ; Thu, 20 Dec 2018 18:38:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8A85620811 for ; Thu, 20 Dec 2018 18:38:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="GAuPK3M3" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389461AbeLTSiz (ORCPT ); Thu, 20 Dec 2018 13:38:55 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:50346 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731280AbeLTSiz (ORCPT ); Thu, 20 Dec 2018 13:38:55 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=n7kHmUd9NWsVcSQjDie0de8H/6KifCjV/t7SEMIprHM=; b=GAuPK3M3GhPbWSmQD9yxlNrAV atVUsYHU1uG8HtVBEt5FIk9fUImoviog1al6vwFl2Ag+CnqsGQFgOQ5X9CwShCeCxffYpKkXQ7HQn nUL+zhQ5k7UWXPYlizVd9PtXmgNepfJudD5komJn4qI97s/9qbusrvVilzhpki3Ba7wiCyRpKsgs5 jpuPZe9cY4zgq1CR3Z7HX/hUA8xd1vIBFB1uiiqslYlccKLBxLnGjfLPBuRvlC21Bnbs96rYyFvd2 BdwHCrvKHgMHeqNWshdlh9mweE9pn1C0oziMW/MUz8T/vdjyrDXy9ru8ozooyEjyy0LxYXD/Y6RZi T0onVamLA==; Received: from hch by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1ga3Df-0007D9-KH; Thu, 20 Dec 2018 18:38:51 +0000 Date: Thu, 20 Dec 2018 10:38:51 -0800 From: Christoph Hellwig To: Daniel Vetter Cc: Christoph Hellwig , Oleksandr Andrushchenko , Juergen Gross , Oleksandr Andrushchenko , Linux Kernel Mailing List , dri-devel , Daniel Vetter , xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent Message-ID: <20181220183851.GA27025@infradead.org> References: <20181127103252.20994-1-andr2000@gmail.com> <17640791-5306-f7e4-8588-dd39c14e975b@tronnes.org> <20181220153646.GA22997@infradead.org> <20181220173950.GA19641@infradead.org> <20181220183311.GA30801@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 20, 2018 at 07:35:15PM +0100, Daniel Vetter wrote: > > Err, with streaming DMA buffer sharing is trivial. The coherent DMA > > allocator is what causes all kinds of horrible hacks that can't actually > > work on various platforms. > > Hm, I thought the streaming dma api is the one that causes bounce > buffers and all that fun. If you're unlucky at least. Yes it may. But even if that happens everything will actually work, just slower. While the dma coherent API is simply broken. But if you don't want bounce buffering you need to use the dma noncoherent allocator as proposed here: https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031982.html which combines allocating memory that doesn't need to be bounce buffered with a sharing scheme that can actually work.