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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,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 05640C0044C for ; Wed, 31 Oct 2018 05:38:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88B2C20838 for ; Wed, 31 Oct 2018 05:38:22 +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="egjyJGXx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 88B2C20838 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728323AbeJaOez (ORCPT ); Wed, 31 Oct 2018 10:34:55 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:46942 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725955AbeJaOey (ORCPT ); Wed, 31 Oct 2018 10:34:54 -0400 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=hV4CoD4NAAjdCFYh7pqqU5BKJHTrpP5sN+V7I8qzmDM=; b=egjyJGXxR/iyWp1LCPilFc4dk VpnvtncFWHm9XFC6rTZQ0itq7QXSOOl6QWk0jp9j13w7IhjNdwcDMQzZM6ZvPrnAlOrYxDUU704Xj wK0EvPxDysKxkPo14XAmdFOaR5WEMh3AHw1Fm3S2JMgeerqKD6+dqswLUS4db2kgzxJ6hZxqYDNrD SMNHnUbkVqcqrbXswzYcgsQK22WLf982JQ63Gw+Cx8hh75pIwmq4C9SejjkP4fsvbzPCO/WyGyS1K NRc0wngoVPsFwDKX48Mxxb/y7OzRUGUURlbm8ZWfx9BVk4vt0QkmrQcPtmCmB34ymiD6RtlGsJY80 cZP2ZL5jQ==; Received: from hch by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gHjCi-0000r4-DV; Wed, 31 Oct 2018 05:38:08 +0000 Date: Tue, 30 Oct 2018 22:38:08 -0700 From: Christoph Hellwig To: Laurent Pinchart Cc: "Matwey V. Kornilov" , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, matwey.kornilov@gmail.com, tfiga@chromium.org, stern@rowland.harvard.edu, ezequiel@collabora.com, hdegoede@redhat.com, hverkuil@xs4all.nl, mchehab@kernel.org, rostedt@goodmis.org, mingo@redhat.com, isely@pobox.com, bhumirks@gmail.com, colin.king@canonical.com, kieran.bingham@ideasonboard.com, keiichiw@chromium.org, Christoph Hellwig Subject: Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer Message-ID: <20181031053808.GB22504@infradead.org> References: <20180821170629.18408-1-matwey@sai.msu.ru> <20180821170629.18408-3-matwey@sai.msu.ru> <2213616.rQm4DhIJ7U@avalon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2213616.rQm4DhIJ7U@avalon> 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 Wed, Oct 31, 2018 at 12:00:12AM +0200, Laurent Pinchart wrote: > As discussed before, we're clearly missing a proper non-coherent memory > allocation API. As much as I would like to see a volunteer for this, I don't > think it's a reason to block the performance improvement we get from this > patch. > > This being said, I'm a bit concerned about the allocation of 16kB blocks from > kmalloc(), and believe that the priority of the non-coherent memory allocation > API implementation should be increased. Christoph, you mentioned in a recent > discussion on this topic that you are working on removing the existing non- > coherent DMA allocation API, what is your opinion on how we should gllobally > solve the problem that this patch addresses ? I hope to address this on the dma-mapping side for this merge window. My current idea is to add (back) add dma_alloc_noncoherent-like API (name to be determindes). This would be very similar to to the DMA_ATTR_NON_CONSISTENT to dma_alloc_attrs with the following differences: - it must actually be implemented by every dma_map_ops instance, no falling back to dma_alloc_coherent like semantics. For all actually coherent ops this is trivial as there is no difference in semantics and we can fall back to the 'coherent' semantics, for non-coherent direct mappings it also is mostly trivial as we generally can use dma_direct_alloc. The only instances that will need real work are IOMMUs that support non-coherent access, but there is only about a handfull of those. - instead of using the only vaguely defined dma_cache_sync for ownership transfers we'll use dma_sync_single_* which are well defined and available everywhere I'll try to prioritise this to get done early in the merge window, but I'll need someone else do the USB side. > > + dma_sync_single_for_cpu(&urb->dev->dev, > > + urb->transfer_dma, > > + urb->transfer_buffer_length, > > + DMA_FROM_DEVICE); > > + > > As explained before as well, I think we need dma_sync_single_for_device() > calls, and I know they would degrade performances until we fix the problem on > the DMA mapping API side. This is not a reason to block the patch either. I > would appreciate, however, if a comment could be added to the place where > dma_sync_single_for_device() should be called, to explain the problem. Yes, as a rule of thumb every dma_sync_single_for_cpu call needs to pair with a previous dma_sync_single_for_device call. (Exceptions like selective use of DMA_ATTR_SKIP_CPU_SYNC proove the rule)