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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 76B48C48BDF for ; Tue, 22 Jun 2021 07:33:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 596086128E for ; Tue, 22 Jun 2021 07:33:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230046AbhFVHf1 (ORCPT ); Tue, 22 Jun 2021 03:35:27 -0400 Received: from verein.lst.de ([213.95.11.211]:45444 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229628AbhFVHf0 (ORCPT ); Tue, 22 Jun 2021 03:35:26 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id A866167373; Tue, 22 Jun 2021 09:33:08 +0200 (CEST) Date: Tue, 22 Jun 2021 09:33:08 +0200 From: Christoph Hellwig To: Tomasz Figa Cc: Christoph Hellwig , Sergey Senozhatsky , Hans Verkuil , Ricardo Ribalda , Mauro Carvalho Chehab , Linux Media Mailing List , Linux Kernel Mailing List Subject: Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations Message-ID: <20210622073308.GA32231@lst.de> References: <10a0903a-e295-5cba-683a-1eb89a0804ed@xs4all.nl> <20210617080107.GA1422@lst.de> <20210617085233.GA4702@lst.de> <20210617100656.GA11107@lst.de> <20210618042526.GA17794@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 18, 2021 at 01:44:08PM +0900, Tomasz Figa wrote: > > Well, dma_alloc_coherent users want a non-cached mapping. And while > > some architectures provide that using a vmap with "uncached" bits in the > > PTE to provide that, this: > > > > a) is not possibly everywhere > > b) even where possible is not always the best idea as it creates mappings > > with differnet cachability bets > > I think this could be addressed by having a dma_vmap() helper that > does the right thing, whether it's vmap() or dma_common_pages_remap() > as appropriate. Or would be this still insufficient for some > architectures? It can't always do the right thing. E.g. for the case where uncached memory needs to be allocated from a special boot time fixed pool. > > And even without that dma_alloc_noncoherent causes less overhead than > > dma_alloc_noncontigious if you only need a single contiguous range. > > > > Given that behind the scenes dma_alloc_noncontiguous() would also just > call __dma_alloc_pages() for devices that need contiguous pages, would > the overhead be basically the creation of a single-entry sgtable? In the best case: yes. > > So while I'm happy we have something useful for more complex drivers like > > v4l I think the simple dma_alloc_coherent API, including some of the less > > crazy flags for dma_alloc_attrs is the right thing to use for more than > > 90% of the use cases. > > One thing to take into account here is that many drivers use the > existing "simple" way, just because there wasn't a viable alternative > to do something better. Agreed, though, that we shouldn't optimize for > the rare cases. While that might be true for a few drivers, it is absolutely not true for the wide majority. I think you media people are a little special, with only the GPU folks contending for "specialness" :) (although media handles it way better, gpu folks just create local hacks that can't work portably).