From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 20756900138 for ; Wed, 17 Aug 2011 08:49:52 -0400 (EDT) MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: text/plain; charset=us-ascii Received: from spt2.w1.samsung.com ([210.118.77.14]) by mailout4.w1.samsung.com (Sun Java(tm) System Messaging Server 6.3-8.04 (built Jul 29 2009; 32bit)) with ESMTP id <0LQ200FXSOZ14460@mailout4.w1.samsung.com> for linux-mm@kvack.org; Wed, 17 Aug 2011 13:49:49 +0100 (BST) Received: from linux.samsung.com ([106.116.38.10]) by spt2.w1.samsung.com (iPlanet Messaging Server 5.2 Patch 2 (built Jul 14 2004)) with ESMTPA id <0LQ2007H9OZ03S@spt2.w1.samsung.com> for linux-mm@kvack.org; Wed, 17 Aug 2011 13:49:48 +0100 (BST) Date: Wed, 17 Aug 2011 14:49:13 +0200 From: Marek Szyprowski Subject: RE: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings In-reply-to: Message-id: <008d01cc5cdc$11392520$33ab6f60$%szyprowski@samsung.com> Content-language: pl References: <1313146711-1767-1-git-send-email-m.szyprowski@samsung.com> <201108161528.48954.arnd@arndb.de> <20110816135516.GC17310@n2100.arm.linux.org.uk> <201108161626.26130.arnd@arndb.de> Sender: owner-linux-mm@kvack.org List-ID: To: Marek Szyprowski , 'Arnd Bergmann' , 'Russell King - ARM Linux' Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org, linux-mm@kvack.org, linaro-mm-sig@lists.linaro.org, 'Michal Nazarewicz' , 'Kyungmin Park' , 'Andrew Morton' , 'KAMEZAWA Hiroyuki' , 'Ankita Garg' , 'Daniel Walker' , 'Mel Gorman' , 'Jesse Barker' , 'Jonathan Corbet' , 'Shariq Hasnain' , 'Chunsang Jeong' Hello, On Wednesday, August 17, 2011 10:01 AM Marek Szyprowski wrote: > On Tuesday, August 16, 2011 4:26 PM Arnd Bergmann wrote: > > On Tuesday 16 August 2011, Russell King - ARM Linux wrote: > > > On Tue, Aug 16, 2011 at 03:28:48PM +0200, Arnd Bergmann wrote: > > > > Hmm, I don't remember the point about dynamically sizing the pool for > > > > ARMv6K, but that can well be an oversight on my part. I do remember the > > > > part about taking that memory pool from the CMA region as you say. > > > > > > If you're setting aside a pool of pages, then you have to dynamically > > > size it. I did mention during our discussion about this. > > > > > > The problem is that a pool of fixed size is two fold: you need it to be > > > sufficiently large that it can satisfy all allocations which come along > > > in atomic context. Yet, we don't want the pool to be too large because > > > then it prevents the memory being used for other purposes. > > > > > > Basically, the total number of pages in the pool can be a fixed size, > > > but as they are depleted through allocation, they need to be > > > re-populated from CMA to re-build the reserve for future atomic > > > allocations. If the pool becomes larger via frees, then obviously > > > we need to give pages back. > > > > Ok, thanks for the reminder. I must have completely missed this part > > of the discussion. > > > > When I briefly considered this problem, my own conclusion was that > > the number of atomic DMA allocations would always be very low > > because they tend to be short-lived (e.g. incoming network packets), > > so we could ignore this problem and just use a smaller reservation > > size. While this seems to be true in general (see "git grep -w -A3 > > dma_alloc_coherent | grep ATOMIC"), there is one very significant > > case that we cannot ignore, which is pci_alloc_consistent. > > > > This function is still called by hundreds of PCI drivers and always > > does dma_alloc_coherent(..., GFP_ATOMIC), even for long-lived > > allocations and those that are too large to be ignored. > > > > So at least for the case where we have PCI devices, I agree that > > we need to have the dynamic pool. > > Do we really need the dynamic pool for the first version? I would like to > know how much memory can be allocated in GFP_ATOMIC context. What are the > typical sizes of such allocations? > > Maybe for the first version a static pool with reasonably small size > (like 128KiB) will be more than enough? This size can be even board > depended or changed with kernel command line for systems that really > needs more memory. > > I noticed one more problem. The size of the CMA managed area must be > the multiple of 16MiBs (MAX_ORDER+1). This means that the smallest CMA area > is 16MiB. These values comes from the internals of the kernel memory > management design and page blocks are the only entities that can be managed > with page migration code. I'm really sorry for the confusion. This 16MiB value worried me too much and I've checked the code once again and found that this MAX_ORDER+1 value was a miscalculation, which appeared in v11 of the patches. The true minimal CMA area size is 8MiB for ARM architecture. I believe this shouldn't be an issue for the current ARMv6+ based machines. I've checked it with "mem=16M cma=8M" kernel arguments. System booted fine and CMA area has been successfully created. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org