From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754147AbaI2MJ6 (ORCPT <rfc822;w@1wt.eu>);
	Mon, 29 Sep 2014 08:09:58 -0400
Received: from mailout32.mail01.mtsvc.net ([216.70.64.70]:45360 "EHLO
	n23.mail01.mtsvc.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org
	with ESMTP id S1753412AbaI2MJ5 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 29 Sep 2014 08:09:57 -0400
Message-ID: <54294C0B.1060705@hurleysoftware.com>
Date: Mon, 29 Sep 2014 08:09:47 -0400
From: Peter Hurley <peter@hurleysoftware.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.2
MIME-Version: 1.0
To: Akinobu Mita <akinobu.mita@gmail.com>
CC: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        David Woodhouse <dwmw2@infradead.org>, Don Dutile <ddutile@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
        x86@kernel.org, iommu@lists.linux-foundation.org
Subject: Re: [PATCH v3 0/5] enhance DMA CMA on x86
References: <1397567329-3771-1-git-send-email-akinobu.mita@gmail.com>	<5426CA0A.7000806@hurleysoftware.com> <CAC5umyhgs8---HZLa7_DOSbqW0uPbLgqTfBweScZSR9oWbG9xg@mail.gmail.com>
In-Reply-To: <CAC5umyhgs8---HZLa7_DOSbqW0uPbLgqTfBweScZSR9oWbG9xg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
X-Authenticated-User: 990527 peter@hurleysoftware.com
X-MT-ID: 8FA290C2A27252AACF65DBC4A42F3CE3735FB2A4
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 09/27/2014 08:31 PM, Akinobu Mita wrote:
> 2014-09-27 23:30 GMT+09:00 Peter Hurley <peter@hurleysoftware.com>:
>> On 04/15/2014 09:08 AM, Akinobu Mita wrote:
>>> This patch set enhances the DMA Contiguous Memory Allocator on x86.

[...]

>> What this patchset does is restrict all iommu configurations which can
>> map all of system memory to one _very_ small physical region, thus disabling
>> the whole point of an iommu.
>>
>> Now I know why my GPU is causing paging to disk! And why my RAID controller
>> stalls for ages when I do a git log at the same time as a kernel build!
> 
> The solution I have for this is that instead of trying to
> dma_alloc_from_contiguous() firstly, call alloc_pages() in dma_alloc_coherent().
> dma_alloc_from_contiguous() should be called only when alloc_pages() is failed
> or DMA_ATTR_FORCE_CONTIGUOUS is specified in dma_attr.

Why is all this extra complexity being added when there are no X86 users
of DMA_ATTR_FORCE_CONTIGUOUS?


>> And the apparent goal of this patchset is to enable DMA allocation below
>> 4GB, which is already supported in the existing page allocator with the
>> GFP_DMA32 flag?!
> 
> The goal of this patchset is to enable huge DMA allocation which
> alloc_pages() can't (> MAX_ORDER) for the devices that require it.

What x86 devices need > MAX_ORDER DMA allocation and why can't they allocate
directly from dma_alloc_from_contiguous()?

Regards,
Peter Hurley