From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752028AbcENIFA (ORCPT ); Sat, 14 May 2016 04:05:00 -0400 Received: from mga11.intel.com ([192.55.52.93]:11717 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750975AbcENIEz (ORCPT ); Sat, 14 May 2016 04:04:55 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,617,1455004800"; d="scan'208";a="976296947" Date: Sat, 14 May 2016 13:41:06 +0530 From: Vinod Koul To: Kuninori Morimoto Cc: Laurent Pinchart , Geert Uytterhoeven , Laurent Pinchart , sakato , OSD2 ML , dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] dma: rcar-dmac: use list_add() on rcar_dmac_desc_put() Message-ID: <20160514081106.GE23734@localhost> References: <87r3dywh6u.wl%kuninori.morimoto.gx@renesas.com> <20160502093712.GE2274@localhost> <1529191.mKmgdgIODm@avalon> <87wpn1th2q.wl%kuninori.morimoto.gx@renesas.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wpn1th2q.wl%kuninori.morimoto.gx@renesas.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 11, 2016 at 03:28:04AM +0000, Kuninori Morimoto wrote: > > Hi Laurent, Vinod, > > > > > From: Kuninori Morimoto > > > > > > > > Current rcar_dmac_desc_put() is using list_add_tail() in order to > > > > push used descriptor to list of free descriptors, and next DMA transfer > > > > try to reuse it from this list. But because it is using *_tail(), > > > > this reuse effect can't be obtained without using all of them. > > > > For a longer-term solution, we should allocate hardware descriptors > > > > using GFP_KERNEL instead of GFP_NOWAIT, but it is difficult today. > > > > This patch uses list_add() instead of list_add_tail() for short-term > > > > solution. > > > > > > So how does reuse case help by not moving the descriptor to tail. > > > > > > Also you are not reusing descriptor, you are reusing a descriptor memory, > > > these are two different things. > > > > > > Lastly how does this help? Something doesn't seem right > > > > For each descriptor, in addition to the memory used by the descriptors > > structure itself, the driver allocates a list of chunks as well as a buffer > > for hardware descriptors. Descriptors themselves are preallocated, and > > allocation of the chunks and buffer is performed the first time the descriptor > > is used. The memory isn't freed when the transfer is completed, as the chunks > > and buffer will be needed again when the descriptor is reused internally, so > > the driver keeps the memory around. > > > > If only a few descriptors are used concurrently, the current list_add_tail() > > implementation will result in all preallocated descriptors being used before > > going back to the first one, and will thus allocate chunks and a buffer for > > all preallocated descriptors. Using list_add() will put the complete > > descriptor at the head of the list of available descriptors, so the next > > transfer will be more likely to reuse a descriptor that already has associated > > memory instead of one that has never been used before. > > Laurent, thank you for your help > Vinod, does above clear for you ? Yes makese sense now. But please add these details in the changelog. This helps people know why a line was modified down the line -- ~Vinod