From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Williams Subject: Re: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch" allocator Date: Tue, 15 Sep 2009 10:28:46 -0700 Message-ID: <4AAFCECE.9070302@intel.com> References: <20090904064308.7141.30576.stgit@dwillia2-linux.ch.intel.com> <20090904064445.7141.18693.stgit@dwillia2-linux.ch.intel.com> <129600E5E5FB004392DDC3FB599660D7B5548785@irsmsx504.ger.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <129600E5E5FB004392DDC3FB599660D7B5548785@irsmsx504.ger.corp.intel.com> Sender: linux-raid-owner@vger.kernel.org To: "Sosnowski, Maciej" Cc: "linux-kernel@vger.kernel.org" , "linux-raid@vger.kernel.org" List-Id: linux-raid.ids Sosnowski, Maciej wrote: > Williams, Dan J wrote: >> Channel switching is problematic for some dmaengine drivers as the >> architecture precludes separating the ->prep from ->submit. In these >> cases the driver can select ASYNC_TX_DISABLE_CHANNEL_SWITCH to modify >> the async_tx allocator to only return channels that support all of the >> required asynchronous operations. >> >> For example MD_RAID456=y selects support for asynchronous xor, xor >> validate, pq, pq validate, and memcpy. When >> ASYNC_TX_DISABLE_CHANNEL_SWITCH=y any channel with all these >> capabilities is marked DMA_ASYNC_TX allowing async_tx_find_channel() to >> quickly locate compatible channels with the guarantee that dependency >> chains will remain on one channel. When >> ASYNC_TX_DISABLE_CHANNEL_SWITCH=n async_tx_find_channel() may select >> channels that lead to operation chains that need to cross channel >> boundaries using the async_tx channel switch capability. >> >> Signed-off-by: Dan Williams >> --- > > Signed-off-by: Maciej Sosnowski > > with one question: > >> + /* note: this only matters in the >> + * CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH=y case >> + */ >> + if (device_has_all_tx_types(device)) >> + dma_cap_set(DMA_ASYNC_TX, device->cap_mask); > > Why not to put this part plus device_has_all_tx_types() > into #ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH then? > Because device_has_all_tx_types() already has the necessary ifdefs per operation type, and it does not rule out the possibility that code outside of async_tx can make use of DMA_ASYNC_TX capability. We don't gain anything by eliminating this call... although we would save an entry in the dmaengine channel table by turning off DMA_ASYNC_TX in enum dma_transaction_type. But we have already cleaned up dma_transaction_type in this patchset so the net change from 2.6.31 to 2.6.32 is still positive in this area. -- Dan