linux-kernel - RE: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch" allocator

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Message-ID: <129600E5E5FB004392DDC3FB599660D7B5548785@irsmsx504.ger.corp.intel.com>
Date:	Tue, 15 Sep 2009 17:05:28 +0100
From:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: RE: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch"
 allocator

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 <dan.j.williams@...el.com>
> ---

Signed-off-by: Maciej Sosnowski <maciej.sosnowski@...el.com>

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?

Thanks,
Maciej--
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