From: vinod.koul@intel.com (Vinod Koul)
To: linux-arm-kernel@lists.infradead.org
Subject: DMA engine API issue (was: [PATCH/RFC 0/5] R-Car Gen2 DMAC hardware descriptor list support)
Date: Fri, 1 Aug 2014 22:39:18 +0530 [thread overview]
Message-ID: <20140801170918.GE8181@intel.com> (raw)
In-Reply-To: <20140801143020.GQ30282@n2100.arm.linux.org.uk>
On Fri, Aug 01, 2014 at 03:30:20PM +0100, Russell King - ARM Linux wrote:
> On Fri, Aug 01, 2014 at 10:51:26AM +0200, Laurent Pinchart wrote:
> > I'll take this opportunity to question why we have a separation between
> > tx_submit and issue_pending. What's the rationale for that, especially given
> > that dma_issue_pending_all() might kick in at any point and issue pending
> > transfers for all devices. A driver could thus see its submitted but not
> > issued transactions being issued before it explicitly calls
> > dma_async_issue_pending().
>
> A prepared but not submitted transaction is not a pending transaction.
>
> The split is necessary so that a callback can be attached to the
> transaction. This partially comes from the async-tx API, and also
> gets a lot of use with the slave API.
>
> The prepare function allocates the descriptor and does the initial
> setup, but does not mark the descriptor as a pending transaction.
> It returns the descriptor, and the caller is then free to add a
> callback function and data pointer to the descriptor before finally
> submitting it. This sequence must occur in a timely manner as some
> DMA engine implementations hold a lock between the prepare and submit
> callbacks (Dan explicitly permits this as part of the API.)
>
> > The DMA_PRIVATE capability flag seems to play a role here, but it's far from
> > being clear how that mechanism is supposed to work. This should be documented
> > as well, and any light you could shed on this dark corner of the API would
> > help.
>
> Why do you think that DMA_PRIVATE has a bearing in the callbacks? It
> doesn't. DMA_PRIVATE is all about channel allocation as I explained
> yesterday, and whether the channel is available for async_tx usage.
>
> A channel marked DMA_PRIVATE is not available for async_tx usage at
> any moment. A channel without DMA_PRIVATE is available for async_tx
> usage until it is allocated for the slave API - at which point the
> generic DMA engine code will mark the channel with DMA_PRIVATE,
> thereby taking it away from async_tx API usage. When the slave API
> frees the channel, DMA_PRIVATE will be cleared, making the channel
> available for async_tx usage.
>
> So, basically, DMA_PRIVATE set -> async_tx usage not allowed.
> DMA_PRIVATE clear -> async_tx usage permitted. It really is that
> simple.
>
> > Similarly, the DMA engine API is split in functions with different
> > prefixes (mostly dmaengine_*, dma_async_*, dma_*, and various
> > unprefixed niceties such as async_tx_ack or txd_lock. If there's a
> > rationale for that (beyond just historical reasons) it should also
> > be documented, otherwise a cleanup would help all the confused DMA
> > engine users (myself included).
>
> dmaengine_* are generally the interface functions to the DMA engine code,
> which have been recently introduced to avoid the multiple levels of
> pointer indirection having to be typed in every driver.
>
> dma_async_* are the DMA engine interface functions for the async_tx API.
>
> dma_* predate the dmaengine_* naming, and are probably too generic, so
> should probably end up being renamed to dmaengine_*.
>
> txd_* are all about the DMA engine descriptors.
>
> async_tx_* are the higher level async_tx API functions.
Ah looks like I repeated the good answers from you. Should have read all
replied first
--
~Vinod
next prev parent reply other threads:[~2014-08-01 17:09 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1406032431-3807-1-git-send-email-laurent.pinchart+renesas@ideasonboard.com>
[not found] ` <87d2cwvmxa.wl%kuninori.morimoto.gx@gmail.com>
[not found] ` <3361994.F0HgAeJkR9@avalon>
2014-07-23 11:07 ` DMA engine API issue (was: [PATCH/RFC 0/5] R-Car Gen2 DMAC hardware descriptor list support) Laurent Pinchart
2014-07-24 0:46 ` Kuninori Morimoto
2014-07-24 1:35 ` Kuninori Morimoto
2014-07-24 4:53 ` Vinod Koul
2014-07-24 4:52 ` Vinod Koul
2014-08-01 8:51 ` Laurent Pinchart
2014-08-01 14:30 ` Russell King - ARM Linux
2014-08-01 17:09 ` Vinod Koul [this message]
2014-08-04 13:47 ` Geert Uytterhoeven
2014-08-04 17:00 ` Laurent Pinchart
2014-08-04 17:54 ` Russell King - ARM Linux
2014-08-05 23:19 ` Laurent Pinchart
2014-08-06 7:17 ` Geert Uytterhoeven
2014-08-06 11:04 ` Arnd Bergmann
2014-08-01 17:07 ` Vinod Koul
2014-08-04 16:50 ` Laurent Pinchart
2014-08-04 18:03 ` DMA engine API issue Lars-Peter Clausen
2014-08-04 18:32 ` Russell King - ARM Linux
2014-08-04 23:12 ` Laurent Pinchart
2014-08-05 16:56 ` DMA engine API issue (was: [PATCH/RFC 0/5] R-Car Gen2 DMAC hardware descriptor list support) Vinod Koul
2014-07-24 12:29 ` [alsa-devel] DMA engine API issue Lars-Peter Clausen
2014-07-24 12:51 ` DMA engine API issue (was: [PATCH/RFC 0/5] R-Car Gen2 DMAC hardware descriptor list support) Russell King - ARM Linux
2014-08-01 9:24 ` Laurent Pinchart
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