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From: "Dan Williams" <dan.j.williams@intel.com>
To: "Olof Johansson" <olof@lixom.net>
Cc: linuxppc-dev@ozlabs.org, pasemi-linux@ozlabs.org, "Nelson,
	Shannon" <shannon.nelson@intel.com>,
	linux-kernel@vger.kernel.org, hskinnemoen@atmel.com
Subject: Re: [PATCH] pasemi_dma: Driver for PA Semi PWRficient on-chip DMAengine
Date: Thu, 13 Mar 2008 17:06:19 -0700	[thread overview]
Message-ID: <e9c3a7c20803131706m188f9494m1cdcda8354a65f4e@mail.gmail.com> (raw)
In-Reply-To: <20080313231442.GA4639@lixom.net>

On Thu, Mar 13, 2008 at 4:14 PM, Olof Johansson <olof@lixom.net> wrote:
>  > Dummy descriptors that do nothing but cause an interrupt is the intent.
>
>  Well, it'd be slightly more efficient to do add the interrupt attribute
>  to the last issued descriptor when it's known in advance. If the
>  underlying driver doesn't support it, adding a separate descriptor would
>  be a good fallback.
>

When it is known in advance the interrupt attribute *is* set,
otherwise the descriptor may already be in flight.

>  Anyway, this isn't likely to be a performance bottleneck. If it turns
>  out to be, I'll refactor it and submit patches.
>

Would not hurt to have another pair of eyes on this part of the code.
A rewrite of the channel switch mechanism is currently pending in
async_tx.git#upstream.

[..]
>  > >  Can't do that if it's called both from the polling as well as the IRQ
>  > >  context, it'd need to hold off irqs. I.e. once I add the DMA_INTERRUPT
>  > >  support it will be needed.
>  > >
>  >
>  > ...do it in a tasklet.
>
>  Why? That just adds overhead and latency.
>

The original ioat_dma code used nothing heavier than spin_lock_bh.
Async_tx now assumes that local_bh_disable prevents races with any
channel's cleanup routine.  Clients can place the same kind of code in
an async_tx callback as they would in a timer callback.  The
assumption is that code using async_tx can afford its extra overhead,
which is true for raid.  This is also why you don't see async_memcpy
calls in net_dma.

--
Dan

  reply	other threads:[~2008-03-14  0:06 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-06 23:39 [PATCH] pasemi_dma: Driver for PA Semi PWRficient on-chip DMA engine Olof Johansson
2008-03-07  0:31 ` Stephen Rothwell
2008-03-07  1:35   ` Olof Johansson
2008-03-11  7:06 ` Andrew Morton
2008-03-11 14:25   ` Olof Johansson
2008-03-11 17:53     ` Andrew Morton
2008-03-11 18:15       ` Dan Williams
2008-03-11 18:29         ` Kumar Gala
2008-03-11 20:37           ` Dan Williams
2008-03-11 17:04 ` [PATCH] pasemi_dma: Driver for PA Semi PWRficient on-chip DMAengine Dan Williams
2008-03-13 19:54   ` Olof Johansson
2008-03-13 22:29     ` Dan Williams
2008-03-13 23:14       ` Olof Johansson
2008-03-14  0:06         ` Dan Williams [this message]
2008-03-16 21:30 ` [PATCH v2] pasemi_dma: Driver for PA Semi PWRficient on-chip DMA engine Olof Johansson
2008-03-17 18:46   ` Dan Williams
2008-03-18  0:27     ` Olof Johansson
2008-03-17 20:34   ` [PATCH v2] pasemi_dma: Driver for PA Semi PWRficient on-chip DMAengine Nelson, Shannon
2008-03-18  0:21     ` Olof Johansson

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