linux-kernel - Re: [PATCH v8 1/5] PM / Runtime: Add getter for querying the IRQ safe option

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Message-ID: <20141103170408.GD4042@n2100.arm.linux.org.uk>
Date:	Mon, 3 Nov 2014 17:04:08 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>,
	Jonathan Corbet <corbet@....net>,
	Dan Williams <dan.j.williams@...el.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-pm@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v8 1/5] PM / Runtime: Add getter for querying the IRQ
 safe option

On Mon, Nov 03, 2014 at 09:57:28PM +0530, Vinod Koul wrote:
> On Sat, Nov 01, 2014 at 02:29:42AM +0200, Laurent Pinchart wrote:
> > Many other drivers suffer from the same problem. While I won't reject your 
> > proposed fix, I would prefer a more generic approach.
> > 
> > One option that has been discussed previously was to use a work queue to delay 
> > starting the DMA transfer to an interruptible context where 
> > pm_runtime_get_sync() could be called. However, as Russell pointed out [1], 
> > even that won't work in all cases as the DMA slave might need the transfer to 
> > be started before enabling part of its hardware (OMAP audio seem to be such a 
> > case).
> > 
> > I've heard a rumor of a possible DMA engine rework to forbid calling the 
> > descriptor preparation API from atomic context. This could be used as a base 
> > to implement runtime PM, as DMA slave drivers should not prepare descriptors 
> > if they don't need to use them. However that's a long term plan, and we need a 
> > solution sooner than that.
> 
> Well it is not a rumour :)
> 
> I have been contemplating that now that async_tx will be killed so we dont
> have to worry about that usage. For the slave dma usage, we can change the
> prepare API to be non atomic. I think the users will be okay with approach.
> This way drivers can use runtime pm calls in prepare.

Except we /do/ have a fair number of places where the prep calls are made
from atomic contexts, particularly in serial drivers.  You'd need to
introduce a tasklet into almost every serial driver which doesn't
already have one to restart RX DMA after an error or pause.  Eg,

drivers/tty/serial/amba-pl011.c
drivers/tty/serial/pch_uart.c
drivers/tty/serial/imx.c

Probably also:

drivers/net/ethernet/micrel/ks8842.c

There could well be other places as well, I've not gone through and
checked exhaustively.

-- 
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according to speedtest.net.
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