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

[Russell King - ARM Linux]

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.

-- 
FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up
according to speedtest.net.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/