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Date: Fri, 9 Dec 2016 19:23:17 +0100 From: Mason <slash.tmp@...e.fr> To: Vinod Koul <vinod.koul@...el.com> Cc: Sebastian Frias <sf84@...oste.net>, Russell King <linux@....linux.org.uk>, dmaengine@...r.kernel.org, Linus Walleij <linus.walleij@...aro.org>, Dan Williams <dan.j.williams@...el.com>, LKML <linux-kernel@...r.kernel.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Jon Mason <jdmason@...zu.us>, Mark Brown <broonie@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, Lee Jones <lee.jones@...aro.org>, Laurent Pinchart <laurent.pinchart@...asonboard.com>, Arnd Bergmann <arnd@...db.de>, Maxime Ripard <maxime.ripard@...e-electrons.com>, Dave Jiang <dave.jiang@...el.com>, Peter Ujfalusi <peter.ujfalusi@...com>, Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>, Thibaud Cornic <thibaud_cornic@...madesigns.com> Subject: Re: Tearing down DMA transfer setup after DMA client has finished [ Dropping Mans to preserve his peace-of-mind ] On 09/12/2016 18:56, Vinod Koul wrote: > On Fri, Dec 09, 2016 at 06:34:15PM +0100, Mason wrote: >> On 09/12/2016 18:17, Vinod Koul wrote: >> >>> On Fri, Dec 09, 2016 at 11:25:57AM +0100, Sebastian Frias wrote: >>>> >>>> What concrete solution do you propose? >>> >>> I have already proposed two solutions. >>> >>> A) Request a channel only when you need it. Obviously we can't do virtual >>> channels with this (though we should still use virt-channels framework). >>> The sbox setup and teardown can be done as part of channel request and >>> freeup. PL08x already does this. >>> >>> Downside is that we can only have as many consumers at a time as channels. >>> >>> I have not heard any technical reason for not doing this apart from drivers >>> grab the channel at probe, which is incorrect and needs to be fixed >>> irrespective of the problem at hand. >>> >>> This is my preferred option. >> >> There is one important drawback with this solution. If a driver calls >> dma_request_chan() when no channels are currently available, it will >> get -EBUSY. If there were a flag in dma_request_chan to be put to >> sleep (with timeout) until a channel is available, then it would >> work. But busy waiting in the client driver is a waste of power. > > Right, but in that case the fallback would be PIO mode, and if that is > not availble (IIRC some f your devices don't) then reject the usage with > EAGAIN. Maybe I'm missing something, but I don't see how that would help. Take the NAND Flash controller driver, for instance. PIO is not an option, because the ECC engine is tied to DMA. And failing with -EAGAIN doesn't help the busy looping situation. The caller should be put on some kind of queue to wait for a "channel ready" event. Regards.
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