linux-kernel - Re: [RFC v2 5/5] Atmel MCI: Driver for Atmel on-chip MMC controllers

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Message-ID: <20080213220646.081583d6@siona>
Date:	Wed, 13 Feb 2008 22:06:46 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	"Dan Williams" <dan.j.williams@...el.com>
Cc:	linux-kernel@...r.kernel.org,
	"Shannon Nelson" <shannon.nelson@...el.com>,
	"David Brownell" <david-b@...bell.net>, kernel@...32linux.org,
	"Francis Moreau" <francis.moro@...il.com>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Vladimir A. Barinov" <vbarinov@...mvista.com>,
	"Pierre Ossman" <drzeus-list@...eus.cx>
Subject: Re: [RFC v2 5/5] Atmel MCI: Driver for Atmel on-chip MMC
 controllers

On Wed, 13 Feb 2008 12:11:58 -0700
"Dan Williams" <dan.j.williams@...el.com> wrote:

> > +               desc = chan->device->device_prep_slave(chan,
> > +                               sg_dma_address(sg), direction,
> > +                               DMA_SLAVE_WIDTH_32BIT,
> > +                               sg_dma_len(sg), dma_flags);
> > +               desc->txd.callback = NULL;
> > +               list_add_tail(&desc->client_node,
> > +                               &host->dma.data_descs);
> > +       }  
> 
> Need to handle device_prep_slave returning NULL?

You're right, we definitely need to handle that. Which probably means
we need to prepare an interrupt descriptor first that we can throw in
when we're unable to obtain more descriptors, and submit the rest from
the callback.

Except we're not allowed to submit anything from the callback. Ouch.

How can we solve that? Set up a work queue and submit it from there?
Trigger a different tasklet?

In any case, I guess I need to implement support for interrupt
descriptors in the dw_dmac driver.

Haavard
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