linux-kernel - Re: [PATCH] spi/pxa2xx-pci: Enable DMA binding through device name

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Date:	Fri, 25 Jul 2014 10:38:36 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Kweh Hock Leong <hock.leong.kweh@...el.com>,
	Eric Miao <eric.y.miao@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Mark Brown <broonie@...nel.org>,
	Chew Chiau Ee <chiau.ee.chew@...el.com>,
	Darren Hart <dvhart@...ux.intel.com>, chiauee85@...il.com,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] spi/pxa2xx-pci: Enable DMA binding through device name

On Friday 25 July 2014 11:22:49 Mika Westerberg wrote:
> > All you need to do is change your filter function to take the
> > slave id from pxa_spi_info and stick it in there, e.g.
> > 
> > static bool pxa2xx_spi_dw_dma_filter(struct dma_chan *chan, void *param)
> > {       
> >         const struct pxa2xx_spi_master *pdata = param;
> >         struct dw_dma_chan *dwc = to_dw_dma_chan(chan);
> >         
> >         dwc->request_line = fargs->req;
> >         dwc->src_master = 0;
> >         dwc->dst_master = 0;
> >         
> >         return 1;
> > }           
> 
> Oh man. That makes pxa2xx_spi dependent on a certain specific DMA engine
> driver.

I think you can improve this by putting the filter function (and a pointer
to it) into the pxa2xx_spi_master data provided by the PCI driver.

On ARM, we usually provide those pointers through platform_data from
the board file.

> > Note that the filter function by definition is specific to the dma
> > controller, not the dma slave (that's why most people define it in
> > the dmaengine driver), and the pxa2xx_spi_dma_filter() function used
> > in spi-pxa2xx-dma.c looks like it was written for another dma engine:
> 
> I wonder what's the rationale that passing slave_id with
> dma_slave_config is wrong? The current code works fine with that and is
> is independent of the DMA engine driver (even though we know that it is
> going to be dw-dma).
> 
> The dw-dma handles slave_id in its implementation of
> dmaengine_slave_config().

The main point is that a single 'slave_id' is not actually enough to
identify what a slave needs. In case of dw_dma, the dma engine actually
requires three numbers (request line, source master, destination master)
to identify it, and there is no good way to put all that information into
a single integer. Other dma engines require a different set of data.

There are only two drivers left that actually use slave_id in this way,
and only for the legacy (no DT or ACPI) case, every other driver uses
either DT or a filter function. I believe the shmobile part will soon
be done with, after shmobile has been converted to DT, and after that
we should remove the slave_id field from the dma_slave_config interface.

	Arnd
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