linux-kernel - Re: [PATCH 1/3] dma: add dma_flags_set/get

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Message-ID: <20071017160327.GP5601@sgi.com>
Date:	Wed, 17 Oct 2007 09:03:27 -0700
From:	akepner@....com
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tony Luck <tony.luck@...el.com>,
	Grant Grundler <grundler@...isc-linux.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Jes Sorensen <jes@....com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Roland Dreier <rdreier@...co.com>,
	James Bottomley <James.Bottomley@...eleye.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] dma: add dma_flags_set/get_*() interfaces


[reply to the series of three mails below]

On Tue, Oct 16, 2007 at 08:27:28PM -0700, Andrew Morton wrote:
> On Tue, 16 Oct 2007 18:41:28 -0700 akepner@....com wrote:
> 
> > +#define DMA_BARRIER_ATTR	0x1
> > +#ifndef ARCH_USES_DMA_ATTRS
> > +static inline int dma_flags_set_attr(u32 attr, enum dma_data_direction dir) 
> > +{
> > +	return dir;
> > +}
> 
> This function takes an `enum dma_data_direction' as its second arg, but your
> ia64 implementation takes an 'int'.
> 

This is because the dma_data_direction enum type isn't available 
at the point the ia64 implementation is defined. 


> > .....
> >  dma_addr_t sn_dma_map_single(struct device *dev, void *cpu_addr, size_t size,
> > -                          int direction)
> > +                          int flags)
> >  {
> >       dma_addr_t dma_addr;
> >       unsigned long phys_addr;
> >       struct pci_dev *pdev = to_pci_dev(dev);
> >       struct sn_pcibus_provider *provider = SN_PCIDEV_BUSPROVIDER(pdev);
> > +     int dmabarrier = dma_flags_get_attr(flags) & DMA_BARRIER_ATTR;
> 
> So we take an `enum data_direction' and then wedge it into a word alongside
> some extra flags?
> 
> Can we do something nicer than that?

Changing the type of the last argument to dma_map_* functions 
to be a bitmask? Or adding an additional argument? (Both of 
which you mention below.)

> > .....

> > +DMA_BARRIER_ATTR would be set when the memory region is mapped for DMA,
> > +e.g.:
> > +
> > +     int count;
> > +     int flags = dma_flags_set_attr(DMA_BARRIER_ATTR, DMA_BIDIRECTIONAL);
> > +     ....
> > +     count = dma_map_sg(dev, sglist, nents, flags);
> > +
> 
> Isn't this rather a kludge?

I prefer the term "hack".

> 
> What would be the cost of doing this cleanly and either redefining
> dma_data_direction to be a field-of-bits or just leave dma_data_direction
> alone (it is quite unrelated to this work, isn't it?) and adding new
> fields/arguments to manage this new functionality?

It'd be significantly more work to do change or add arguments 
to the dma_map_* functions. But if that's what I need to do to 
get this accepted, I'll back up and have another go at it.

-- 
Arthur

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