linux-kernel - Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

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Date:	Mon, 3 Sep 2012 23:27:37 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sjur Brændeland <sjurbren@...il.com>
Cc:	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

On Mon, Sep 03, 2012 at 04:57:45PM +0200, Sjur Brændeland wrote:
> Hi Michael,
> 
> > How does access to descriptors work in this setup?
> 
> When the ring is setup by remoteproc the descriptors are
> also allocated using dma_alloc_coherent().
> 
> >> -static void free_buf(struct port_buffer *buf)
> >> +/* Allcoate data buffer from DMA memory if requested */
> >
> > typo
> 
> Thanks.
> 
> >> +static inline void *
> >> +alloc_databuf(struct virtio_device *vdev, size_t size, dma_addr_t *dma_handle,
> >> +                gfp_t flag)
> >>  {
> >> -     kfree(buf->buf);
> >> +#ifdef CONFIG_HAS_DMA
> >> +     if (virtio_has_feature(vdev, VIRTIO_CONSOLE_F_DMA_MEM)) {
> >> +             struct device *dev = &vdev->dev;
> >> +             /*
> >> +              * Allocate DMA memory from ancestors. Finding the ancestor
> >> +              * is a bit quirky when DMA_MEMORY_INCLUDES_CHILDREN is not
> >> +              * implemented.
> >> +              */
> >> +             dev = dev->parent ? dev->parent : dev;
> >> +             dev = dev->parent ? dev->parent : dev;
> >> +             return dma_alloc_coherent(dev, size, dma_handle, flag);
> >> +     }
> >> +#endif
> >
> > Are these ifdefs really needed? If DMA_MEM is set,
> > can't we use dma_alloc_coherent
> > unconditionally?
> 
> If an architecture do not support DMA you will get
> a link error: "unknown symbol" for dma_alloc_coherent.
> 
> Regards,
> Sjur

Yes, it even seems intentional.
But I have a question: can the device work without DMA?
Does not VIRTIO_CONSOLE_F_DMA_MEM mean dma api is required?
If yes you should just fail load.

Also let's add a wrapper at top of file so ifdefs
do not litter the code like this. For example:

#ifdef CONFIG_HAS_DMA
#define VIRTIO_CONSOLE_HAS_DMA (1)
#else
#define VIRTIO_CONSOLE_HAS_DMA (0)
#endif

Now use if instead of ifdef.

-- 
MST
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