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

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Date:	Tue, 4 Sep 2012 16:50:22 +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 Tue, Sep 04, 2012 at 01:28:36PM +0200, Sjur Brændeland wrote:
> Hi Michael,
> 
> >> If an architecture do not support DMA you will get
> >> a link error: "unknown symbol" for dma_alloc_coherent.
> >
> > Yes, it even seems intentional.
> > But I have a question: can the device work without DMA?
> 
> The main dependency is actually on the dma-allocation.
> In my case I do dma_declare_coherent_memory() with
> the memory area shared with the remote device as argument.
> Subsequent calls to dma_alloc_coherent() will then allocate
> from this memory area.
> 
> > Does not VIRTIO_CONSOLE_F_DMA_MEM mean dma api is required?
> 
> Yes.dma_alloc_coherent to work.
> 
> > If yes you should just fail load.
> 
> Agree, I'll check on VIRTIO_CONSOLE_HAS_DMA before adding
> VIRTIO_CONSOLE_F_DMA_MEM to the feature array.
> 
> > 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
> 
> OK, good point. Perhaps we could even exploit gcc's
> ability to remove dead code and do something like this:
> 
>        if (VIRTIO_CONSOLE_HAS_DMA &&
>            virtio_has_feature(vdev, VIRTIO_CONSOLE_F_DMA_MEM)) {
> ...
> 
> This does not give any linker warnings when compiling for UML (no DMA).
> 
> Regards,
> Sjur

Exactly. Though if we just fail load it will be much less code.

Generally, using a feature bit for this is a bit of a problem though:
normally driver is expected to be able to simply ignore
a feature bit. In this case driver is required to
do something so a feature bit is not a good fit.
I am not sure what the right thing to do is.

-- 
MST
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