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

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Date:	Tue, 4 Sep 2012 13:28:36 +0200
From:	Sjur Brændeland <sjurbren@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.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

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
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