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,

quoted

quoted

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.

quoted

Does not VIRTIO_CONSOLE_F_DMA_MEM mean dma api is required?

Yes.dma_alloc_coherent to work.

quoted

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.

quoted

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