Messages in this thread | | | Date | Tue, 4 Sep 2012 13:28:36 +0200 | Subject | Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation | From | Sjur Brændeland <> |
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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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