About dma_sync_single_for_{cpu,device}

On 7/31/12, Russell King - ARM Linux [off-list ref] wrote:

On Tue, Jul 31, 2012 at 09:31:13PM +0200, Karl Beldan wrote:

quoted

On 7/31/12, Russell King - ARM Linux [off-list ref] wrote:

quoted

On Tue, Jul 31, 2012 at 08:45:57AM +0200, Karl Beldan wrote:

quoted

I was expecting the following to work:
	addr = dma_map_single(dev, buffer, size, DMA_TO_DEVICE);
	dma_sync_single_for_device(dev, buffer, pattern_size,
DMA_FROM_DEVICE);
	dev_send(buffer);
	// wait for irq (don't peek in the buffer) ... got irq
	dma_sync_single_for_cpu(dev, buffer, pattern_size, DMA_FROM_DEVICE);
	if (!xfer_done(buffer)) // not RAM value
		dma_sync_single_for_device(dev, buffer, pattern_size,
DMA_FROM_DEVICE);
	[...]

Hi Russell,

quoted

First point is that you clearly do not understand the DMA API at all.
The
DMA API has the idea of buffer ownership.  Only the owner may access
the
buffer:

Are you saying that this scenario does not work ?
We are taking some liberties with the DMA API, we're more using some
of its funcs rather than _using_ it ;).
The question was not whether this was a proper usage of the API, but
why that scenario would not lead to the expected results .. and now
I've found the culprit peek I am happy.

If you abuse the API don't expect your stuff to work in future kernel
versions.

Of course.

It seems that the overall tone of your reply is "what we have now works,
we don't care if it's correct, sod you."

Not at all : {
On 7/31/12, Karl Beldan [off-list ref] wrote:

I might use something different in a not so distant future, yet, for
the time being, this way of doing as its advantages.

} and during this time I might stick to the API.
I am not at ease telling people how they should take things,
especially if I asked for their help, all the more when they put
efforts within the exchange while being expert on the matter, yet
please, do not assume I did not care for your advices, which I deem of
the most valuable, as, needless to say, do the community.

Fine, I won't spend any more time on this.  Just don't ever think about
merging it into mainline, thanks.

Merge submission while taking such liberties .. that I would not dare
;) this really was a down to the ground technical question not the
start of a disguised start of a merging request.

I am sure that taking such liberties and feeling its limits before
sticking to a super safe API is not a bad thing, e.g it might trigger
easierly nasty hidden bugs, it is often beneficial to me at least.

Thanks for your feedback,

Karl