dma_find_channel(DMA_MEMCPY) on ioat

[Carlos Sergio Bederian <carlos.bederian@xxxxxxxxxx>]

I work at an HPC center and I've been trying to figure out why the
knem intra-node communication kernel module stopped being able to use
IOAT to offload memcpy at some point in time, presumably a long time
ago.
The knem module uses dma_find_channel(DMA_MEMCPY) to get a dma_chan so
I wrote a test kernel module that tries to grab a dma_chan using both
dma_find_channel and dma_request_channel and then submits a memcpy.
dma_request_channel succeeds in returning a DMA_MEMCPY channel, but
dma_find_channel never does, regardless of order. This is on a Debian
6.12.9 kernel.
Is there anything I'm missing?

static struct dma_chan* dma_req(void) {
    struct dma_chan* chan = NULL;
    dma_cap_mask_t mask;
    dma_cap_zero(mask);
    dma_cap_set(DMA_MEMCPY, mask);
    chan = dma_request_channel(mask, NULL, NULL);
    if (!chan) {
        pr_err("dmacopy: dma_request_channel didn't return a channel");
    } else {
        pr_info("dmacopy: dma_request_channel succeeded");
    }
    return chan;
}

static struct dma_chan* dma_find(void) {
    struct dma_chan* chan = NULL;
    dmaengine_get();
    chan = dma_find_channel(DMA_MEMCPY);
    if (!chan) {
        pr_err("dmacopy: dma_find_channel didn't return a channel");
        dmaengine_put();
    } else {
        pr_info("dmacopy: dma_find_channel succeeded");
    }
    return chan;
}