On Fri, Feb 28, 2025 at 6:39 PM Dave Jiang <dave.jiang@xxxxxxxxx> wrote:
>
>
>
> On 2/28/25 2:23 PM, Carlos Sergio Bederian wrote:
> > 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?
>
> Does dmatest work for you?
Yup, I've just compiled 6.12.17 with dmatest and it ran fine on every channel
listed in /sys/class/dma. No changes wrt dma_find_channel.
> Also, make sure dmatest isn't loaded when you have your module loaded.
dmatest wasn't even built.
> Or any other kernel module that uses dma like ntb_transport isn't claiming
> the channels.
No users AFAICT.
>
> DJ
> >
> > 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;
> > }
> >
>
