Re: [PATCH v4 3/3] selftests: pci_endpoint: Migrate to Kselftest framework

Manivannan Sadhasivam
2025-01-16T04:47:35Z

On Thu, Jan 02, 2025 at 03:23:14PM +0100, Niklas Cassel wrote:
> Hello Mani, Vinod,
>
> On Thu, Jan 02, 2025 at 12:34:04PM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Dec 31, 2024 at 08:33:57PM +0100, Niklas Cassel wrote:
> > >
> > > I have some patches that adds DMA_MEMCPY to dw-edma, but I'm not sure if the DWC eDMA hardware supports having both src and dst as PCI addresses, or if only one of them can be a PCI address (with the other one being a local address).
> > >
> > > If only one of them can be a PCI address, then I'm not sure if your suggested patch is correct.
> > >
> >
> > I don't see why that would be an issue. DMA_MEMCPY is independent of PCI/local
> > addresses. If a dmaengine driver support doing MEMCPY, then the dma cap should
> > be sufficient. As you said, if a controller supports both SLAVE and MEMCPY, the
> > test currently errors out, which is wrong.
>
> While I am okay with your suggested change to pci-epf-test.c:
> > >- if (epf_test->dma_private) {
> > >+ if (!dma_has_cap(DMA_MEMCPY, epf_test->dma_chan_tx->device->cap_mask)) {
>
> Since this will ensure that a DMA driver implementing DMA_MEMCPY,
> which cannot be shared (has DMA_PRIVATE set), will not error out.
>
>
> What I'm trying to explain is that in:
> https://lore.kernel.org/linux-pci/Z2BW4CjdE1p50AhC@vaman/
> https://lore.kernel.org/linux-pci/20241217090129.6dodrgi4tn7l3cod@thinkpad/
>
> Vinod (any you) suggested that we should add support for prep_memcpy()
> (which implies also setting cap DMA_MEMCPY) in the dw-edma DMA driver.
>
> However, from section "6.3 Using the DMA" in the DWC databook,
> the DWC eDMA hardware only supports:
> - Transfer (copy) of a block of data from local memory to remote memory.
> - Transfer (copy) of a block of data from remote memory to local memory.
>
>
> Currently, we have:
> https://github.com/torvalds/linux/blob/v6.13-rc5/include/linux/dmaengine.h#L843-L844
> https://github.com/torvalds/linux/blob/v6.13-rc5/drivers/dma/dw-edma/dw-edma-core.c#L215-L231
>
> Where we can expose per-channel capabilities, so we set MEM_TO_DEV/DEV_TO_MEM
> per channel, however, these are returned in a struct dma_slave_caps *caps,
> so this is AFAICT only for DMA_SLAVE, not for DMA_MEMCPY.
>
> Looking at:
> https://github.com/torvalds/linux/blob/v6.13-rc5/include/linux/dmaengine.h#L975-L979
> it seems that DMA_MEMCPY is always assumed to be MEM_TO_MEM.
>
> To me, it seems that we would either need a new dma_transaction_type (e.g. DMA_COPY)
> where we can set dir:
> MEM_TO_DEV, DEV_TO_MEM, or DEV_TO_DEV. (dw-edma would not support DEV_TO_DEV.)
>
> Or, if we should stick with DMA_MEMCPY, we would need another way of telling
> client drivers that only src or dst can be a remote address.
>
> Until this is solved, I think I will stop my work on adding DMA_MEMCPY to the
> dw-edma driver.
>

I think your concern is regarding setting the DMA transfer direction for MEMCPY,
right? And you are saying that even if we use tx/rx channels, currently we
cannot set DEV_TO_DEV like directions?

But I'm somewhat confused about what is blocking you from adding MEMCPY support
to the dw-edma driver since that driver cannot support DEV_TO_DEV. In your WIP
driver, you were setting the direction based on the channel. Isn't that
sufficient enough?

- Mani

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