Re: dma_alloc_coherent bug in latest ntb_tool changes

[Serge Semin <fancer.lancer@gmail.com>]

On Thu, Jun 07, 2018 at 04:17:02PM -0700, Dave Jiang <dave.jiang@intel.com> wrote:
> 
> 
> On 06/07/2018 04:10 PM, Logan Gunthorpe wrote:
> > Hey,
> > 
> > I'm just running ntb_test on recent kernels with switchtec hardware and
> > found that ntb_tool and ntb_perf no longer work. I get this kernel warning:
> > 
> > [   64.077406] WARNING: CPU: 3 PID: 2231 at
> > ./include/linux/dma-mapping.h:516 tool_mw_trans_write+0x16d/0x332 [ntb_tool]
> > 
> > Indicating that the dma_coherent mask is not set on the device. (Not
> > sure how much testing that patch set got before it was merged upstream...)
> > 
> > It seems one of the changes that slipped through is that it now
> > allocates DMA memory using the NTB struct device instead of the PCI
> > struct device that was used before the patchset.
> > 
> > Seeing nothing in the NTB tree sets the coherent mask on the NTB device
> > we get a warning and fail to allocate DMA-able memory.
> > 
> > I'm not sure the proper fix here: should we be using the NTB device for
> > DMA operations or roll back to using the PCI device? It seems cleaner to
> > use the NTB device but then we at least have to set the coherent_mask to
> > something sane (probably safe to set it to 64bits for NTB hardware).
> > However, I'm not sure if, for all arches, DMA ops depend on anything in
> > the device besides the mask.
> > 
> > Logan
> > 
> 
> I seem to recall having this discussion previously but don't remember
> the outcome. I still think we should be dealing with the PCI device when
> it comes to hardware stuff like DMA since that is the actual device. The
> NTB device is just a an abstract device to make it work with the NTB bus
> isn't it?

>> The NTB device is just a an abstract device to make it work with the NTB bus isn't it?

It is, and due to being abstraction, the client drivers shouldn't know, what
is hidden behind the scene. It might be PCIe, but may be something else. That's
why we decided to use dma_coerce_mask_and_coherent() and to use NTB device for
DMA allocations. I even had to alter ntb.c file so the ntb_register_device()
wouldn't memset the ntb-structure with zeros.

-Sergey