On Monday 04 November 2013, Jason Gunthorpe wrote:
> On Mon, Nov 04, 2013 at 02:43:00PM -0700, Stephen Warren wrote:
> > For DMA, it does look like Arnd's code was requesting it too, but that
> > should also be fine; as long as no transactions are actually issued
> > against that DMA slave channel, then the HW state shouldn't matter?
>
> TBH, I'm not really familiar with how the DMA slave API works.
>
> As long as the API and HW guarantees that the channel cannot do any
> DMAs no matter what the connected IP does, it is obviously fine..
Right, to initiate a DMA, you need to at least request a channel,
configure it, and submit a descriptor. This does only the first
step, so I'm pretty it's ok.
> But not all DMA is like that, eg bus master DMA in PCI requires
> drivers to call pci_enable_device only after they disable DMA in the
> device, which is why I mentioned it..
Yes, a DMA bus master device is different here and needs to be
handled as you exlain.
> It would be nice to see a general API like this unambiguously make
> clear the steps required:
>
> - Gain access to registers
> - Gain control of the device
> - Enable DMA, bind interrupts, etc
> - Finalize device setup
> - Make the device visible to the rest of the system
>
> I've seen lots of drivers where the above is just not done right.
I agree this would be nice, but it's probably out of scope of what
we're trying to do here, unless you have better ideas for how to
structure it.
Arnd
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