Re: [PATCH V2 1/2] Documentation: DT: Add binding documentation for NVIDIA ADMA

[Stephen Warren]

On 10/09/2015 04:20 AM, Jon Hunter wrote:
> On 08/10/15 15:27, Stephen Warren wrote:
> > On 10/08/2015 03:58 AM, Jon Hunter wrote:
>
> [snip]
>
> > > That's fine. From my perspective I don't have a strong objection either
> > > way, however, I can see that given that the name indicates rx or tx,
> > > then the direction in the binding could be seen as redundant.
> > >
> > > So to confirm you are happy with the client bindings being as follows?
> > >
> > > tegra_admaif: admaif@0x702d0000 {
> > >         ...
> > >        dmas = <&adma 1>, <&adma 1>, <&adma 2>, <&adma 2>,
> > >               <&adma 3>, <&adma 3>, <&adma 4>, <&adma 4>,
> > >               <&adma 5>, <&adma 5>, <&adma 6>, <&adma 6>,
> > >               <&adma 7>, <&adma 7>, <&adma 8>, <&adma 8>,
> > >               <&adma 9>, <&adma 9>, <&adma 10>, <&adma 10>;
> > >        dma-names = "rx1", "tx1", "rx2", "tx2", "rx3", "tx3",
> > >                    "rx4", "tx4", "rx5", "tx5", "rx6", "tx6",
> > >                    "rx7", "tx7", "rx8", "tx8", "rx9", "tx9",
> > >                    "rx10", "tx10";
> > >        ...
> > > };
> >
> > Yes, that looks good for the client binding.
>
> One more clarifying question ... should the xlate verify that no other
> dma channel is using the same hardware request signal?
>
> I understand that typically the xlate decodes the binding to get the
> channel info, but because this is invoked by dmaengine while allocating
> a channel, I was wondering if we should prevent dmaengine allocating
> more than one channel to be used with the same hardware request? If so,
> then passing the direction to the xlate would be necessary (so I can
> determine in the xlate that no one else is currently using this, which
> is what I currently do).
>
> Alternatively, I could check that no one else is using the request
> signal at a later when the transfer is being prepared.

I think that handling this at prepare/usage time is probably most 
appropriate. That is the time when the resource conflict /actually/ occurs.

The only time when two clients would be given the same DMA request 
signal is if there are multiple different drivers that can DMA into the 
same FIFO in a time-multiplexed fashion. That seems pretty unlikely off 
the top of my head, but I don't think we want to actively ban that, in 
case we come up with a cunning use-case for it.

> If you are wondering why I am worried about this, I my mind I think that
> the driver should be robust enough to check for conflicts in the request
> signals used by the various channels.

Sure, makes sense.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/