[PATCH 0/3] arm64: dts: r8a7796: Add CAN/CAN FD support

[Geert Uytterhoeven]

Hi Chris,

On Thu, Nov 24, 2016 at 3:25 PM, Chris Paterson
<Chris.Paterson2 at renesas.com> wrote:
> From: Simon Horman [mailto:horms at verge.net.au]
> Sent: 24 November 2016 10:18
>> On Thu, Nov 24, 2016 at 10:05:08AM +0000, Chris Paterson wrote:
>> > From: Simon Horman [mailto:horms at verge.net.au]
>> > > Regarding the arch/arm64/boot/dts/renesas/ portion, I would like
>> > > some consideration given to what effect enabling memory above 4Gb
>> > > (64bit
>> > > addressing) would have.
>> >
>> > Can you give me some guidance here? I'm not sure what you're referring
>> > to. As far as I know the DT reg definition here is 64-bit, or are you
>> > referring to DMA usage? If the later, neither CAN driver uses DMA.
>>
>> Sorry for not being clearer.
>>
>> What I would like to know is if there are any problems in the CAN driver or
>> hardware that would prevent it from functioning with memory that requires
>> 64bit addressing present.
>>
>> If the CAN hardware cannot use DMA then DMA doesn't need to be taken
>> into account. But if it DMA could be enabled in future for CAN, for example
>> after some driver enhancements, then it would be good to know if 64bit
>> memory can be supported - if not it would imply DMA cannot be enabled.
>
> Thank you for the clarification.
>
> The CAN interface for r8a7795/6 does not support DMA.
>
> With CAN FD there is currently a H/W issue that means DMA is unusable.

Is that issue present on R-Car M3-W, or only on R-Car H3 ES1.x?

> Potentially this issue could be fixed in the future and DMA support could
> be added to the driver. If this happens I can see no reason why the CAN FD
> IP wouldn't be able to handle DMA transfers when using 64bit addressing.

Yep, AFAIK it uses SYS-DMAC, which supports 64-bit addressing.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds