Re: [RFC PATCH 2/2] net: macb: Add 64 bit addressing support for GEM

[Harini Katakam]

Hi Rafal,

On Thu, Nov 17, 2016 at 5:20 PM, Rafal Ozieblo <rafalo@xxxxxxxxxxx> wrote:
> Hello,
> I think, there could a bug in your patch.
>
>> +
>> +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
>> +             dmacfg |= GEM_BIT(ADDR64);
>> +#endif
>
> You enable 64 bit addressing (64b dma bus width) always when appropriate architecture config option is enabled.
> But there are some legacy controllers which do not support that feature. According Cadence hardware team:
> "64 bit addressing was added in July 2013. Earlier version do not have it.
> This feature was enhanced in release August 2014 to have separate upper address values for transmit and receive."
>
>> /* Bitfields in NSR */
>> @@ -474,6 +479,10 @@
>>  struct macb_dma_desc {
>  >      u32     addr;
>>       u32     ctrl;
>> +#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
>> +     u32     addrh;
>> +     u32     resvd;
>> +#endif
>>  };
>
> It will not work for legacy hardware. Old descriptor is 2 words wide, the new one is 4 words wide.
> If you enable CONFIG_ARCH_DMA_ADDR_T_64BIT but hardware doesn't support it at all,
> you will miss every second descriptor.
>

True, this feature is not available in all of Cadence IP versions.
In fact, the IP version Zynq does not support this. But the one in ZynqMP does.
So, we enable kernel config for 64 bit DMA addressing for this SoC and hence
the driver picks it up. My assumption was that if the legacy IP does not support
64 bit addressing, then this DMA option wouldn't be enabled.

There is a design config register in Cadence IP which is being read to
check for 64 bit address support - DMA mask is set based on that.
But the addition of two descriptor words cannot be based on this runtime check.
For this reason, all the static changes were placed under this check.

Regards,
Harini