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

[Rafal Ozieblo]

-----Original Message-----
From: Nicolas Ferre [mailto:nicolas.ferre@xxxxxxxxx] 
Sent: 17 listopada 2016 14:29
To: Harini Katakam; Rafal Ozieblo
Cc: harini.katakam@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [RFC PATCH 2/2] net: macb: Add 64 bit addressing support for GEM

> Le 17/11/2016 Ã 13:21, Harini Katakam a Ãcrit :
> > 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.
>
> We have quite a bunch of options in this driver to determinate what is the real capacity of the underlying hardware.
> If HW configuration registers are not appropriate, and it seems they are not, I would advice to simply use the DT compatibility string.
>
> Best regards,
> --
> Nicolas Ferre

HW configuration registers are appropriate. The issue is that this code doesnât use the capability bit to switch between different dma descriptors (2 words vs. 4 words).
DMA descriptor size is chosen based on kernel configuration, not based on hardware capabilities.

Regards, 
Rafal Ozieblo