Re: [RFC PATCH-for-8.0 3/3] hw/net/xilinx_ethlite: Replace tswap32() by be32_to_cpu()

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Tue, Dec 13, 2022 at 01:53:15PM +0000, Peter Maydell wrote:
> On Tue, 13 Dec 2022 at 12:52, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> >
> > This partly revert commit d48751ed4f ("xilinx-ethlite:
> > Simplify byteswapping to/from brams") which states the
> > packet data is stored in big-endian.
> >
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> 
> > @@ -102,8 +102,8 @@ eth_read(void *opaque, hwaddr addr, unsigned int size)
> >              D(qemu_log("%s " TARGET_FMT_plx "=%x\n", __func__, addr * 4, r));
> >              break;
> >
> > -        default:
> > -            r = tswap32(s->regs[addr]);
> > +        default: /* Packet data */
> > +            r = be32_to_cpu(s->regs[addr]);
> >              break;
> >      }
> >      return r;
> > @@ -160,8 +160,8 @@ eth_write(void *opaque, hwaddr addr,
> >              s->regs[addr] = value;
> >              break;
> >
> > -        default:
> > -            s->regs[addr] = tswap32(value);
> > +        default: /* Packet data */
> > +            s->regs[addr] = cpu_to_be32(value);
> >              break;
> >      }
> >  }
> 
> This is a change of behaviour for this device in the
> qemu-system-microblazeel petalogix-s3adsp1800 board, because
> previously on that system the bytes of the rx buffer would
> appear in the registers in little-endian order and now they
> will appear in big-endian order.
> 
> Edgar, do you know what the real hardware does here ?
> 

Yeah, I think these tx/rx buffers (the default case with tswap32) should be modelled as plain RAM's (they are just RAM's on real HW).
Because we're modeling as MMIO regs, I think we get into endianness trouble when the ethernet
output logic treats the content as a blob (thus the need for byteswaps). Does that make sense?

Cheers,
Edgar