Re: [RFC PATCH v3 04/17] hw/net/xilinx_ethlite: Simplify by having configurable endianness

On 11/8/24 16:43, Philippe Mathieu-Daudé wrote: > The Xilinx 'ethlite' device was added in commit b43848a100 > ("xilinx: Add ethlite emulation"), being only built back > then for a big-endian MicroBlaze target (see commit 72b675caac > "microblaze: Hook into the build-system"). > > I/O endianness access was then clarified in commit d48751ed4f > ("xilinx-ethlite: Simplify byteswapping to/from brams"). Here > the 'fix' was to use tswap32(). Since the machine was built as > big-endian target, tswap32() use means the fix was for a little > endian host. While the datasheet (reference added in file header) > is not precise about it, we interpret such change as the device > expects accesses in big-endian order. > > Instead of having a double swapping, one in the core memory layer > due to DEVICE_NATIVE_ENDIAN and a second one with the tswap calls, > allow the machine code to select the proper endianness desired, > removing the need of tswap(). > > Replace the DEVICE_NATIVE_ENDIAN MemoryRegionOps by a pair of > DEVICE_LITTLE_ENDIAN / DEVICE_BIG_ENDIAN. > Add the "little-endian" property to select the device endianness, > defaulting to little endian. > Set the proper endianness on the single machine using the device. > > Signed-off-by: Philippe Mathieu-Daudé <philmd(a)linaro.org&gt; > --- > RFC until I digest Paolo's review from v1: > https://lore.kernel.org/qemu-devel/34f6fe2f-06e0-4e2a-a361-2d662f6814b5@r... tl;dr: this works but would break migration compatibility with the previous version.  If you want to keep that, you need to add > -            r = tswap32(s->regs[addr]); > +            r = s->regs[addr]; if (s->little_endian_model)     r = cpu_to_le32(r); else     r = cpu_to_be32(r); > @@ -161,23 +165,26 @@ eth_write(void *opaque, hwaddr addr, >               break; >           default: if (s->little_endian_model)     r = le32_to_cpu(r); else     r = be32_to_cpu(r); > -            s->regs[addr] = tswap32(value); > +            s->regs[addr] = value; >               break; These pairs ensure that RAM goes through an even number of swaps.  I don't think they are needed but you decide.

However, I am wondering if the double MemoryRegionOps are needed *at all*.  Since petalogix is arguably a little-endian only machine, can you just use DEVICE_LITTLE_ENDIAN?