Re: [PATCH 1/4] net: sfp: add workaround for Realtek RTL8672 and RTL9601C chips

Andrew Lunn
2020-12-30T17:14:17Z

On Wed, Dec 30, 2020 at 05:05:46PM +0000, Russell King - ARM Linux admin wrote:
> On Wed, Dec 30, 2020 at 05:56:34PM +0100, Pali Rohár wrote:
> > This change is really required for those Realtek chips. I thought that
> > it is obvious that from *both* addresses 0x50 and 0x51 can be read only
> > one byte at the same time. Reading 2 bytes (for be16 value) cannot be
> > really done by one i2 transfer, it must be done in two.
>
> Then these modules are even more broken than first throught, and
> quite simply it is pointless supporting the diagnostics on them
> because we can never read the values in an atomic way.
>
> It's also a violation of the SFF-8472 that _requires_ multi-byte reads
> to read these 16 byte values atomically. Reading them with individual
> byte reads results in a non-atomic read, and the 16-bit value can not
> be trusted to be correct.

Hi Pali

I have to agree with Russell here. I would rather have no diagnostics
than untrustable diagnostics.

The only way this is going to be accepted is if the manufacture says
that reading the first byte of a word snapshots the second byte as
well in an atomic way and returns that snapshot on the second
read. But i highly doubt that happens, given how bad these SFPs are.

Andrew