Re: [PATCH net-next 0/2] net: phy: sfp: Add single-byte SMBus SFP access

[Russell King (Oracle)]

On Sun, Feb 23, 2025 at 06:28:45PM +0100, Maxime Chevallier wrote:
> Hi everyone,
> 
> Some PHYs such as the VSC8552 have embedded "Two-wire Interfaces" designed to
> access SFP modules downstream. These controllers are actually SMBus controllers
> that can only perform single-byte accesses for read and write.

This goes against SFF-8472, and likely breaks atomic access to 16-bit
PHY registers.

For the former, I quote from SFF-8472:

"To guarantee coherency of the diagnostic monitoring data, the host is
required to retrieve any multi-byte fields from the diagnostic
monitoring data structure (e.g. Rx Power MSB - byte 104 in A2h, Rx
Power LSB - byte 105 in A2h) by the use of a single two-byte read
sequence across the 2-wire interface."

So, if using a SMBus controller, I think we should at the very least
disable exporting the hwmon parameters as these become non-atomic
reads.

Whether PHY access works correctly or not is probably module specific.
E.g. reading the MII_BMSR register may not return latched link status
because the reads of the high and low bytes may be interpreted as two
seperate distinct accesses.

In an ideal world, I'd prefer to say no to hardware designs like this,
but unfortunately, hardware designers don't know these details of the
protocol, and all they see is "two wire, oh SMBus will do".

-- 
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