Re: [net-next,PATCH 1/2] net: dsa: microchip: Add emulated MIIM access to switch LED config registers

[Marek Vasut]

On 1/16/25 2:06 PM, Andrew Lunn wrote:
> > This preparatory patch exposes the LED control bits in those Switch Config
> > Registers by mapping them at high addresses in the MIIM space, so the PHY
> > driver can access those registers and surely not collide with the existing
> > MIIM block registers. The two registers which are exposed are the global
> > Register 11 (0x0B): Global Control 9 as MIIM block register 0x0b00 and
> > port specific Register 29/45/61 (0x1D/0x2D/0x3D): Port 1/2/3 Control 10
> > as MIIM block register 0x0d00 . The access to those registers is further
> > restricted only to the LED configuration bits to prevent the PHY driver
> > or userspace tools like 'phytool' from tampering with any other switch
> > configuration through this interface.
>
> I do wounder about using register numbers outside of the 0-31
> range. We never have enforced it, but these clearly break 802.3 C22.

Yes, I agree this is completely non-standard.

The registers I need to access from the PHY driver are neither C22 nor 
C45 registers , they are switch registers accessible directly via SPI 
only. I only needed some way to accesses them from the PHY driver to 
flip the LED bits.

> I wounder if it would be better to emulate pages as well. The PHY
> driver already has lanphy_write_page_reg() and
> lanphy_read_page_reg(). Could you emulate what those need?  It adds a
> lot of complexity, but it should be future proof.
>
> What might be simpler is to expose an emulated C45 register range, and
> put the registers in the vendor section.
What would be the benefit of doing that, compared to the current 
approach? I think in either case, for example phytool could tamper with 
those registers anyway, right ?

Please note that the KSZ9xxx series to my knowledge do not have this PHY 
interface anymore, so that future proofing part is not really applicable 
I think.