Re: [PATCH net-next v3 2/3] net: dsa: qca8k: do not clear MASTER_EN after a failed page select

[Andrew Lunn <andrew@lunn.ch>]

On Sat, Oct 03, 2026 at 07:24:03PM +0200, Yongzhao Chen wrote:
> qca8k_mdio_write() and qca8k_mdio_read() take the common exit path when
> selecting the page of QCA8K_MDIO_MASTER_CTRL fails. That path clears
> MASTER_CTRL[31:16] with a raw write to phy 0x10, reg 0x1f, which only
> addresses MASTER_CTRL while page 0 is selected. After a failed page
> select that did not reach the switch, the previous page remains selected.
> The write can then clear the upper half of an unrelated register at
> page * 0x200 + 0x3c.
> 
> No MDIO master transaction has been started at that point, so there is
> nothing to clean up. Unlock and return the error directly.
> 
> qca8k_set_page() also keeps the old cached page when the page write
> fails. If the write did reach the switch, a later access to the cached
> page skips the page select and reaches the wrong register. Invalidate
> the cache on failure so that the next access selects the page again.
> 
> The Sashiko review of v1 identified the stray cleanup write. A userspace
> model of the driver's functions reproduces both problems by injecting
> page-select failures with and without a hardware page change; these cases
> pass after this change.
> 
> The cleanup write comes from commit 759bafb8a322 ("net: dsa: qca8k: add
> support for internal phy and internal mdio"), and the stale page cache
> from commit ba5707ec58cf ("net: dsa: qca8k: handle qca8k_set_page
> errors").
> 
> Signed-off-by: Yongzhao Chen <yongzhao.derek@gmail.com>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew