linux-kernel - Re: [PATCH net] net: dsa: microchip: fix register write order in ksz8_ind

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Date: Tue, 5 Mar 2024 12:40:40 +0100
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: tobias.jakobi.compleo@...il.com
Cc: Woojung Huh <woojung.huh@...rochip.com>, UNGLinuxDriver@...rochip.com,
	Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: microchip: fix register write order in
 ksz8_ind_write8()

On Mon, Mar 04, 2024 at 04:41:35PM +0100, tobias.jakobi.compleo@...il.com wrote:
> From: "Tobias Jakobi (Compleo)" <tobias.jakobi.compleo@...il.com>
> 
> This bug was noticed while re-implementing parts of the kernel
> driver in userspace using spidev. The goal was to enable some
> of the errata workarounds that Microchip describes in their
> errata sheet [1].
> 
> Both the errata sheet and the regular datasheet of e.g. the KSZ8795
> imply that you need to do this for indirect register accesses:
> - write a 16-bit value to a control register pair (this value
>   consists of the indirect register table, and the offset inside
>   the table)
> - either read or write an 8-bit value from the data storage
>   register (indicated by REG_IND_BYTE in the kernel)
> 
> The current implementation has the order swapped. It can be
> proven, by reading back some indirect register with known content
> (the EEE register modified in ksz8_handle_global_errata() is one of
> these), that this implementation does not work.
> 
> Private discussion with Oleksij Rempel of Pengutronix has revealed
> that the workaround was apparantly never tested on actual hardware.
> 
> [1] https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/Errata/KSZ87xx-Errata-DS80000687C.pdf
> 
> Signed-off-by: Tobias Jakobi (Compleo) <tobias.jakobi.compleo@...il.com>

The subject should have [PATCH net] for stable and Fixes tag:
Fixes: 7b6e6235b664 ("net: dsa: microchip: ksz8795: handle eee specif erratum")

Reviewed-by: Oleksij Rempel <o.rempel@...gutronix.de>

Thank you!
Regards,
Oleksij
-- 
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