linux-kernel - Re: [PATCH net-next v4 4/9] net: phy: micrel: apply resume errata workaround for ksz8873 and ksz8863

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Message-ID: <20210612151330.nvin5ldcx6xunexx@skbuf>
Date:   Sat, 12 Jun 2021 18:13:30 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        Michael Grzeschik <m.grzeschik@...gutronix.de>,
        kernel@...gutronix.de, Jakub Kicinski <kuba@...nel.org>,
        UNGLinuxDriver@...rochip.com,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v4 4/9] net: phy: micrel: apply resume errata
 workaround for ksz8873 and ksz8863

On Sat, Jun 12, 2021 at 06:26:39AM +0200, Oleksij Rempel wrote:
> On Fri, Jun 11, 2021 at 10:20:10PM +0300, Vladimir Oltean wrote:
> > On Fri, Jun 11, 2021 at 09:15:22AM +0200, Oleksij Rempel wrote:
> > > The ksz8873 and ksz8863 switches are affected by following errata:
> > > 
> > > | "Receiver error in 100BASE-TX mode following Soft Power Down"
> > > |
> > > | Some KSZ8873 devices may exhibit receiver errors after transitioning
> > > | from Soft Power Down mode to Normal mode, as controlled by register 195
> > > | (0xC3) bits [1:0]. When exiting Soft Power Down mode, the receiver
> > > | blocks may not start up properly, causing the PHY to miss data and
> > > | exhibit erratic behavior. The problem may appear on either port 1 or
> > > | port 2, or both ports. The problem occurs only for 100BASE-TX, not
> > > | 10BASE-T.
> > > |
> > > | END USER IMPLICATIONS
> > > | When the failure occurs, the following symptoms are seen on the affected
> > > | port(s):
> > > | - The port is able to link
> > > | - LED0 blinks, even when there is no traffic
> > > | - The MIB counters indicate receive errors (Rx Fragments, Rx Symbol
> > > |   Errors, Rx CRC Errors, Rx Alignment Errors)
> > > | - Only a small fraction of packets is correctly received and forwarded
> > > |   through the switch. Most packets are dropped due to receive errors.
> > > |
> > > | The failing condition cannot be corrected by the following:
> > > | - Removing and reconnecting the cable
> > > | - Hardware reset
> > > | - Software Reset and PCS Reset bits in register 67 (0x43)
> > > |
> > > | Work around:
> > > | The problem can be corrected by setting and then clearing the Port Power
> > > | Down bits (registers 29 (0x1D) and 45 (0x2D), bit 3). This must be done
> > > | separately for each affected port after returning from Soft Power Down
> > > | Mode to Normal Mode. The following procedure will ensure no further
> > > | issues due to this erratum. To enter Soft Power Down Mode, set register
> > > | 195 (0xC3), bits [1:0] = 10.
> > > |
> > > | To exit Soft Power Down Mode, follow these steps:
> > > | 1. Set register 195 (0xC3), bits [1:0] = 00 // Exit soft power down mode
> > > | 2. Wait 1ms minimum
> > > | 3. Set register 29 (0x1D), bit [3] = 1 // Enter PHY port 1 power down mode
> > > | 4. Set register 29 (0x1D), bit [3] = 0 // Exit PHY port 1 power down mode
> > > | 5. Set register 45 (0x2D), bit [3] = 1 // Enter PHY port 2 power down mode
> > > | 6. Set register 45 (0x2D), bit [3] = 0 // Exit PHY port 2 power down mode
> > > 
> > > This patch implements steps 2...6 of the suggested workaround. During
> > > (initial) switch power up, step 1 is executed by the dsa/ksz8795
> > > driver's probe function.
> > > 
> > > Note: In this workaround we toggle the MII_BMCR register's BMCR_PDOWN
> > > bit, this is translated to the actual register and bit (as mentioned in
> > > the arratum) by the ksz8_r_phy()/ksz8_w_phy() functions.
> > 
> > s/arratum/erratum/
> > 
> > Also, the commit message is still missing this piece of information you
> > gave in the previous thread:
> > 
> > | this issue was seen  at some early point of development (back in 2019)
> > | reproducible on system start. Where switch was in some default state or
> > | on a state configured by the bootloader. I didn't tried to reproduce it
> > | now.
> > 
> > Years from now, some poor souls might struggle to understand why this
> > patch was done this way. If it is indeed the case that the issue is only
> > seen during the handover between bootloader and kernel, there is really
> > no reason to implement the ERR workaround in phy_resume instead of doing
> > it once at probe time.
> 
> Ok, i'll drop this patch for now.

I mean, you don't have to drop it, you just have to provide a competent
explanation for how the patch addresses the ERR as described by Microchip.
Do you still have a board with this switch?