Date:   Wed, 22 Feb 2023 19:12:21 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Marek Vasut <marex@...x.de>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, UNGLinuxDriver@...rochip.com,
        Vladimir Oltean <olteanv@...il.com>,
        Woojung Huh <woojung.huh@...rochip.com>, stable@...r.kernel.org
Subject: Re: [PATCH] net: dsa: microchip: Fix gigabit set and get function
 for KSZ87xx

On Wed, Feb 22, 2023 at 07:43:35PM +0100, Marek Vasut wrote:
> On 2/22/23 17:39, Russell King (Oracle) wrote:
> > On Wed, Feb 22, 2023 at 05:30:43PM +0100, Marek Vasut wrote:
> > > On 2/22/23 16:56, Russell King (Oracle) wrote:
> > > > On Wed, Feb 22, 2023 at 04:10:33PM +0100, Marek Vasut wrote:
> > > > > On 2/22/23 14:03, Russell King (Oracle) wrote:
> > > > > > On Wed, Feb 22, 2023 at 12:50:07PM +0000, Russell King (Oracle) wrote:
> > > > > > > On Wed, Feb 22, 2023 at 04:17:38AM +0100, Marek Vasut wrote:
> > > > > > > > diff --git a/drivers/net/dsa/microchip/ksz_common.c b/drivers/net/dsa/microchip/ksz_common.c
> > > > > > > > index 729b36eeb2c46..7fc2155d93d6e 100644
> > > > > > > > --- a/drivers/net/dsa/microchip/ksz_common.c
> > > > > > > > +++ b/drivers/net/dsa/microchip/ksz_common.c
> > > > > > > > @@ -319,7 +319,7 @@ static const u16 ksz8795_regs[] = {
> > > > > > > >     	[S_BROADCAST_CTRL]		= 0x06,
> > > > > > > >     	[S_MULTICAST_CTRL]		= 0x04,
> > > > > > > >     	[P_XMII_CTRL_0]			= 0x06,
> > > > > > > > -	[P_XMII_CTRL_1]			= 0x56,
> > > > > > > > +	[P_XMII_CTRL_1]			= 0x06,
> > > > > > > 
> > > > > > > Looking at this driver, I have to say that it looks utterly vile
> > > > > > > from the point of view of being sure that it is correct, and I
> > > > > > > think this patch illustrates why.
> > > > > > > 
> > > > > > > You mention you're using a KSZ8794. This uses the ksz8795_regs
> > > > > > > array, and ksz8_dev_ops. You claim this is about the P_GMII_1GBIT_M
> > > > > > > bit, which is bit 6.
> > > > > > > 
> > > > > > > This bit is accessed only by ksz_get_gbit() and ksz_set_gbit().
> > > > > > > 
> > > > > > > Firstly, ksz_set_gbit() is only called from ksz_port_set_xmii_speed(),
> > > > > > > which is only called from ksz9477_phylink_mac_link_up(). This is only
> > > > > > > referenced by ksz9477_dev_ops and lan937x_dev_ops, but not ksz8_dev_ops.
> > > > > > > Therefore, ksz_set_gbit() is not called for KSZ8794.
> > > > > > > 
> > > > > > > ksz_get_gbit() is only referenced by ksz9477.c in
> > > > > > > ksz9477_get_interface(), called only by ksz9477_config_cpu_port().
> > > > > > > This is only referenced by ksz9477_dev_ops, but not ksz8_dev_ops.
> > > > > > > 
> > > > > > > Therefore, my conclusion is that neither of the ksz_*_gbit()
> > > > > > > functions are called on KSZ8794, and thus your change has no effect
> > > > > > > on the driver's use of P_GMII_1GBIT_M - I think if you put some
> > > > > > > debugging printk()s into both ksz_*_gbit() functions, it'll prove
> > > > > > > that.
> > > > > > > 
> > > > > > > There's other places that P_XMII_CTRL_1 is accessed - ksz_set_xmii()
> > > > > > > and ksz_get_xmii(). These look at the P_MII_SEL_M, P_RGMII_ID_IG_ENABLE
> > > > > > > and P_RGMII_ID_EG_ENABLE bits - bits 0, 1, 3 and 4.
> > > > > > > 
> > > > > > > ksz_get_xmii() is only called by ksz9477_get_interface(), which we've
> > > > > > > already looked at above as not being called.
> > > > > > > 
> > > > > > > ksz_set_xmii() is only called by ksz_phylink_mac_config(), which is
> > > > > > > always called irrespective of the KSZ chip.
> > > > > > > 
> > > > > > > Now, let's look at functions that access P_XMII_CTRL_0. These are
> > > > > > > ksz_set_100_10mbit() and ksz_duplex_flowctrl(). The former
> > > > > > > accesses bit P_MII_100MBIT_M, which is bit 4. The latter looks at
> > > > > > > bits 6, bit 5, and possibly bit 3 depending on the masks being used.
> > > > > > > KSZ8795 uses ksz8795_masks, which omits bit 3, so bits 5 and 6.
> > > > > > > Note... bit 6 is also P_GMII_1GBIT_M. So if ksz_duplex_flowctrl()
> > > > > > > is ever called for the KSZ8795, then we have a situation where
> > > > > > > the P_GMII_1GBIT_M will be manipulated.
> > > > > > > 
> > > > > > > ksz_set_100_10mbit() is only called from ksz_port_set_xmii_speed(),
> > > > > > > which we've established won't be called.
> > > > > > > 
> > > > > > > ksz_duplex_flowctrl() is only called from ksz9477_phylink_mac_link_up()
> > > > > > > which we've also established won't be called.
> > > > > > > 
> > > > > > > So, as far as I can see, P_XMII_CTRL_0 won't be accessed on this
> > > > > > > device.
> > > > > > > 
> > > > > > > Now, what about other KSZ devices - I've analysed this for the KSZ8795,
> > > > > > > but what about any of the others which use this register table? It
> > > > > > > looks to me like those that use ksz8795_regs[] all use ksz8_dev_ops
> > > > > > > and the same masks and bitvals, so they should be the same.
> > > > > > > 
> > > > > > > That is a hell of a lot of work to prove that setting both
> > > > > > > P_XMII_CTRL_0 and P_XMII_CTRL_1 to point at the same register is
> > > > > > > in fact safe. Given the number of registers, the masks, and bitval
> > > > > > > arrays, doing this to prove every combination and then analysing
> > > > > > > the code is utterly impractical - and thus why I label this driver
> > > > > > > as "vile". Is there really no better option to these register
> > > > > > > arrays, bitval arrays and mask arrays - something that makes it
> > > > > > > easier to review and prove correctness?
> > > > > > > 
> > > > > > > I'm not going to give a reviewed-by for this, because... I could
> > > > > > > have made a mistake in the above analysis given the vile nature
> > > > > > > of this driver.
> > > > > > 
> > > > > > However, I should add that - as a result of neither ksz_*_gbit()
> > > > > > functions being used, I consider at least the subject line to be
> > > > > > rather misleading! While it may be something that you spotted,
> > > > > > I suspect the other bits that are actually written are more the
> > > > > > issue you're fixing.
> > > > > 
> > > > > Thank you for the lengthy review, I agree the driver and the register offset
> > > > > calculation are hideous.
> > > > > 
> > > > > However, I did spent quite a bit of time on it already and checked both
> > > > > P_XMII_CTRL_0 and P_XMII_CTRL_1 mappings with printks and by dumping the
> > > > > register values via regmap debugfs interface.
> > > > > 
> > > > > Also note that KSZ8794 and KSZ8795 seem to be the same chip die, just
> > > > > different package (the former has fewer ports) and different chip ID.
> > > > 
> > > > It's not clear what you think of my review and whether you are going to
> > > > take any action at all... So, let me try again...
> > > > 
> > > > The fundamental question that my review raises was whether this gigabit
> > > > bit is actually used, and your response remains silent on that point.
> > > > 
> > > > As the gigabit bit is not actually used given the code structure, it
> > > > is irrelevant for this commit, despite Is_Gbit being the thing that
> > > > lead to the patch.
> > > > 
> > > > Therefore, I believe that the patch description needs to be updated
> > > > to state what the effective fix for this change is (which is to fix
> > > > ksz_set_xmii()) rather than making it sound like it's fixing a wrong
> > > > access for Is_Gbit.
> > > > 
> > > > The reason I think this is important is that if we need to look back
> > > > at the history, current description leads one to think that this
> > > > change is about fixing the Is_Gbit bit - but that isn't used as the
> > > > code stands. The effective change that this patch makes is to the
> > > > only access the driver makes to this register in ksz_set_xmii(),
> > > > and I think that needs to be explained as the primary reason for
> > > > this patch. Fixing Is_Gbit seems to be merely incidental.
> > > 
> > > On the hardware I use here, the P_XMII_CTRL_1 register ends up being
> > > populated with all bits set, 0xff. Without this change, the driver writes to
> > > non-existent register when it attempts to access P_XMII_CTRL_1 .
> > 
> > Why is this so difficult?
> > 
> > I'm *not* disagreeing with the patch. I'm disagreeing with your
> > commit description.
> 
> Why not comment on the commit message part which you disagree with, and
> suggest an improved wording you do agree with ? Then I can send a V2 with
> that part changed, and be done with it.

I have made it crystal clear which bit of the commit message I don't
agree with.

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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