Date:   Wed, 22 Feb 2023 17:30:43 +0100
From:   Marek Vasut <marex@...x.de>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
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 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 .

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