netdev - Re: Issue with commit fea23fb591cc "net: phy: convert read-modify-write to phy

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Date:   Thu, 4 Jan 2018 20:16:44 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Issue with commit fea23fb591cc "net: phy: convert
 read-modify-write to phy_modify()"

Am 04.01.2018 um 12:44 schrieb Russell King - ARM Linux:
> On Thu, Jan 04, 2018 at 08:00:53AM +0100, Heiner Kallweit wrote:
>> Parameter mask of phy_modify() holds the bits to be cleared.
>> In the mentioned commit parameter mask seems to be inverted in
>> few cases, what IMO is wrong (see example).
> 
> I'd be grateful if you could list those that you think are wrong please.
> For function __phy_modify documentation and implementation conflict.
Documentation states "(value & mask) | set" whilst implementation is
"(value & ~mask) | set". Based on the subsequent patches I assume
that your intention is what is documented.

Personally I find "ret & ~mask" more intuitive (see also set_mask_bits
in include/linux/bitops.h) but this may be a question of personal taste.
In kernel code both flavors are used.

+ * Unlocked helper function which allows a PHY register to be modified as
+ * new register value = (old register value & mask) | set
+ */
+int __phy_modify(struct phy_device *phydev, u32 regnum, u16 mask, u16 set)
+{
+	int ret, res;
+
+	ret = __phy_read(phydev, regnum);
+	if (ret >= 0) {
+		res = __phy_write(phydev, regnum, (ret & ~mask) | set);
+		if (res < 0)
+			ret = res;
+	}
+
+	return ret;
+}

Could you please advise whether documentation or implementation reflect
your intention? Then I'll check again which changes I'd consider to
be wrong.

Regards, Heiner

>> Maybe I miss something, could you please check?
> 
> It's entirely possible that some are wrong - the patch started out as
> having the mask argument inverted, but during its evolution, that was
> corrected, and I thought all places had been updated - maybe they were
> initially wrong.
> 
> I did go through the patch several times before sending it to try to
> ensure that it was correct, but must have overlooked some, because the
> one you quote is one I definitely looked at several times.  It's highly
> likely that if I have another look through the patch, I still won't
> spot those that you've found.
> 
>> And somehow related:
>> When adding such helpers, wouldn't it make sense to add
>> helpers for setting / clearing bits too? Something like:
>> phy_set_bits(phydev, reg, val) -> phy_modify(phydev, reg, 0, val)
> 
> Maybe, but lets try and solve the problems with the existing patch
> first.
> 
> Thanks for reporting this, and sorry for the hassle.
>