netdev - RE: -Wsizeof-array-div warnings in ethernet drivers

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Date:   Tue, 17 Sep 2019 10:36:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jose Abreu' <Jose.Abreu@...opsys.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Giuseppe Cavallaro" <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        "David S. Miller" <davem@...emloft.net>
CC:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        David Bolvansky <david.bolvansky@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: RE: -Wsizeof-array-div warnings in ethernet drivers

From: Jose Abreu
> Sent: 17 September 2019 08:59
> From: Nathan Chancellor <natechancellor@...il.com>
> Date: Sep/17/2019, 08:32:32 (UTC+00:00)
> 
> > Hi all,
> >
> > Clang recently added a new diagnostic in r371605, -Wsizeof-array-div,
> > that tries to warn when sizeof(X) / sizeof(Y) does not compute the
> > number of elements in an array X (i.e., sizeof(Y) is wrong). See that
> > commit for more details:
...
> > ../drivers/net/ethernet/amd/xgbe/xgbe-dev.c:361:49: warning: expression
> > does not compute the number of elements in this array; element type is
> > 'u8' (aka 'unsigned char'), not 'u32' (aka 'unsigned int')
> > [-Wsizeof-array-div]
> >         unsigned int key_regs = sizeof(pdata->rss_key) / sizeof(u32);
> >                                        ~~~~~~~~~~~~~~  ^
...
> > What is the reasoning behind having the key being an array of u8s but
> > seemlingly converting it into an array of u32s? It's not immediately
> > apparent from reading over the code but I am not familiar with it so I
> > might be making a mistake. I assume this is intentional? If so, the
> > warning can be silenced and we'll send patches to do so but we want to
> > make sure we aren't actually papering over a mistake.
> 
> This is because we write 32 bits at a time to the reg but internally the
> driver uses 8 bits to store the array. If you look at
> dwxgmac2_rss_configure() you'll see that cfg->key is casted to u32 which
> is the value we use in HW writes. Then the for loop just does the math
> to check how many u32's has to write.

That stinks of a possible misaligned data access.....

	David

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