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

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Date:   Tue, 17 Sep 2019 07:58:36 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     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: 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:
> 
> https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_llvm_llvm-2Dproject_commit_3240ad4ced0d3223149b72a4fc2a4d9b67589427&d=DwIBAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=yaVFU4TjGY0gVF8El1uKcisy6TPsyCl9uN7Wsis-qhY&m=XFx3a9bS-B05voQh4LJquxHuP4TOowjC82tcMo4gPz0&s=NpzuPgHoSG2p4Mg6ko4MgHHV3TpTwEGm2XNTyw-s3XY&e= 
> 
> Some ethernet drivers have an instance of this warning due to receive
> side scaling support:
> 
> 
> ../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);
>                                        ~~~~~~~~~~~~~~  ^
> ../drivers/net/ethernet/amd/xgbe/xgbe-dev.c:361:49: note: place
> parentheses around the 'sizeof(u32)' expression to silence this warning
> 
> 
> ../drivers/net/ethernet/stmicro/stmmac/dwxgmac2_core.c:537:36: 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]
>         for (i = 0; i < (sizeof(cfg->key) / sizeof(u32)); i++) {
>                                 ~~~~~~~~  ^
> ../drivers/net/ethernet/stmicro/stmmac/dwxgmac2_core.c:537:36: note:
> place parentheses around the 'sizeof(u32)' expression to silence this
> warning
> 
> 
> ../drivers/net/ethernet/synopsys/dwc-xlgmac-hw.c:2329:49: 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);
>                                        ~~~~~~~~~~~~~~  ^
> ../drivers/net/ethernet/synopsys/dwc-xlgmac-hw.c:2329:49: note: place
> parentheses around the 'sizeof(u32)' expression to silence this warning
> 
> 
> 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.

---
Thanks,
Jose Miguel Abreu