From: Nathan Chancellor <natechancel...@gmail.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
