Messages in this thread | | | Date | Wed, 18 May 2022 09:18:19 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] can: mcp251xfd: silence clang's -Wunaligned-access warning |
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On Thu, May 19, 2022 at 01:15:04AM +0900, Vincent MAILHOL wrote: > On Tue. 19 May 2022 at 01:08, Nathan Chancellor <nathan@kernel.org> wrote: > > Hi Vincent, > > > > On Wed, May 18, 2022 at 08:43:57PM +0900, Vincent Mailhol wrote: > > > clang emits a -Wunaligned-access warning on union > > > mcp251xfd_tx_ojb_load_buf. > > > > > > The reason is that field hw_tx_obj (not declared as packed) is being > > > packed right after a 16 bits field inside a packed struct: > > > > > > | union mcp251xfd_tx_obj_load_buf { > > > | struct __packed { > > > | struct mcp251xfd_buf_cmd cmd; > > > | /* ^ 16 bits fields */ > > > | struct mcp251xfd_hw_tx_obj_raw hw_tx_obj; > > > | /* ^ not declared as packed */ > > > | } nocrc; > > > | struct __packed { > > > | struct mcp251xfd_buf_cmd_crc cmd; > > > | struct mcp251xfd_hw_tx_obj_raw hw_tx_obj; > > > | __be16 crc; > > > | } crc; > > > | } ____cacheline_aligned; > > > > > > Starting from LLVM 14, having an unpacked struct nested in a packed > > > struct triggers a warning. c.f. [1]. > > > > > > This is a false positive because the field is always being accessed > > > with the relevant put_unaligned_*() function. Adding __packed to the > > > structure declaration silences the warning. > > > > > > [1] https://github.com/llvm/llvm-project/issues/55520 > > > > > > Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> > > > --- > > > Actually, I do not have llvm 14 installed so I am not able to test > > > (this check was introduced in v14). But as explained in [1], adding > > > __packed should fix the warning. > > > > Thanks for the patch! This does resolve the warning (verified with LLVM > > 15). > > Great, thanks for the check! Does this mean we can add you Tested-by > (I assume yes, c.f. below, if not the case, please raise your voice).
Sure, see below.
> > > Because this is a false positive, I did not add a Fixes tag, nor a > > > Reported-by: kernel test robot. > > > > I think that the Reported-by tag should always be included but I agree > > that a Fixes tag is not necessary for this warning, as we currently have > > it under W=1, so it should not be visible under normal circumstances. > > ACK. > Marc, can you directly add below tags to the patch: > > Reported-by: kernel test robot <lkp@intel.com> > Tested-by: Nathan Chancellor <nathan@kernel.org>
Please use:
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
To make it clear that I didn't perform anything more than a build test to see that the warning is fixed.
Cheers, Nathan
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