Re: [PATCH] soc: fsl: Remove bogus packed attributes from qman.h

Re: [PATCH] soc: fsl: Remove bogus packed attributes from qman.h

[Herbert Xu]

On Tue, Sep 01, 2020 at 01:50:38AM +0000, Leo Li wrote:
>
> Sorry for the late response.  I missed this email previously.
> 
> These structures are descriptors used by hardware, we cannot have _ANY_ padding from the compiler.  The compiled result might be the same with or without the __packed attribute for now, but I think keep it there probably is safer for dealing with unexpected alignment requirements from the compiler in the future.
> 
> Having conflicting alignment requirements warning might means something is wrong with the structure in certain scenario.  I just tried a ARM64 build but didn't see the warnings.  Could you share the warning you got and the build setup?  Thanks.

Just do a COMPILE_TEST build on x86-64:

In file included from ../drivers/crypto/caam/qi.c:12:
../include/soc/fsl/qman.h:259:1: warning: alignment 1 of ‘struct qm_dqrr_entry’ is less than 8 [-Wpacked-not-aligned]
 } __packed;
 ^
../include/soc/fsl/qman.h:292:2: warning: alignment 1 of ‘struct <anonymous>’ is less than 8 [-Wpacked-not-aligned]
  } __packed ern;
  ^

In any case, those packed markers are completely unnecessary because
those structs contain no holes.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] soc: fsl: Remove bogus packed attributes from qman.h

[Li Yang]

On Mon, Aug 31, 2020 at 8:57 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Tue, Sep 01, 2020 at 01:50:38AM +0000, Leo Li wrote:
> >
> > Sorry for the late response.  I missed this email previously.
> >
> > These structures are descriptors used by hardware, we cannot have _ANY_ padding from the compiler.  The compiled result might be the same with or without the __packed attribute for now, but I think keep it there probably is safer for dealing with unexpected alignment requirements from the compiler in the future.
> >
> > Having conflicting alignment requirements warning might means something is wrong with the structure in certain scenario.  I just tried a ARM64 build but didn't see the warnings.  Could you share the warning you got and the build setup?  Thanks.
>
> Just do a COMPILE_TEST build on x86-64:
>
> In file included from ../drivers/crypto/caam/qi.c:12:

Looks like the CAAM driver and dependent QBMAN driver doesn't support
COMPILE_TEST yet.  Are you trying to add the support for it?

I changed the Kconfig to enable the COMPILE_TEST anyway and updated my
toolchain to gcc-10 trying to duplicate the issue.  The issues can
only be reproduced with "W=1".

> ../include/soc/fsl/qman.h:259:1: warning: alignment 1 of ‘struct qm_dqrr_entry’ is less than 8 [-Wpacked-not-aligned]
>  } __packed;
>  ^
> ../include/soc/fsl/qman.h:292:2: warning: alignment 1 of ‘struct <anonymous>’ is less than 8 [-Wpacked-not-aligned]
>   } __packed ern;
>   ^

I think this is a valid concern that if the parent structure doesn't
meet certain alignment requirements, the alignment for the
sub-structure cannot be guaranteed.  If we just remove the __packed
attribute from the parent structure, the compiler could try to add
padding in the parent structure to fulfill the alignment requirements
of the sub structure which is not good.  I think the following changes
are a better fix for the warnings:

diff --git a/include/soc/fsl/qman.h b/include/soc/fsl/qman.h
index cfe00e08e85b..9f484113cfda 100644
--- a/include/soc/fsl/qman.h
+++ b/include/soc/fsl/qman.h
@@ -256,7 +256,7 @@ struct qm_dqrr_entry {
        __be32 context_b;
        struct qm_fd fd;
        u8 __reserved4[32];
-} __packed;
+} __packed __aligned(64);
 #define QM_DQRR_VERB_VBIT              0x80
 #define QM_DQRR_VERB_MASK              0x7f    /* where the verb contains; */
 #define QM_DQRR_VERB_FRAME_DEQUEUE     0x60    /* "this format" */
@@ -289,7 +289,7 @@ union qm_mr_entry {
                __be32 tag;
                struct qm_fd fd;
                u8 __reserved1[32];
-       } __packed ern;
+       } __packed __aligned(64) ern;
        struct {
                u8 verb;
                u8 fqs;         /* Frame Queue Status */


Regards,
Leo

Re: [PATCH] soc: fsl: Remove bogus packed attributes from qman.h

[Herbert Xu]

On Tue, Sep 01, 2020 at 04:40:16PM -0500, Li Yang wrote:
>
> Looks like the CAAM driver and dependent QBMAN driver doesn't support
> COMPILE_TEST yet.  Are you trying to add the support for it?

Yes.

> I think this is a valid concern that if the parent structure doesn't
> meet certain alignment requirements, the alignment for the
> sub-structure cannot be guaranteed.  If we just remove the __packed
> attribute from the parent structure, the compiler could try to add
> padding in the parent structure to fulfill the alignment requirements
> of the sub structure which is not good.  I think the following changes
> are a better fix for the warnings:

This works for me.  Thanks!
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt