[PATCH] soc/fsl: qbman: fix conflicting alignment attributes

[Arnd Bergmann]

On Fri, Mar 26, 2021 at 3:17 AM Li Yang <leoyang.li at nxp.com> wrote:
> On Tue, Mar 23, 2021 at 8:17 AM Arnd Bergmann <arnd at kernel.org> wrote:
> >
> > From: Arnd Bergmann <arnd at arndb.de>
> >
> > When building with W=1, gcc points out that the __packed attribute
> > on struct qm_eqcr_entry conflicts with the 8-byte alignment
> > attribute on struct qm_fd inside it:
> >
> > drivers/soc/fsl/qbman/qman.c:189:1: error: alignment 1 of 'struct qm_eqcr_entry' is less than 8 [-Werror=packed-not-aligned]
> >
> > I assume that the alignment attribute is the correct one, and
> > that qm_eqcr_entry cannot actually be unaligned in memory,
> > so add the same alignment on the outer struct.
> >
> > Fixes: c535e923bb97 ("soc/fsl: Introduce DPAA 1.x QMan device driver")
> > Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> > ---
> >  drivers/soc/fsl/qbman/qman.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/soc/fsl/qbman/qman.c b/drivers/soc/fsl/qbman/qman.c
> > index a1b9be1d105a..fde4edd83c14 100644
> > --- a/drivers/soc/fsl/qbman/qman.c
> > +++ b/drivers/soc/fsl/qbman/qman.c
> > @@ -186,7 +186,7 @@ struct qm_eqcr_entry {
> >         __be32 tag;
> >         struct qm_fd fd;
> >         u8 __reserved3[32];
> > -} __packed;
> > +} __packed __aligned(8);
>
> The EQCR structure is actually aligned on 64-byte from the manual.
> But probably 8 is enough to let the compiler not complain.

The important bit is that all members inside are at most 8-byte long
and naturally aligned, so the compiler can now optimize the accesses
because it can assume that each member can be updated atomically.

Marking a structure as unaligned with the __packed attribute can lead
to rather unoptimized code, which is particularly important when the
structure is mapped as uncached -- accessing it one byte at a time
means you have eight times the latency for a simple read. I think on
arm64, the compiler doesn't actually have to do that, but at least if you
run a 32-bit kernel, the packing would prevent the use of ldrd or ldm
instructions that need an aligned word.

       Arnd