Re: [PATCH] soc/fsl: qbman: fix conflicting alignment attributes
From: Arnd Bergmann On Tue, 23 Mar 2021 14:15:23 +0100, Arnd Bergmann wrote: > 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. Applied to arm/fixes [1/1] soc/fsl: qbman: fix conflicting alignment attributes commit: b9ab2619047edc1c6c07dc68963d84a5979f0abe Arnd
Re: [PATCH] soc/fsl: qbman: fix conflicting alignment attributes
On Fri, Mar 26, 2021 at 3:17 AM Li Yang wrote:
> On Tue, Mar 23, 2021 at 8:17 AM Arnd Bergmann wrote:
> >
> > From: Arnd Bergmann
> >
> > 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
> > ---
> > 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
Re: [PATCH] soc/fsl: qbman: fix conflicting alignment attributes
On Tue, Mar 23, 2021 at 8:17 AM Arnd Bergmann wrote:
>
> From: Arnd Bergmann
>
> 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
> ---
> 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.
> #define QM_EQCR_VERB_VBIT 0x80
> #define QM_EQCR_VERB_CMD_MASK 0x61/* but only one value; */
> #define QM_EQCR_VERB_CMD_ENQUEUE 0x01
> --
> 2.29.2
>
[PATCH] soc/fsl: qbman: fix conflicting alignment attributes
From: Arnd Bergmann
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
---
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);
#define QM_EQCR_VERB_VBIT 0x80
#define QM_EQCR_VERB_CMD_MASK 0x61/* but only one value; */
#define QM_EQCR_VERB_CMD_ENQUEUE 0x01
--
2.29.2
