Alignment on XScale ARM

Alignment on XScale ARM

[Marcus Tangermann]

Hi,

we use an XScale 422 (ARM) with a big endian Linux system based on the buildroot environment. It seems there is an alignment problem we have. I've tracked down the problem to the following. 
1. The data received via a netlink communication from the kernel is stored into a buffer buf. The data is correct.
2. The address of the buffer is converted to a pointer to nlmsghdr:
         struct nlmsghdr *h;
         h = (struct nlmsghdr*) buf; 
         printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
         printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
   The results of printf are nonsens. The first fields seem to have an offset of 2 bytes, at the end there also seem to be swapped bytes.
3. To test what might happen, I've created an own struct
       struct aligntest{
                __u32 first;
                __u32 int second;
                __u32 third;
                __u32 fourth;
       };
     When you now set a point to a buffer, again the values are wrong:
         s1 = (struct aligntest*) buf;
         printf("1: %08X\n", s1->first);
         printf("2: %08X\n", s1->second);
         printf("3: %08X\n", s1->third);
         printf("4: %08X\n", s1->fourth);

It seems, that there is a problem with the unsigned int values. When you add the attribute "packed" to the definition of struct aligntest ( __attribute__(__packed__) ), all values are displayed correctly. 
So, any hints what the problem can be?

Regards
Marcus
____________________________________________________________________
Psssst! Schon vom neuen WEB.DE MultiMessenger gehört? 
Der kann`s mit allen: http://www.produkte.web.de/messenger/?did=3123

Re: Alignment on XScale ARM

[Will Newton]

On Fri, Nov 7, 2008 at 10:52 PM, Marcus Tangermann
<Marcus.Tangermann@web.de> wrote:
> Hi,
>
> we use an XScale 422 (ARM) with a big endian Linux system based on the buildroot environment. It seems there is an alignment problem we have. I've tracked down the problem to the following.
> 1. The data received via a netlink communication from the kernel is stored into a buffer buf. The data is correct.
> 2. The address of the buffer is converted to a pointer to nlmsghdr:
>         struct nlmsghdr *h;
>         h = (struct nlmsghdr*) buf;
>         printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
>         printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
>   The results of printf are nonsens. The first fields seem to have an offset of 2 bytes, at the end there also seem to be swapped bytes.
> 3. To test what might happen, I've created an own struct
>       struct aligntest{
>                __u32 first;
>                __u32 int second;
>                __u32 third;
>                __u32 fourth;
>       };
>     When you now set a point to a buffer, again the values are wrong:
>         s1 = (struct aligntest*) buf;
>         printf("1: %08X\n", s1->first);
>         printf("2: %08X\n", s1->second);
>         printf("3: %08X\n", s1->third);
>         printf("4: %08X\n", s1->fourth);
>
> It seems, that there is a problem with the unsigned int values. When you add the attribute "packed" to the definition of struct aligntest ( __attribute__(__packed__) ), all values are displayed correctly.
> So, any hints what the problem can be?

I'm not sure what the problem is? You can't generally misalign a
struct pointer safely.

Re: Alignment on XScale ARM

[Nicolas Pitre]

On Fri, 7 Nov 2008, Marcus Tangermann wrote:

> Hi,
> 
> we use an XScale 422 (ARM) with a big endian Linux system based on the 
> buildroot environment. It seems there is an alignment problem we have. 
> I've tracked down the problem to the following. 1. The data received 
> via a netlink communication from the kernel is stored into a buffer 
> buf. The data is correct. 2. The address of the buffer is converted to 
> a pointer to nlmsghdr:
>          struct nlmsghdr *h;
>          h = (struct nlmsghdr*) buf; 
>          printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
>          printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
> The results of printf are nonsens. The first fields seem to have an 
> offset of 2 bytes, at the end there also seem to be swapped bytes. 3. 
> To test what might happen, I've created an own struct
>        struct aligntest{
>                 __u32 first;
>                 __u32 int second;
>                 __u32 third;
>                 __u32 fourth;
>        };
> When you now set a point to a buffer, again the values are wrong:
>          s1 = (struct aligntest*) buf;
>          printf("1: %08X\n", s1->first);
>          printf("2: %08X\n", s1->second);
>          printf("3: %08X\n", s1->third);
>          printf("4: %08X\n", s1->fourth);
> 
> It seems, that there is a problem with the unsigned int values. When 
> you add the attribute "packed" to the definition of struct aligntest ( 
> __attribute__(__packed__) ), all values are displayed correctly. So, 
> any hints what the problem can be?

The ARM processor traditionally cannot access misaligned short and int 
values in memory.  By using the packed attribute you tell the compiler 
that the structure should not have inserted padding to align its 
members, and that accesses are to be performed wth byte accesses and the 
value reconstructed that way.


Nicolas

Re: Alignment on XScale ARM

[David Brownell]

On Saturday 08 November 2008, Nicolas Pitre wrote:
> The ARM processor traditionally cannot access misaligned short and int 
> values in memory.  By using the packed attribute you tell the compiler 
> that the structure should not have inserted padding to align its 
> members, and that accesses are to be performed wth byte accesses and the 
> value reconstructed that way.

... but then when you pass "&misaligned_ptr->member" to some other
function, that other function generally won't know it needs to use
byte access.  So, be careful.  The "packed" struct attribute isn't
a panacea.

General Linux kernel policy is to use <asm/unaligned.h> accessors,
but obviously "packed" has advantages when you've got to pull
structures out of protocol buffers.  The userspace headers don't
seem to support such accessors though.

Also, the network stack is known for making assumptions about its
data being properly aligned.  See how NET_IP_ALIGN is used inside
the kernel; you might be happier aligning your RX buffers before
reading data into them from the kernel.

- Dave

Re: Alignment on XScale ARM

[Marcus Tangermann]

Hello,

due to your hints I've found a solution. The problem was simple: The buffer was not word aligned as the Xscale requires. 
With
char buffer[8192] __attribute__((aligned(4)));
things are working.  Sometimes you don't see the wood for the trees.
So thanks to all for the good hints!

Marcus

> -----Ursprüngliche Nachricht-----
> Von: "Marcus Tangermann" <Marcus.Tangermann@web.de>
> Gesendet: 07.11.08 23:52:43
> An: linux-embedded@vger.kernel.org
> Betreff: Alignment on XScale ARM

> Hi,
> 
> we use an XScale 422 (ARM) with a big endian Linux system based on the buildroot environment. It seems there is an alignment problem we have. I've tracked down the problem to the following. 
> 1. The data received via a netlink communication from the kernel is stored into a buffer buf. The data is correct.
> 2. The address of the buffer is converted to a pointer to nlmsghdr:
>          struct nlmsghdr *h;
>          h = (struct nlmsghdr*) buf; 
>          printf("IPSECTOOLS: Length  %04d\n", h->nlmsg_len);
>          printf("IPSECTOOLS: Type %02d\n", h->nlmsg_type);
>    The results of printf are nonsens. The first fields seem to have an offset of 2 bytes, at the end there also seem to be swapped bytes.
> 3. To test what might happen, I've created an own struct
>        struct aligntest{
>                 __u32 first;
>                 __u32 int second;
>                 __u32 third;
>                 __u32 fourth;
>        };
>      When you now set a point to a buffer, again the values are wrong:
>          s1 = (struct aligntest*) buf;
>          printf("1: %08X\n", s1->first);
>          printf("2: %08X\n", s1->second);
>          printf("3: %08X\n", s1->third);
>          printf("4: %08X\n", s1->fourth);
> 
> It seems, that there is a problem with the unsigned int values. When you add the attribute "packed" to the definition of struct aligntest ( __attribute__(__packed__) ), all values are displayed correctly. 
> So, any hints what the problem can be?
> 
> Regards
> Marcus
> ____________________________________________________________________
> Psssst! Schon vom neuen WEB.DE MultiMessenger gehört? 
> Der kann`s mit allen: http://www.produkte.web.de/messenger/?did=3123
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-embedded" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 


__________________________________________________________________
"Run, Fatboy, Run" sowie "Rails & Ties" kostenlos anschauen!
Blockbuster-Gutscheine sichern unter http://www.blockbuster.web.de