From: John Youn
> Sent: 10 November 2016 03:28
> From: Vahram Aharonyan <vahrama@xxxxxxxxxxxx>
>
> Make the DMA descriptor structure packed to guarantee alignment and size
> in memory.
>
> Signed-off-by: John Youn <johnyoun@xxxxxxxxxxxx>
> ---
> drivers/usb/dwc2/hw.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/usb/dwc2/hw.h b/drivers/usb/dwc2/hw.h
> index 6031efe..ee827e8 100644
> --- a/drivers/usb/dwc2/hw.h
> +++ b/drivers/usb/dwc2/hw.h
> @@ -802,7 +802,7 @@
> struct dwc2_dma_desc {
> u32 status;
> u32 buf;
> -};
> +} __packed;
Nack (not that my nacks have much force).
You clearly do not understand what __packed means.
The dma descriptor almost certainly has to be 32bit aligned (if not 64).
By adding __packed you are telling the compiler that the structure can be
aligned to any boundary (eg 4n+1) so that it must use byte accesses and
shifts to access the members on systems that can't do misaligned accesses.
I can't help feeling this isn't what you want.
I suspect you are trying to tell the compiler not to add hidden padding.
While the C language allows arbitrary padding after structure elements,
the implementations for specific architectures define when such padding
is added.
Padding will not be added if an item is already on its natural boundary
in any of the sane architecture specific definitions.
Typical variations are whether 64bit items only need 32bit alignment.
Some odd architectures specify 32bit alignment for all structs (don't
think linux has any like that).
By using __packed you are relying on a compiler extension - and so
can assume the compiler is used a sane structure layout.
If you are being really picky you could add a compile-time assert
that the structure is the correct size - but that isn't worth it
for 8 bytes.
David
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