Re: [PATCH] EmbeddedPkg/MmcDxe: Add alignment for ECSD data

[Andrew Fish <afish@apple.com>]

> On Jun 8, 2017, at 1:18 AM, Jun Nie <jun.nie@linaro.org> wrote:
> 
> Add alignment for ECSD data for DMA access. Otherwise
> the data is corrupted on Sanechips platform.
> 
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Jun Nie <jun.nie@linaro.org <mailto:jun.nie@linaro.org>>
> ---
> EmbeddedPkg/Universal/MmcDxe/Mmc.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/EmbeddedPkg/Universal/MmcDxe/Mmc.h b/EmbeddedPkg/Universal/MmcDxe/Mmc.h
> index 8a7d5a3..ca1a9d5 100644
> --- a/EmbeddedPkg/Universal/MmcDxe/Mmc.h
> +++ b/EmbeddedPkg/Universal/MmcDxe/Mmc.h
> @@ -319,7 +319,7 @@ typedef struct  {
>   OCR       OCRData;
>   CID       CIDData;
>   CSD       CSDData;
> -  ECSD      ECSDData;                         // MMC V4 extended card specific
> +  ECSD      ECSDData __attribute__((aligned(8)));  // MMC V4 extended card specific
> } CARD_INFO;
> 

Jun,

This structure does not look portable. 
1) CARD_TYPE is an ENUM and the size of an enum is not a standard thing in C. Compiler is probably picking int and it seems to work. 
2) I don't think  __attribute__((aligned(8))) is supported by all the edk2 compilers (VC++ for example). It is an GNU extensions to C, not standard C, so we normally don't use it in edk2 code. 

While the alignment of types is not defined by the C standard, it is defined in the EFI ABI to be natural alignment. So you can add padding elements to a structure. You can also union with a UINT64 for force 8 byte alignment. 

typedef struct  {
  UINT16    RCA;
  UINT32	CardType;   //CARD_TYPE
  OCR       OCRData;
  CID       CIDData;
  CSD       CSDData;
  UINT32    Pad;
  ECSD      ECSDData;                         // MMC V4 extended card specific
} CARD_INFO;

Thanks,

Andrew Fish

> typedef struct _MMC_HOST_INSTANCE {
> -- 
> 1.9.1
> 
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