2015-11-10 16:34 GMT+13:00
979746084@qq.com <
979746084@qq.com>:
> Which is Yaffs2's on flash spare structure?
> For yaffs1, yaffs_spare is the actual on-flash spare structure
> struct yaffs_spare {
> u8 tb1;
> u8 tb0; //前四个字节表示yaffs_tag信息
> u8 tb2;
> u8 tb3;
> u8 page_status; //一个字节作为删除标记 /* set to 0 to delete the chunk */
> u8 block_status; //一个字节作为坏块标记位，非0xff表示坏块
> u8 tb4; //中间两个字节表示yaffs_tag信息
> u8 tb5;
> u8 ecc1[3]; //数组，三个字节存放相应chunk的后256字节ECC校验结果
> u8 tb6; //接着又两个字节的yaffs_tag信息
> u8 tb7;
> u8 ecc2[3]; //数组，三个字节存放相应chunk的前256字节的ECC校验结果
> };
>
> the following two, yaffs_tags and yaffs_packed_tags1 are the logical
> structure,why there are two ,they are nearly the same?
>
> struct yaffs_tags {
> u32 chunk_id:20; //20位用来表示chunk_id
> u32 serial_number:2; //2位用来表示序列号
> u32 n_bytes_lsb:10; //10位用来表示该页内有效的字节数
> u32 obj_id:18; //18位用来表示obj_id，唯一标识一个对象
> u32 ecc:12; //yaffs_tags本身的ECC校验和
> u32 n_bytes_msb:2; //还有两位未使用，保持为1
> };
>
> struct yaffs_packed_tags1 {
> u32 chunk_id:20;//20位的chunkID
> u32 serial_number:2;//2位的serial num序列号
> u32 n_bytes:10;//10位表示有效字节数
> u32 obj_id:18;//18位表示objID
> u32 ecc:12;//12位表示ECC校验值
> u32 deleted:1;//1位的删除标记
> u32 unused_stuff:1;//1位，未使用
> unsigned should_be_ff;//为全1
>
> };
>
>
> For yaffs2, yaffs_packed_tags2_tags_only and yaffs_packed_tags are only
> the tags of yaffs2, not the whole yaffs2 spare, So whtat's the real
> on-flash yaffs2's spare structure?
>
> struct yaffs_packed_tags2_tags_only {
> unsigned seq_number;//顺序号
> unsigned obj_id;//对象ID
> unsigned chunk_id;//chunkID
> unsigned n_bytes;//字节数
> };
>
> struct yaffs_packed_tags2 {
> struct yaffs_packed_tags2_tags_only t;//tags标签
> struct yaffs_ecc_other ecc;//ECC校验值，12字节，存储的是上面的tags标签的ECC校验值
> };
>
>
What is important is the information that is stored: ie. it must store the
yaffs_packed_tags2_tags_only as a minimum.
If the flash space where the tags is stored is already ECC corrected, then
that is enough. If not, then the other option is to
instead store the yaffs_packed_tags2 structure which is both the tags + an
ECC block to protect the tags.
Which to use will depend on the type of flash and ECC you are using and
whether you are using inband tags etc.
For example, consider the following scenarios:
1. Using inband tags:
The tags (yaffs_packed_tags2_tags_only) are stored alongside the data in
the data portion of the NAND page. The whole data portion (including the
tags) is
protected by ECC stored in the spare area.
2. Using out of band tags, but ECC is already available on spare area:
Some parts (eg. some Micron ONFI parts) have built in ECC and have a fixed
layout where some of the spare bytes are used for ECC but other bytes are
still
available for storing tags and these bytes are protected by the ECC. In
this case you can store just the tags since there is no need for extra ECC
on the tags.
3. Some parts do not provide any ECC on the spare area. If you want to
store tags in the spare area (ie. out of band tags) then it is advisable to
use yaffs_packed_tags2
since that also performs extra ECC on the tags.
Regards
-- Charles
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