Re: `disk` level IO: how it works?

[Robert Millan]

On Tue, Jan 22, 2008 at 08:01:56AM +0000, Oleg Strikov wrote:
> Good day!
> I'm trying to look into grub low-level disk IO.
> I've started exploration with my own  low-level FS block reader, but it
> works too strange:
> 
> /* I know that such code is incorrect due to low-level FS operations but i'm
> trying to understand some principals of low level IO :) */
> 
> file = grub_file_open (FILE);
> fs_data = (struct grub_ext2_data *) file->data;
> disk = fs_data->disk;
> 
> fsize = grub_file_size(file);
> block_num = (fsize >> LOG2_BLOCK_SIZE (fs_data));
> if (((grub_off_t) block_num << LOG2_BLOCK_SIZE (fs_data)) != fsize)
>     block_num++;
> 
> log2_blksz = LOG2_EXT2_BLOCK_SIZE (fs_data);
> 
> blocks = grub_malloc (sizeof (int) * block_num);
> 
> fshelp_node = &fs_data->diropen;
> 
> for (i = 0; i < block_num; i++)
>   {
>     blocks[i] = grub_ext2_read_block_wrapper (fshelp_node, i);
>   }
> 
> grub_disk_read(fs_data->disk,
>   \
>            grub_le_to_cpu32 (blocks[i]) << log2_blksz,               \
>            0,
>                                                                     \
>            sizeof (struct my_entry),
>          \
>            (char *) entry);
> 
> grub_ext2_read_block_wrapper its my exported wrapper from ext2.c that calls
> original grub_ext2_read_block with the same parameters.

I find this very confusing.  Could you rephrase your question to make it more
specific?  Like, what are you trying to do or which function you don't
understand.

> Problem: I have extended partition with two logical (hd0,5) & (hd0,6)
> If I place FILE on (hd0,5) (at the begining of my physical disk) - all is OK
> If I place FILE on (hd0,6) i get `out of partition` at grub_disk_read.

Did you check if debug options give you anything useful?  Try "set debug=disk",
and maybe also "set debug=fs".

What's the size of your disk, and your partitions?

> P.S. I'm right that `disk` represents one partition (logical or primary),
> not a group?

I think grub_disk_t can either be a partition or a disk.

-- 
Robert Millan

<GPLv2> I know my rights; I want my phone call!
<DRM> What use is a phone call… if you are unable to speak?
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