Re: [PATCH v3 2/2] disk: increase sector size up to 127 for LBA reads

[Daniel Kiper]

On Tue, Jan 30, 2024 at 10:34:33PM +0300, ValdikSS via Grub-devel wrote:
> Increase the value from 63 to speed up reading process.
>
> This commit increases two limits: the low-level int 13h reading code
> and a high-level reading code with disk cache.
> The disk cache imposes an overall limitation of a higher-layer reading
> code. The original comment regarding 16K is incorrect, it was
> 512<<6 = 32768, and now it is 512<<7 = 65536.
>
> According to [Wikipedia][1] and [OSDev][2], the upper safe value for LBA
> read using IBM/MS INT13 Extensions is 127 sectors due to the
> limitations of some BIOSes.
> GRUB [already enforced][3] the limit, but it was no-op due to other
> constraints. This value is also used in [syslinux][4].
>
> As we're now reading up to 127 sectors of 512 bytes, we need to be able
> to store in the cache up to 65024 bytes. Without this change, GRUB
> wouldn't try to read more than 64 sectors at once
> (even if the lower reading layer allows it).
>
> [1]: 
> https://en.wikipedia.org/wiki/INT_13H#INT_13h_AH=42h:_Extended_Read_Sectors_From_Drive
> [2]: 
> https://wiki.osdev.org/Disk_access_using_the_BIOS_(INT_13h)#LBA_in_Extended_Mode
> [3]: 
> https://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/disk/i386/pc/biosdisk.c?h=grub-2.12-rc1#n430
> [4]: 
> https://repo.or.cz/syslinux.git/blob/refs/tags/syslinux-6.03:/core/fs/diskio_bios.c#l349
>
> Signed-off-by: ValdikSS <iam@valdikss.org.ru>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Daniel