Re: [PATCH 0/2] disk: use maximum number of sectors for LBA

[ValdikSS]

On 05.10.2023 22:09, Glenn Washburn wrote:
> > GRUB2 limits number of sectors read at once in LBA mode to the
> > number reported in CHS disk geometry by BIOS.
> > This is unnecessary, as IBM/MS INT13 Extensions allows reading
> > up to 127 sectors in a single call.
>
> Please provide more documentation for this. Wikipedia[1] suggests that
> IBM/MS INT13 Extensions allows reading up to 64K sectors in a single
> call. But that "some Phoenix BIOSes are limited to a maximum of 127
> sectors". So the above seems misleading.

Rephrased as "upper safe value for LBA read". Check v2.

> > This fixes greatly increased boot times on WYSE C10LE x86 thin
> > client with Phoenix bios.
> >
> > See: https://lists.gnu.org/archive/html/grub-devel/2023-07/msg00001.html
>
> First, this seems risky to me to be added for the release.  Why do we
> believe that some other bios doesn't have a limit less than 127?

I've checked Wikipedia and OSDev, as well as Russian forum (don't 
remember which), and tested on two of my old machines with Phoenix and 
Award BIOS.

syslinux also limits to 127 if EBIOS is detected:

        hard_max_transfer = 63;
        ...
        if (!(oreg.eflags.l & EFLAGS_CF) &&
            oreg.ebx.w[0] == 0xaa55 && (oreg.ecx.b[0] & 1)) {
            ebios = true;
            hard_max_transfer = 127;

https://github.com/geneC/syslinux/blob/5e426532210bb830d2d7426eb8d8c154d9dfcba6/core/fs/diskio_bios.c#L349

That's why I assume it should be safe to enable.

> Also, based on reading the email in the link above, am I correct in
> concluding that your bios set the CHS values returned in
> grub_biosdisk_rw_int13_extensions() from the values in the MBR?

Yes, this is correct.

> Glenn
>
> [1]
> https://en.wikipedia.org/wiki/INT_13H#INT_13h_AH=42h:_Extended_Read_Sectors_From_Drive

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