[RFC PATCH] disk: Don't loop over MAX_SEARCH_PARTITIONS in part_create_block_devices()

[AKASHI Takahiro]

On Wed, Jan 11, 2023 at 09:05:05AM +0100, Stefan Roese wrote:
> On 1/11/23 02:32, AKASHI Takahiro wrote:
> > Hi,
> > 
> > Thank you for catching this issue.
> > 
> > On Tue, Jan 10, 2023 at 09:00:38AM +0100, Stefan Roese wrote:
> > > I've noticed that the first ext4 file loading from a MMC partition for
> > > ZynqMP takes quite some time (~ 1 second). Debugging showed, that the
> > > MMC driver reads the partition info 128 time (MAX_SEARCH_PARTITIONS)
> > > resulting in this boot delay. To fix this, let's just end creating the
> > > block drives in part_create_block_devices() when no more valid partition
> > > is found. This reduces the first file reading from ~0.9s to ~0.3s.
> > > 
> > > Signed-off-by: Stefan Roese <sr at denx.de>
> > > Cc: Simon Glass <sjg at chromium.org>
> > > Cc: AKASHI Takahiro <takahiro.akashi at linaro.org>
> > > Cc: Heinrich Schuchardt <xypron.glpk at gmx.de>
> > > ---
> > > Reasoning for RFC:
> > > I did not dig into the current disk / partition stuff too deeply, so I'm
> > > not 100% sure that this patch does not break anything.
> > 
> > I'm afraid that this fix won't work for all the partition types,
> > especially for those in which entries in a partition table can be sparsely filled,
> > or in other words, valid partition numbers may not always be contiguous
> > even if they don't reach a maximum number.
> 
> So what is the maximum number? Where originates the
> MAX_SEARCH_PARTITIONS = 128 from?

commit bc314f8e5f9b ("cmd: part: list all 128 GPT partitions")

But GPT may accept arbitrary number of partitions.
The GPT header has "num_partition_entries" for the table size.

The max depends on partition types.

> > I somehow confirmed this against a GPT partition by using gdisk.
> 
> How exactly did you confirm this?

Created partitions for, say, #3 and #5 by 'n' command.

-Takahiro Akashi

> Thanks,
> Stefan
> 
> > -Takahiro Akashi
> > 
> > > ---
> > >   disk/disk-uclass.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/disk/disk-uclass.c b/disk/disk-uclass.c
> > > index d32747e2242d..2999f7285b5a 100644
> > > --- a/disk/disk-uclass.c
> > > +++ b/disk/disk-uclass.c
> > > @@ -36,7 +36,7 @@ int part_create_block_devices(struct udevice *blk_dev)
> > >   	/* Add devices for each partition */
> > >   	for (count = 0, part = 1; part <= MAX_SEARCH_PARTITIONS; part++) {
> > >   		if (part_get_info(desc, part, &info))
> > > -			continue;
> > > +			break;
> > >   		snprintf(devname, sizeof(devname), "%s:%d", blk_dev->name,
> > >   			 part);
> > > -- 
> > > 2.39.0
> > > 
> 
> Viele Grüße,
> Stefan Roese
> 
> -- 
> DENX Software Engineering GmbH,      Managing Director: Erika Unter
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