[PATCH] UBI: only read UBI_VID_HDR_SIZE when reading the vid_hdr

[Sascha Hauer]

On Thu, Jun 23, 2016 at 05:16:06PM +0200, Richard Weinberger wrote:
> Am 23.06.2016 um 17:06 schrieb Sascha Hauer:
> >>>  	p = (char *)vid_hdr - ubi->vid_hdr_shift;
> >>>  	read_err = ubi_io_read(ubi, p, pnum, ubi->vid_hdr_aloffset,
> >>> -			  ubi->vid_hdr_alsize);
> >>> +			  UBI_VID_HDR_SIZE);
> >>
> >> Hmm, I fear this will break as soon ubi->vid_hdr_shift is non-zero.
> > 
> > Ok, just tried and indeed it does break. Would it be an option to read
> > UBI_VID_HDR_SIZE + ubi->vid_hdr_shift bytes instead?
> 
> Well, you need to satisfy the trick UBI does.
> Please read the huge comment on it on top of io.c.
> 
> Since in most cases ubi->vid_hdr_shift is 0 we could also do a fast path.
> i.e.
> if (ubi->vid_hdr_shift)
> 	read_len = ubi->vid_hdr_alsize
> else
> 	read_len = UBI_VID_HDR_SIZE;

Yes. I thought reading UBI_VID_HDR_SIZE + ubi->vid_hdr_shift has the
advantage that even with vid_hdr_shift != 0 we can profit from reading
subpages. I tested it with a vid hdr offset of 512 and it works ok.

> 
> But first I have to review a view call sites. :-)

Yes, please. It lowers the chance that I break the kernel ;)

> 
> Can you tell a bit more on the NAND you're facing that speedup?
> I find it surprising that you gain a full second.

It's a Micron Nand with a page size of 8192+448. On an i.MX6 we only
have to read 512 bytes when reading the vid header.

Sascha

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