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

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Fri, 24 Jun 2016 08:10:37 +0200
Sascha Hauer <s.hauer@pengutronix.de> wrote:

> 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.

Not so surprising to me. I tried the same trick on a 16k page NAND a
while a ago, and it drastically decreased the attach time (don't recall
the exact numbers).

> 
> 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
>