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

[Richard Weinberger]

Am 24.06.2016 um 09:39 schrieb Boris Brezillon:
> On Fri, 24 Jun 2016 08:10:37 +0200
> Sascha Hauer <s.hauer at 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).

I assumed it will give you only on large pages (as found on MLC NAND)
a speedup.

Thanks,
//richard