[U-Boot] [PATCH] board_f: fix noncached reservation calculation

[Vikas MANOCHA]

Hi,

> -----Original Message-----
> From: Stephen Warren <swarren at wwwdotorg.org>
> Sent: Tuesday, August 27, 2019 7:50 PM
> To: Vikas MANOCHA <vikas.manocha at st.com>; Tom Rini
> <trini at konsulko.com>
> Cc: twarren at wwwdotorg.org; u-boot at lists.denx.de; Stephen Warren
> <swarren at nvidia.com>
> Subject: Re: [PATCH] board_f: fix noncached reservation calculation
> 
> On 8/27/19 6:01 PM, Vikas MANOCHA wrote:
> > Stephen Warren wrote at Tuesday, August 27, 2019 3:50 PM
> >> On 8/27/19 4:10 PM, Vikas MANOCHA wrote:
> >>> Stephen Warren wrote at Tuesday, August 27, 2019 10:55 AM
> >>>> The current code in reserve_noncached() has two issues:
> >>>>
> >>>> 1) The first update of gd->start_addr_sp always rounds down to a
> >>>> section start. However, the equivalent calculation in
> >>>> cache.c:noncached_init() always first rounds up to a section start,
> >>>> then subtracts a section size.
> >>>> These two calculations differ if the initial value is already
> >>>> rounded to section alignment.
> >>>
> >>> It shouldn't cause any issue, first one round down to section size.
> >>> Second
> >>> one(cache.c: noncached_init()) rounds up, so needs section size
> >>> subtraction.
> >>
> >> Here's an example where it fails, based on code before my patch:
> >>
> >> Assume that MMU section size is 2, and that mem_malloc_start and
> >> gd->start_addr_sp are both 1000M on entry to the functions, and the
> >> noncached region is 1 (what Jetson TX1 uses). The example uses values
> >> assumed to be multiples of 1M to make the numbers easier to read.
> >>
> >> noncached_init:
> >>
> >> // mem_malloc_start = 1000
> >> end = ALIGN(mem_malloc_start, MMU_SECTION_SIZE) -
> MMU_SECTION_SIZE;
> >> // end = 1000 - 2 = 998 // was already aligned, so 1000 not 1002 size
> >> = ALIGN(CONFIG_SYS_NONCACHED_MEMORY,
> >> MMU_SECTION_SIZE); // size = 2 start = end - size; // start = 998 - 2
> >> = 996 // region is 996...998
> >
> > Thanks for this example, it definitely seems a bug.  Just that we are fixing it
> by adding this gap in the reserve_noncached() also.
> > Better would be to fix this subtraction of MMU_SECTION_SIZE by aligning
> down "end" location, like:
> >
> > end = ALIGN_DOWN(mem_malloc_start, MMU_SECTION_SIZE); // end =
> 1000
> > size = ALIGN(CONFIG_SYS_NONCACHED_MEMORY, MMU_SECTION_SIZE);
> // size =
> > 2 start = end -size; // start = 998
> 
> That would change yet another piece of code that's been stable for a while.
> It's late in the U-Boot release cycle, so I think we should be conservative, and
> not change any more code than necessary. Changing lots of extra code
> would run the risk of introducing more regressions. I'd rather (a) apply the
> original change I posted, which adjusts only the code that caused the
> regression, or (b) revert the patch that caused the regression.

Ok, Either way is fine.

> 
> If you want to adjust the code in noncached_init, we can do that
> immediately after the release, to give maximum time for any regressions to
> be debugged and fixed before the next release.

Ok.

Cheers,
Vikas