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

[Tom Rini]

On Wed, Aug 28, 2019 at 07:22:36PM +0000, Vikas MANOCHA wrote:
> 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.

So this patch keeps your use case working and fixes Stephen's problem,
to be clear?  Thanks guys!

-- 
Tom
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