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

[Vikas MANOCHA]

Hi Stephen,

> -----Original Message-----
> From: Stephen Warren <swarren at wwwdotorg.org>
> Sent: Tuesday, August 27, 2019 3:50 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 4:10 PM, Vikas MANOCHA wrote:
> > Hi Stephen,
> >
> >> -----Original Message-----
> >> From: Stephen Warren <swarren at wwwdotorg.org>
> >> Sent: Tuesday, August 27, 2019 10:55 AM
> >> To: Tom Rini <trini at konsulko.com>
> >> Cc: twarren at wwwdotorg.org; u-boot at lists.denx.de; Stephen Warren
> >> <swarren at nvidia.com>; Vikas MANOCHA <vikas.manocha at st.com>
> >> Subject: [PATCH] board_f: fix noncached reservation calculation
> >>
> >> From: Stephen Warren <swarren at nvidia.com>
> >>
> >> 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

> 
> reserve_noncached:
> 
> // gd->start_addr_sp = 1000
> gd->start_addr_sp &= ~(MMU_SECTION_SIZE - 1);
> // gd->start_addr_sp = 1000
> gd->start_addr_sp -= CONFIG_SYS_NONCACHED_MEMORY;

Here CONFIG_SYS_NONCACHED_MEMORY needs to be aligned to MMU SECTION SIZE before 
subtracting from start_addr_sp to fix the second issue you highlighted.

gd->start_addr_sp -= ALIGN(CONFIG_SYS_NONCACHED_MEMORY, MMU_SECTION_SIZE);  // start of non cached = 998.

> // gd->start_addr_sp = 1000 - 1 = 999
> // region is 999...1000

> 
> So, the end of the region that's been reserved is 1000, yet the code that sets
> up the noncached region believes the end of the region is at 998. Even
> ignoring the difference in size calculation due to issue (2) below, that still
> means the reservation is in the wrong place, and the stack can end up
> overlaid with the noncached reservation, or even other data below it.
> 
> >> 2) The second update of gd->start_addr_sp subtracts exactly
> >> CONFIG_SYS_NONCACHED_MEMORY, whereas the equivalent
> calculation in
> >> cache.c:noncached_init() rounds the noncached size up to section
> >> alignment before subtracting it. The two calculations differ if the
> >> noncached region size is not a multiple of the MMU section size.
> >
> > Never thought CONFIG_SYS_NON_CACACHED_MEMORY could be non-
> multiple of
> > MMU section size for basic MMU setup in u-boot. It has granularity of
> section size.
> > Is it the case with Jetson TX1 ?
> 
> Yes, on Jetson TX1, the MMU section size is 2M, yet the noncached region is
> 1M. Nothing in the README docs for the nocached region state or imply that
> the noncached region needs to be a multiple of the MMU section size,

MMU setup granularity is section size, configuring any memory area less than 
section size is not possible in this basic mmu setup. Your patch rounds up this
noncached area to SECTION area which makes it robust.

> and
> all code that uses the config symbol before your patch rounds the config
> symbol to MMU section size, implying that its value doens't need to be
> rounded already.

It was using stack area well below the stack pointer, so was working fine. The patch
just didn’t take care of the case where configured noncached area is not multiple of section size.

> 
> >> In practice, one/both of those issues causes a practical problem on
> >> Jetson TX1; U-Boot triggers a synchronous abort during
> >> initialization, likely due to overlapping use of some memory region.
> >>
> >> This change fixes both these issues by duplicating the exact
> >> calculations from
> >> noncached_init() into reserve_noncached().
> >>
> >> However, this fix assumes that gd->start_addr_sp on entry to
> >> reserve_noncached() exactly matches mem_malloc_start on entry to
> >> noncached_init(). I haven't traced the code to see whether it
> >> absolutely guarantees this in all (or indeed any!) cases.
> >> Consequently, I added some comments in the hope that this condition
> will continue to be true.
> >
> > It is enforced it in the code, reserve_noncached is called from
> > reserve_malloc()after malloc area reservation.
> 
> That's a bit implicit still; nothing in reserve_malloc sets or uses the value of
> mem_malloc_start, so the two could easily become decoupled if the
> reservation calculations don't match the code which actually sets up the
> region usage, which incidentally is exactly what happened here, and hence
> why I found this bug.

Ok

> 
> >> diff --git a/common/board_r.c b/common/board_r.c index
> >> b7f68bba4a7e..d6fb5047a265 100644
> >> --- a/common/board_r.c
> >> +++ b/common/board_r.c
> >> @@ -247,6 +247,10 @@ static int initr_malloc(void)
> >>   	      gd->malloc_ptr / 1024);
> >>   #endif
> >>   	/* The malloc area is immediately below the monitor copy in DRAM
> >> */
> >> +	/*
> >> +	 * This value MUST match the value of gd->start_addr_sp in
> >> board_f.c:
> >> +	 * reserve_noncached().
> >> +	 */
> >
> > minor cosmetic suggestion:
> > gd->start_addr_sp is moving pointer, difficult to comprehend sometimes
> > gd->here, same is true for malloc
> > area also, how about merging two comments like:
> >
> > 	/* The malloc area is immediately below the monitor copy in DRAM
> followed by noncached
> > 	*/
> 
> I'd rather have an explicit separate comment which mentions the other
> function and variable names; if someone searches the code later, it's more
> likely they'll find this comment that way. I guess I could replace the
> intermediate /* and */ lines with just * to merge the comments without
> changing the text in them if you want.

Sure.

Cheers,
Vikas