On 2012?05?31? 13:19, Catalin Marinas wrote:

On 31 May 2012 13:06, bill4carson[off-list ref]  wrote:

quoted

On 2012?05?31? 11:58, Catalin Marinas wrote:

quoted

I still didn't fully understand what the problem is. So, to make sure,
if you run some applications from flash using a yaffs filesystem, you
get random crashes. Is this correct? If yes, a solution is to actually
call flush_dcache_page() on the CPU that does the page copying from
flash into RAM, which could be the yaffs filesystem.

The story goes like this:
function "flush_dcache_page" should be global effective
but in ARMv6 MPCore, it was not, it was just local effective due
to hardware design.

Yes, I know this.

Then, why not fix "flush_dcache_page" to make it globally effective?

quoted

This may cause error in some cases for example:

1) Task running on Core-0 loading text section into memory.
   It was preempted and then migrate into Core-1;

BTW, do you have CONFIG_PREEMPT enabled?

Yes, CONFIG_PREEMPT enabled. Thus cause the task was preempted. :-)

To be clear - is your application reading some data from flash and
trying to execute or it's the kernel doing the load via the
page/prefetch abort mechanism?

In my current case, it is yaffs root file system.

If the latter, task running on core 0 gets a prefetch abort when
trying to execute some code. The kernel reads the page from flash (via
mtd, block layer, VFS) and copies it into RAM. It can be on any CPU as
long as it calls flush_dcache_page on the same CPU that copied the
data.

No matter where the task was running or migrated to, if the code doing
the copy also called flush_dcache_page() on the same core, there is no
data left in the D-cache for that page.

Yes, I agree with it.
But how to flush the data cache on the same core with PREEMPT enabled?
And, I think according to the design, there is no such operation that
guarantee it.

quoted

2) On Core-1, this task continue loading it and then
   "flush_dcache_page" to make sure the loaded text section write
   into main memory.

The flush_dcache_page() must be called by the code doing the copy. If
that copy happened on core 0, the call is done there and not where the
task migrated. We don't do lazy flushing on ARM11MPCore.

WHY?

quoted

3) Task tend to the loaded text section and running it.

If the "flush_dcache_page" was not global effective,
there maybe data still in Core-0's data cache, not write
into main memory. Thus in step 3, error instruction maybe
fetched thus cause strange error.

This can only happen if you have either preempt enabled (so that the
kernel code doing the copy is migrated) or the mtd driver or fs do not
call flush_dcache_page().

PREEMPT + MTD root file system. :-)

But any way, I think flush_dcache_page should be global effective.
If ARMv6 MPCore didn't make it, we should try to accomplish it.



-- 
Love each day!

--bill