[PATCH 07/11] fsmc/nand: Provide contiguous buffers to dma

[Artem Bityutskiy]

On Mon, 2012-10-15 at 09:27 -0700, Brian Norris wrote:
> > I'd really prefer to just switch to kmalloc() everywhere instead of
> > adding hacks like this to the drivers. Then if this is a problem for
> > someone, he can fix it by either switching to smaller buffers (possible
> > in many places), or by improving memory fragmentation issues on his
> > system, or by just using CMA.
> 
> I think I can suggest that this is already a problem on real systems.
> In bringing up a board on v3.3 kernel, I experienced a kernel memory
> allocation error when trying to memdup_user() in eraseblock-size
> regions. I believe I would experience more of these if all
> eraseblock-sized buffers were kmalloc()'d.
> 
> See the commit description for the following commit in mtd-utils.git:
> 
>     commit 71c76e74661492b4f68f670514866cfc85f47089
>     libmtd: fix mtd_write() issues for large data-only writes

Well, this is another work-around. The better way to fix this would be
to change kernel's 'mtdchar_write_ioctl()' to be iterative and avoid
calling 'memdup_user(len)' for arbitrary 'len' passed from user-space.

> I would prefer not building a solution that hopes kmalloc() can get a
> large contiguous buffer (remember, eraseblock sizes come as large as
> 2MB these days). A real solution like CMA or scatter-gather seems like
> a better idea.

Me too. But this does not happen. People bring this up for years. Of
course it is easier to hack drivers, and very understandable. So what
I would like to do is somehow force people to fix this issue.

So I thought about something like introducing an mtd_alloc() which
would:
   a. Try doing CMA allocation: dma_alloc_from_contiguous()
   b. If it fails, use kmalloc().

This function would probably need a cookie which it returns and which
the mtd_free() function would use to call either kfree() or
dma_release_from_contiguous()...

If both fail - bad luck. Go look at the code and switch to S-G lists
instead of allocating contiguous regions. As I said, in many places it
is easy to do, and there are few places when you would need to spend a
bit more time.

How does this sound to you?

-- 
Best Regards,
Artem Bityutskiy


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