[PATCH v2 3/4] mtd: nand: support Micron READ RETRY

[Brian Norris]

On Tue, Dec 17, 2013 at 01:11:25PM +0800, Huang Shijie wrote:
> On Mon, Dec 16, 2013 at 09:06:13PM -0800, Brian Norris wrote:
> > On Tue, Dec 17, 2013 at 12:23:08PM +0800, Huang Shijie wrote:
> > > On Mon, Dec 16, 2013 at 08:43:42PM -0800, Brian Norris wrote:
> > > > > 
> > > > > The DMA warning only occurs when we enable the CONFIG_DMA_API_DEBUG.
> > Particularly, I think it is quite reasonable to restrict DMA to
> > mtd_read()/mtd_write()-like operations -- so this trickles down to
> > read_page/write_page, but not to device configuration operations like
> > ONFI set_features/get_features/read parameter tables, etc.
> > 
> > Do you disagree? Do you see some need to perform DMA on all operations?
> 
> The gpmi is much coupled with the MXS-DMA engine. the gpmi can uses the
> mxs-dma to set the gpmi registers.
> 
> Use the DMA makes the GPMI code very simple, it really makes the code very
> _ugly_ if we do not use the DMA for read_buf/write_buf.

It's not just about ugliness, it's about maintainability. I can clean up
a few cases here and there that don't conform to your driver's needs now
(notice I pointed out another instance of this in Uwe's patch), but
unless we establish better guidelines for NAND, this is fragile.

As a diversion, let's look at atmel_nand for an example. It uses DMA as
well, but it dodges some of your problems using a heuristic, so that it
only uses DMA for larger-than-OOB-size transactions (see
atmel_read_buf). Additionally, it has a hack-y check for highmem, to try
to confirm that the region it's trying to map is, in fact, DMA-able.

So, atmel_nand does not have an ideal solution, but it at least attempts
to solve parts of this problem. GPMI, on the other hand, ignores it
entirely.

> If you insist that it is okay to use the local array in the nand_base.c,
> I can use kmalloc/memcpy in the gpmi's read_buf/write_buf hooks. In such a 
> way, we can make the gpmi looks more normal.

I think a bounce buffer is necessary, if you really want to use DMA all
the time; some transactions are just not worth initializing DMA-able
buffers in the high-level code. But even better, can GPMI easily utilize
some sort of PIO mode for small/slow-path transactions like this, or is
it inextricably tied to the MXS-DMA engine?

Brian