[PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

[computersforpeace@gmail.com (Brian Norris)]

On Wed, Jul 15, 2015 at 07:15:50PM +0200, Marek Vasut wrote:
> On Wednesday, July 15, 2015 at 05:59:46 PM, Brian Norris wrote:
> > 1. Fix up the SPI driver so that it knows how to break large SPI
> > transfers up into smaller segments that its constituent hardware (DMA
> > controllers, fast clocks, etc.) can handle.

BTW, Mark Brown already commented on this approach:

http://lists.infradead.org/pipermail/linux-mtd/2015-May/059364.html

I quote:

| With modern drivers using transfer_one() where we have control of the
| chip select we do also have the ability to add support to the core for
| chopping large transfers up into smaller ones that the hardware can
| handle transparently.  That won't for everything though since it depends
| on us being able to manually control the chip select which not all
| hardware can do.

> I think this might actually be easier -- just do a transfer where you
> don't toggle CS and just stops the clock at the last bit, then do another
> (multiple) transfers which don't toggle CS at all, then finally do a
> transfer which toggles a CS at the end.

Sounds OK to me. And as Mark noted, this could probably be done in the
core. I suppose Mark could suggest whether the most expedient path is to
hack the buggy driver to do this, or whether reworking the SPI core to
have the appropriate spi_master field(s) (and caveats) to support this.

> This should be pretty trivial
> to do and I think for example spi-mxs.c does this.

I don't think spi-mxs.c really does this; it chooses between PIO and DMA
based on length, but with either option, it runs through each SPI
transfer in one go. You might be confused by the fact that this driver
implements ->transfer_one_message instead of ->transfer_one, so it has
to loop through all transfers in the message.

(IIUC, spi-mxs.c could easily be rewritten to use ->transfer, and some
of that not-too-complicated boilerplate could be killed off.)

Brian