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

[Vinod Koul <vinod.koul@intel.com>]

On Sun, Jul 19, 2015 at 09:01:34PM +0200, Michal Suchanek wrote:
> Hello,
> 
> On 15 July 2015 at 17:59, Brian Norris <computersforpeace@gmail.com> wrote:
> > Hi Michal,
> >
> > On Wed, Jul 15, 2015 at 01:52:27PM +0200, Marek Vasut wrote:
> >> The problem is, if you add a new DT binding, you'd have to support it
> >> forever, no matter how bad idea that binding turned out to be.
> >
> > Agreed, and a solid NAK to this patch. I could have sworn I gave such a
> > response when this was originally being discussed a month ago.
> >
> > AFAICT, you have one of two general approaches available to you:
> >
> > 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.
> >
> > 2. Utilize/create a parameter within the SPI subsystem to communicate
> > that the SPI master has a limited max transfer size (notably: NOT a
> > per-device DT property, but a SPI API property), and modify SPI device
> > drivers (like m25p80) to honor it. Mark Brown seemed open to this, and I
> > thought he suggested it somewhere.
> 
> It is not known what exactly is limited here.
> 
> It seems that the pl330 fails but it is not possible to transfer that
> much data over the spi bus in one go without the help of the pl330.
> 
> With either approach the limit depends on the SPI transfer settings
> which are known the the SPI driver. The pl330 driver is oblivious to
> these because it just transfers data from one port to another port and
> has no idea that the port is wired to SPI in the SoC.
> 
> On the other hand, AFAICT the SPI driver only allocates a DMA channel
> which it receives through DT binding and is oblivious to the fact the
> DMA channel lives on a pl330. It could probably determine that the
> channel is indeed driven by a pl330. I don't think it's a great idea
> to add device-specific handling to a generic dmaengine driver or a
> dmaengine-spiecific handling to a SPI driver.
> 
> It's technically possible to pass SPI transfer parameters to the
> dmaengine driver prior to transfer and the dmaengine could impose some
> limitation based on those parameters. However, generalising this to
> drivers other than SPI might be problematic. Should this interface
> also handle i2c parameters, VE parameters, audio parameters, ethernet
> parameters, etc?
Or alternatively we could publish the limitations of the channel using
capabilities so SPI knows I have a dmaengine channel and it can transfer max N
length transfers so would be able to break rather than guessing it or coding
in DT. Yes it may come from DT but that should be dmaengine driver rather
than client driver :)

This can be done by dma_get_slave_caps(chan, &caps)

And we add max_length as one more parameter to existing set

Also all this could be handled in generic SPI-dmaengine layer so that
individual drivers don't have to code it in

Let me know if this idea is okay, I can push the dmaengine bits...

Thanks
-- 
~Vinod