[PATCH v2 1/2] spi: spi-imx: enable dma support for escpi controller

[marex@denx.de (Marek Vasut)]

On Tuesday, January 14, 2014 at 10:55:48 PM, Mark Brown wrote:
> On Tue, Jan 14, 2014 at 10:38:32PM +0100, Marek Vasut wrote:
> > I know the MXC SPI controller works in full-duplex mode, thus must have
> > two equally big buffers (one for RX and one for TX) even if used in
> > half-duplex mode, right?
> 
> We should factor this stuff out into the core, a bunch of controllers
> have that limitation.
> 
> > The problem I perceive here is that when I do for example 4 MB long
> > continuous half-duplex transfer with the IMX SPI, your code will happily
> > allocate 4MB big buffer, right ?
> 
> Or try to - the other trick here is getting 4MB of contiguous memory in
> the first place (unless there's an IOMMU making everything pretty).

Oh, but what if I want to make a really looong transfer ? Say, read entire SPI 
NOR of 128MB in size ... Spansion has such big SPI NORs in their new portfolio.

> > Hence my suggestion, won't it be better to split such long transfers into
> > a chain of DMA descriptors AND use a small (say, 16KiB) buffer for the
> > unwanted direction ? This way, you would allocate the small 16KiB block
> > only once (heck, you can even allocate it in probe() ), and each
> > descriptor would point to this block, overwriting it or sourcing zeroes
> > from it .
> 
> Yes, indeed.  I've half started working on some dmaengine code and was
> going to do something like this as part of it - the idea was to have the
> core generate a sg_list to pass to devices with DMAable transfers which
> would enable this sort of rewriting and would also mean we could do
> things like scatter/gather more effectively.  Not got as far as actually
> getting things into a nice shape for that though.  There's a lot of
> common patterns here which we shouldn't be open coding.

Indeed, full ACK on this.

Best regards,
Marek Vasut