Re: SPI driver for m8xx

[Dan Malek <dan@embeddededge.com>]

Jeremy Rosen wrote:

> for the TX and RX buffer, they use memory they get for
> m8xx_cpm_hostalloc... why ?

It's convenient for small, uncached buffer space.  Don't be allocating
large (more than 32-64 bytes) out of this space, as there is only a
page or two shared among all CPM devices.  This is normally used for
things like SCC/SMC UART "FIFOs".

> Does it have something to do with the MMU ?

No, but with caches.

> I would like to avoid doing that, is their a way to use normal ( kernel
> space) memory ?

An alternate and proper approach is to use consistent_alloc().  This will
allocate page size memory, aligned on page boundaries, with cache disabled.
You must use the virtual/physical addresses returned from this function
and keep track of them in the driver.  The virt_to_* or __va/__pa() macros
won't work.

A third approach is to allocate kernel memory either with kmalloc() or
getpages().  This is not cache coherent with CPM DMA, so you need to
call the cache flush/invalidate functions to manage this space.

All of these have advantages and disadvantages you need to trade off in
your design.


	-- Dan


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