[U-Boot] [PATCH] mtd: add altera quadspi driver

[Marek Vasut <marex@denx.de>]

On Thursday, November 05, 2015 at 05:26:25 AM, Thomas Chou wrote:
> HI Chin Liang,
> 
> On 2015?11?05? 11:05, Chin Liang See wrote:
> >>>>> I notice you are writing in word style which might have concern in
> >>>>> performance. As the burst count can go up to 64, we can write larger
> >>>>> data through memcpy. This will avoid redundancy of data header
> >>>>> (opcode + address + dummy).
> >>>> 
> >>>> You cannot do that, memcpy works on memory while write*() operators
> >>>> work on I/O. You should use readsl() and friends then.
> >>> 
> >>> Actually I am thinking to take advantage the cache fill and dump. But
> >>> after rethinking, this might limit some of the use case as we want the
> >>> driver to support NIOS II without cache. With that, just ignore this
> >>> comment for now.
> >> 
> >> I'm not sure I want to ask for details here. I think we're reading data
> >> from some sort of IO device, so we should just use readl() or readsl()
> >> to read them out (and write*() for the other direction). I don't think
> >> cache operations can be involved in any way. Correct me if I'm wrong
> >> please.
> > 
> > Sure, I can share more. Since the read can support up to burst of 64
> > byte, we can use the a cache fill method which eventually trigger a read
> > of 32 byte (which is size of a cache line) to the Quad SPI controller.
> > To ensure we don't read from old data, we need to invalidate that cache
> > line (through address). By doing this, we can gain better performance as
> > we are reading 32 bytes of data instead 4 per transaction.
> > 
> >>> But your comment lead to the fact that the read part is now using
> >>> memcpy. Thomas needs to fix that to use the readl :)
> >> 
> >> Uhm, I don't think I understand this remark, sorry. I never suggested to
> >> use memcpy() in this entire thread, did I ?
> > 
> > Nope, but this trigger me that we need to do the same for read. The
> > memcpy might lead to the driver reading old data that stay on cache
> > instead from controller. Another way to get rid of this is invalidate
> > the cache.
> 
> Thank for the reminding about the read part. I should not use memcpy()
> indeed.
> 
> Maybe we could pull the memcpy_fromio() and memcpy_toio() or the
> asm-generic/io.h from Linux kernel?
> 
> For i/o access, we bypass the cache for u-boot nios2 with ioremap() or
> map_physaddr(uncached). So no worries or advantage about cache.

Oh, you need those memcpy_{from,to}io() for this hardware? In that case,
go ahead and either implement them or use them :) Now I understand why
you cannot use the reads*() function.

Best regards,
Marek Vasut