On Thursday, November 05, 2015 at 05:26:25 AM, Thomas Chou wrote:
HI Chin Liang,
On 2015年11月05日 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