Re: memcpy_toio on i386 using byte writes even when n%2==0

[Robert Hancock]

Chris Lesiak wrote:
> I'm working on a driver for a custom PCI card on the i386 architecture.
> The card uses a PLX9030 pci bridge to link an FPGA to the PCI bus using
> a 16 bit bus.  I found that something broke when moving from 2.6.10 to
> 2.6.17-rc4.  In the driver, I use memcpy_toio to write 14 bytes to a
> memory region in the FPGA.
>
> To copy the 14 bytes, 2.6.10 does three 32 bit writes followed by one 16
> bit write.  2.6.10 does three 32 bit writes followed by two 8 bit write.
>
> The PLX9030 breaks the 32 bit writes into 16 bit writes for its local
> bus just fine.  The problem is that my board doesn't handle byte
> enables.  It was assumed that if all memory transfers were a multiple of
> 2 bytes, then byte accesses wouldn't be used.  This is no longer true in
> 2.6.7-rc4.
>
> I've solved the problem by padding to 16 bytes, but should this be
> considered a bug in the kernel?

It does seem a little bit less efficient, but I don't know think it's 
necessarily a bug. There's no guarantee of what size writes will be used 
with the memcpy_to/fromio functions.

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@xxxxxxxxxxxxx
Home Page: http://www.roberthancock.com/

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