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Subjectdev driver / pci throughput

Dear kernel-list,

I am writing a dev driver in which large amounts of data are
passed from user space to PCI device memory and I am seeing a
far lower throughput than I expected. I know this is likely to
be high architecture dependant but I would appreciate some
general guidance.

The essential bit of code looks like

#define CHUNK 512->4096 depending on implementation

static ssize_t BSL_write(..., const char *buf, size_t count..){
char chunk[CHUNK];
int i,pos,
for(i=0,pos=0;i<amount of data;i++,pos+=CHUNK){
copy_from_user(buff,buf+pos,CHUNK);
/* reorder data */
/* not significant in throughput */
for(k=0;k<CHUNK;k++){
chunk[k]=buff[B_SM(j+k)];
}
memcpy_toio(MEM_reg+pos+i*CHUNK+j,chunk,CHUNK);
}
return count;
}
+ lots of not important details-----

MEM_reg=ioremap(pci_resource_start(dev,B_SM_MEM)&PCI_BASE_ADDRESS_MEM_MASK,REG_SIZE);



Basically it reads a big buffer from user space, scrambles the
byte ordering (according to B_SM) and puts it onto the PCI
device memory.

** I currently get 3-4Mb/s throughput (on a fairly poor x86 pc),
this is about 1/10 of what I expected to achieve.

** The CHUNK size is not an important factor above 512 bytes- it
is contributing to 0.1 percent of the bottle neck. This
leads me to think that the copy_from overheads are small.

** The byte ordering is not significant, removing it or
replacing it with a null command indicates it contributes
~0.1 percent of the bottleneck.

** The transfer size will always exceed any cache sizes in the
system.


I will go onto implementing an mmap method sometime later but as
the essential memory / PCI memory bandwidth remains the same I
don't think it will make much difference- From testing with
different transfer sizes overheads seem small provided a minimum
transfer size of 512 bytes is enforced.

Any thoughts? pointers towards profiling this code, typical
throughputs, improving throughput etc gratefully received.

Thanks matt

--------
Thanks for the previous help on interrupts etc-

-
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