[re-ordered to remove the top posting]
On Sun, 23 Jul 2006, Guillermo Prandi wrote:
> > On Sun, 23 Jul 2006, Guillermo Prandi wrote:
> > > If you omit the O_SYNC keyword,
> > > the system would map a *buffer* to the /dev/mem file rather than to
> > > the actual memory (you could flush the buffers using the fcntl system
> > > call, but that just would be unpractical). The /dev/mem driver
> > > interprets the O_SYNC flag and makes use of the virtual memory mapper
> > > to map real memory locations into the user's virtual memory ranges.
> >
> > All well and good, but why have I and others not found the caching
> > problem? Admittedly I've only used a TS7200, and I'm using an early
> > Technologics kernel.
> >
> The problem would not show up if your program reads or writes and
> then exits (like to be used from a shell script); in such case cache
> wouldn't apply (buffers are flushed on file close, i.e., on process
> exit). The problem would only be noticed if reads/writes are repeated
> within the same process, and before any automatic buffer flush
> happens (because the kernel typically flushes the file buffers every
> now and then).
OK I'm genuinely trying to understand what you are saying here, so i'm
going to run thru' this stage by stage - because I'm not convinced.
I don't understand where your claimed "buffering" comes in,
/dev/mem is not a block device, we are not using read/write system calls
to read this file.
Let's look at some code......
volatile unsigned char *adc_io_cntl;
volatile unsigned char *adc_io_stat;
volatile unsigned char *adc_io_read;
int iofd;
iofd=open("/dev/mem", O_RDWR);
adc_io_stat=(unsigned char *)mmap(0,getpagesize(),
PROT_READ|PROT_WRITE, MAP_SHARED,iofd,0x10800000);
s=*adc_io_stat & 0x80;
Are you saying that this pointer dereference doesn't generate the correct
code to cause the hardware to read the relevant byte from the correct
memory location, that we are somehow reading a buffered copy of "memory"
instead of the real thing? And that somehow if we add O_SYNC to the open
then we read the real memory?
Jim
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