On Thu, Oct 15, 2009 at 02:04:38AM -0700, Amit Uttamchandani wrote: > On Thu, Oct 15, 2009 at 01:49:46AM -0700, Benoit Donnette wrote: > > > Hi, > > > > > > I am trying to understand the magic behind memory mapped I/O in linux > > > device drivers. I understand the concepts behind setting up memory > > > mapped I/O but I am confused about how the data (once written into > > > memory) gets transferred onto the bus and onto the peripheral. > > > > "Memory mapped I/O" means taht through the specified address, mapped in > > the address space, you actually send data to the device. > > > > It means you map I/O buffers on the device into addressable memory. After > > that, everything you write at these addresses will go to the device as set > > up, not to memory. > > > > Ok that definitely makes more sense, everything is written directly > to the registers on the device not main memory. > > > > > > > E.g. many network drivers use memory mapped I/O and this makes it quite > > > generic but how do you specify the bus in all this? > > > > All the memory mapped I/O setup does the job. > > > > I guess this is the part I am getting confused. For example ethoc.c sets > up everything as memory mapped IO. I need to hookup the actual ethoc > device to the processor. This will be done using SPI. So how does the > 'memory mapped I/O setup' know to use SPI to read/write the data. The read/write calls are bus agnostic -- but that also doesn't mean you'd use MMIO for all types of buses. For instance USB devices do not use it. The non-bus agnostic part is the actual MMIO setup. For instance, look at ioremap_nocache() [1] which makes bus memory CPU accessible via the readb/readw/readl/writeb/writew/writel functions. Your PCI driver would use this when using the pci_iomap() for example for MMIO devices, the bus part being done right before the call [2]. I take it SPI has similar wrappers. [1] http://lxr.linux.no/#linux+v2.6.31/arch/x86/mm/ioremap.c#L302 [2] http://lxr.linux.no/#linux+v2.6.31/arch/sh/drivers/pci/pci.c#L309 Luis _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
