Reading/writing SPI devices - linux-device-driver

Question

(I am really unsure of which tags to apply, so apologies in advance if I chose the wrong ones.)
I hope that this is not a too stupid question, but I'm really lost here.
A client lent me an Atmel SAMA5D2 Xplained board with Linux4SAM to play around with. I'm trying to interface the SPI interface on it, but I have no clue where to start.
To be clear, I've used similar boards (not this particular one) bare-metal. I've also used Linux for many years and wrote a few simple devices drivers some years ago. And still I'm lost.
dmesg | grep spi gives me the following output:
[ 1.840000] atmel_spi f8000000.spi: version: 0x311
[ 1.840000] atmel_spi f8000000.spi: Using dma0chan0 (tx) and dma0chan1 (rx) for DMA transfers
[ 1.850000] atmel_spi f8000000.spi: Using FIFO (16 data)
[ 1.860000] atmel_spi f8000000.spi: Atmel SPI Controller at 0xf8000000 (irq 32)
[ 1.860000] m25p80 spi32766.0: at25df321a (4096 Kbytes)
From this I infer that a driver is loaded and that it is configured to use DMA. Yet, looking in /dev/ there is nothing that looks like a SPI device (I was expecting to find something like /dev/spidev or /dev/spi32766.0 or similar.)
Does this mean that there is no actual device driver loaded? Do I have to write one in order to use the SPI?
If I look at the Makefile in the Linux4SAM source tree, I see around line 1171 that the kernel does not support loading of modules. Does this imply I have to recompile the kernel to include my new driver? This seems to be a silly approach; why providing a Linux distribution if I can't access the hardware with it?
What am I missing here?
(I feel rather stupid...)
EDIT To be clear: I want to access the external SPI interface that will be connected to some external device. I think the m25p80 is some internal Flash memory; I'm not interested to read/write there.

Answer 1

spidev is a standard Linux device driver which just exports a low level API to userspace via /dev interface
if you want to access specific SPI client(slave) you should write your driver
according to Linux SPI driver model:
static const struct of_device_id myspi_dt_ids[] = {
{ .compatible = "xxxx,yyyyy" },
{},
static struct spi_driver xxx_spi_driver = {
.driver = {
.name = "myspi",
.owner = THIS_MODULE,
.of_match_table = of_match_ptr(myspi_dt_ids),
},
.probe = myspi_probe,
.remove = myspi_remove,
e.g. define spi_driver struct, of_device_id struct, callbacks(hooks), and register it.
You should bind it to your DT node with compatible property string.
myspi#0 {
compatible = "xxxx,yyyyy";
spi-max-frequency = <5000000>;
reg = <0>;
};
second,
compatible = "linux,spidev";
Linux discourages using this compatible property because nodes should
describe real HW devices not userspace abstractions

Answer 2

#0andriy put me on the right track. I had to add a SPI resource to the Device Tree and flash the compile Device Tree Blob to the board. (Since I didn't know about Device Trees at all, this information is really hard to find...).
I now have a /dev/spidev32765.0. I added this to the device tree:
spi1: spi#fc000000 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_spi1_default>;
status = "okay";
spidev#0 {
compatible = "linux,spidev";
spi-max-frequency = <83000000>;
reg = <0>;
};
};
pinctrl#fc038000 {
pinctrl_spi1_default: spi1_default {
pinmux = <PIN_PD25__SPI1_SPCK>,
<PIN_PD26__SPI1_MOSI>,
<PIN_PD27__SPI1_MISO>,
<PIN_PD28__SPI1_NPCS0>;
bias-disable;
};
};
Although I read that adding the spidev#0 is not really the right thing to do (I indeed see in dmesg output "spidev spi32765.0: buggy DT: spidev listed directly in DT").
Now, if I run spidev_test it still doesn't work (it times out), but I guess that's for another question.