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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* The RISC-V Zkr extension provides CSR seed which provides access to a
* random number generator.
*/
#define LOG_CATEGORY UCLASS_RNG
#include <dm.h>
#include <interrupt.h>
#include <log.h>
#include <rng.h>
#define DRIVER_NAME "riscv_zkr"
enum opst {
/** @BIST: built in self test running */
BIST = 0b00,
/** @WAIT: sufficient amount of entropy is not yet available */
WAIT = 0b01,
/** @ES16: 16bits of entropy available */
ES16 = 0b10,
/** @DEAD: unrecoverable self-test error */
DEAD = 0b11,
};
static unsigned long read_seed(void)
{
unsigned long ret;
__asm__ __volatile__("csrrw %0, seed, x0" : "=r" (ret) : : "memory");
return ret;
}
static int riscv_zkr_read(struct udevice *dev, void *data, size_t len)
{
u8 *ptr = data;
while (len) {
u32 val;
val = read_seed();
switch (val >> 30) {
case BIST:
continue;
case WAIT:
continue;
case ES16:
*ptr++ = val & 0xff;
if (--len) {
*ptr++ = val >> 8;
--len;
}
break;
case DEAD:
return -ENOENT;
}
}
return 0;
}
/**
* riscv_zkr_bind() - check if the seed register is available
*
* If the SBI software has not set mseccfg.sseed=1 or the Zkr extension is not
* available, reading the seed register will result in an exception from which
* this function safely resumes.
*
* @dev: RNG device
* Return: 0 if successfully probed
*/
static int riscv_zkr_bind(struct udevice *dev)
{
struct resume_data resume;
int ret;
u32 val;
/* Check if reading seed leads to interrupt */
set_resume(&resume);
ret = setjmp(resume.jump);
if (ret)
log_debug("Exception %ld reading seed CSR\n", resume.code);
else
val = read_seed();
set_resume(NULL);
if (ret)
return -ENOENT;
return 0;
}
/**
* riscv_zkr_probe() - check if entropy is available
*
* The bind method already checked that the seed register can be read without
* excpetiong. Here we wait for the self test to finish and entropy becoming
* available.
*
* @dev: RNG device
* Return: 0 if successfully probed
*/
static int riscv_zkr_probe(struct udevice *dev)
{
u32 val;
do {
val = read_seed();
val >>= 30;
} while (val == BIST || val == WAIT);
if (val == DEAD)
return -ENOENT;
return 0;
}
static const struct dm_rng_ops riscv_zkr_ops = {
.read = riscv_zkr_read,
};
U_BOOT_DRIVER(riscv_zkr) = {
.name = DRIVER_NAME,
.id = UCLASS_RNG,
.ops = &riscv_zkr_ops,
.bind = riscv_zkr_bind,
.probe = riscv_zkr_probe,
};
U_BOOT_DRVINFO(cpu_riscv_zkr) = {
.name = DRIVER_NAME,
};
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