drivers/char/hw_random/via-rng.c

   1 /*
   2  * RNG driver for VIA RNGs
   3  *
   4  * Copyright 2005 (c) MontaVista Software, Inc.
   5  *
   6  * with the majority of the code coming from:
   7  *
   8  * Hardware driver for the Intel/AMD/VIA Random Number Generators (RNG)
   9  * (c) Copyright 2003 Red Hat Inc <jgarzik@redhat.com>
  10  *
  11  * derived from
  12  *
  13  * Hardware driver for the AMD 768 Random Number Generator (RNG)
  14  * (c) Copyright 2001 Red Hat Inc <alan@redhat.com>
  15  *
  16  * derived from
  17  *
  18  * Hardware driver for Intel i810 Random Number Generator (RNG)
  19  * Copyright 2000,2001 Jeff Garzik <jgarzik@pobox.com>
  20  * Copyright 2000,2001 Philipp Rumpf <prumpf@mandrakesoft.com>
  21  *
  22  * This file is licensed under  the terms of the GNU General Public
  23  * License version 2. This program is licensed "as is" without any
  24  * warranty of any kind, whether express or implied.
  25  */
  26
  27 #include <linux/module.h>
  28 #include <linux/kernel.h>
  29 #include <linux/hw_random.h>
  30 #include <linux/delay.h>
  31 #include <asm/io.h>
  32 #include <asm/msr.h>
  33 #include <asm/cpufeature.h>
  34
  35
  36 #define PFX     KBUILD_MODNAME ": "
  37
  38
  39 enum {
  40         VIA_STRFILT_CNT_SHIFT   = 16,
  41         VIA_STRFILT_FAIL        = (1 << 15),
  42         VIA_STRFILT_ENABLE      = (1 << 14),
  43         VIA_RAWBITS_ENABLE      = (1 << 13),
  44         VIA_RNG_ENABLE          = (1 << 6),
  45         VIA_NOISESRC1           = (1 << 8),
  46         VIA_NOISESRC2           = (1 << 9),
  47         VIA_XSTORE_CNT_MASK     = 0x0F,
  48
  49         VIA_RNG_CHUNK_8         = 0x00, /* 64 rand bits, 64 stored bits */
  50         VIA_RNG_CHUNK_4         = 0x01, /* 32 rand bits, 32 stored bits */
  51         VIA_RNG_CHUNK_4_MASK    = 0xFFFFFFFF,
  52         VIA_RNG_CHUNK_2         = 0x02, /* 16 rand bits, 32 stored bits */
  53         VIA_RNG_CHUNK_2_MASK    = 0xFFFF,
  54         VIA_RNG_CHUNK_1         = 0x03, /* 8 rand bits, 32 stored bits */
  55         VIA_RNG_CHUNK_1_MASK    = 0xFF,
  56 };
  57
  58 /*
  59  * Investigate using the 'rep' prefix to obtain 32 bits of random data
  60  * in one insn.  The upside is potentially better performance.  The
  61  * downside is that the instruction becomes no longer atomic.  Due to
  62  * this, just like familiar issues with /dev/random itself, the worst
  63  * case of a 'rep xstore' could potentially pause a cpu for an
  64  * unreasonably long time.  In practice, this condition would likely
  65  * only occur when the hardware is failing.  (or so we hope :))
  66  *
  67  * Another possible performance boost may come from simply buffering
  68  * until we have 4 bytes, thus returning a u32 at a time,
  69  * instead of the current u8-at-a-time.
  70  */
  71
  72 static inline u32 xstore(u32 *addr, u32 edx_in)
  73 {
  74         u32 eax_out;
  75
  76         asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
  77                 :"=m"(*addr), "=a"(eax_out)
  78                 :"D"(addr), "d"(edx_in));
  79
  80         return eax_out;
  81 }
  82
  83 static int via_rng_data_present(struct hwrng *rng, int wait)
  84 {
  85         u32 bytes_out;
  86         u32 *via_rng_datum = (u32 *)(&rng->priv);
  87         int i;
  88
  89         /* We choose the recommended 1-byte-per-instruction RNG rate,
  90          * for greater randomness at the expense of speed.  Larger
  91          * values 2, 4, or 8 bytes-per-instruction yield greater
  92          * speed at lesser randomness.
  93          *
  94          * If you change this to another VIA_CHUNK_n, you must also
  95          * change the ->n_bytes values in rng_vendor_ops[] tables.
  96          * VIA_CHUNK_8 requires further code changes.
  97          *
  98          * A copy of MSR_VIA_RNG is placed in eax_out when xstore
  99          * completes.
 100          */
 101
 102         for (i = 0; i < 20; i++) {
 103                 *via_rng_datum = 0; /* paranoia, not really necessary */
 104                 bytes_out = xstore(via_rng_datum, VIA_RNG_CHUNK_1);
 105                 bytes_out &= VIA_XSTORE_CNT_MASK;
 106                 if (bytes_out || !wait)
 107                         break;
 108                 udelay(10);
 109         }
 110         return bytes_out ? 1 : 0;
 111 }
 112
 113 static int via_rng_data_read(struct hwrng *rng, u32 *data)
 114 {
 115         u32 via_rng_datum = (u32)rng->priv;
 116
 117         *data = via_rng_datum;
 118
 119         return 1;
 120 }
 121
 122 static int via_rng_init(struct hwrng *rng)
 123 {
 124         struct cpuinfo_x86 *c = &cpu_data(0);
 125         u32 lo, hi, old_lo;
 126
 127         /* Control the RNG via MSR.  Tread lightly and pay very close
 128          * close attention to values written, as the reserved fields
 129          * are documented to be "undefined and unpredictable"; but it
 130          * does not say to write them as zero, so I make a guess that
 131          * we restore the values we find in the register.
 132          */
 133         rdmsr(MSR_VIA_RNG, lo, hi);
 134
 135         old_lo = lo;
 136         lo &= ~(0x7f << VIA_STRFILT_CNT_SHIFT);
 137         lo &= ~VIA_XSTORE_CNT_MASK;
 138         lo &= ~(VIA_STRFILT_ENABLE | VIA_STRFILT_FAIL | VIA_RAWBITS_ENABLE);
 139         lo |= VIA_RNG_ENABLE;
 140         lo |= VIA_NOISESRC1;
 141
 142         /* Enable secondary noise source on CPUs where it is present. */
 143
 144         /* Nehemiah stepping 8 and higher */
 145         if ((c->x86_model == 9) && (c->x86_mask > 7))
 146                 lo |= VIA_NOISESRC2;
 147
 148         /* Esther */
 149         if (c->x86_model >= 10)
 150                 lo |= VIA_NOISESRC2;
 151
 152         if (lo != old_lo)
 153                 wrmsr(MSR_VIA_RNG, lo, hi);
 154
 155         /* perhaps-unnecessary sanity check; remove after testing if
 156            unneeded */
 157         rdmsr(MSR_VIA_RNG, lo, hi);
 158         if ((lo & VIA_RNG_ENABLE) == 0) {
 159                 printk(KERN_ERR PFX "cannot enable VIA C3 RNG, aborting\n");
 160                 return -ENODEV;
 161         }
 162
 163         return 0;
 164 }
 165
 166
 167 static struct hwrng via_rng = {
 168         .name           = "via",
 169         .init           = via_rng_init,
 170         .data_present   = via_rng_data_present,
 171         .data_read      = via_rng_data_read,
 172 };
 173
 174
 175 static int __init mod_init(void)
 176 {
 177         int err;
 178
 179         if (!cpu_has_xstore)
 180                 return -ENODEV;
 181         printk(KERN_INFO "VIA RNG detected\n");
 182         err = hwrng_register(&via_rng);
 183         if (err) {
 184                 printk(KERN_ERR PFX "RNG registering failed (%d)\n",
 185                        err);
 186                 goto out;
 187         }
 188 out:
 189         return err;
 190 }
 191
 192 static void __exit mod_exit(void)
 193 {
 194         hwrng_unregister(&via_rng);
 195 }
 196
 197 module_init(mod_init);
 198 module_exit(mod_exit);
 199
 200 MODULE_DESCRIPTION("H/W RNG driver for VIA chipsets");
 201 MODULE_LICENSE("GPL");