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 <asm/io.h>
  31 #include <asm/msr.h>
  32 #include <asm/cpufeature.h>
  33
  34
  35 #define PFX     KBUILD_MODNAME ": "
  36
  37
  38 enum {
  39         VIA_STRFILT_CNT_SHIFT   = 16,
  40         VIA_STRFILT_FAIL        = (1 << 15),
  41         VIA_STRFILT_ENABLE      = (1 << 14),
  42         VIA_RAWBITS_ENABLE      = (1 << 13),
  43         VIA_RNG_ENABLE          = (1 << 6),
  44         VIA_XSTORE_CNT_MASK     = 0x0F,
  45
  46         VIA_RNG_CHUNK_8         = 0x00, /* 64 rand bits, 64 stored bits */
  47         VIA_RNG_CHUNK_4         = 0x01, /* 32 rand bits, 32 stored bits */
  48         VIA_RNG_CHUNK_4_MASK    = 0xFFFFFFFF,
  49         VIA_RNG_CHUNK_2         = 0x02, /* 16 rand bits, 32 stored bits */
  50         VIA_RNG_CHUNK_2_MASK    = 0xFFFF,
  51         VIA_RNG_CHUNK_1         = 0x03, /* 8 rand bits, 32 stored bits */
  52         VIA_RNG_CHUNK_1_MASK    = 0xFF,
  53 };
  54
  55 /*
  56  * Investigate using the 'rep' prefix to obtain 32 bits of random data
  57  * in one insn.  The upside is potentially better performance.  The
  58  * downside is that the instruction becomes no longer atomic.  Due to
  59  * this, just like familiar issues with /dev/random itself, the worst
  60  * case of a 'rep xstore' could potentially pause a cpu for an
  61  * unreasonably long time.  In practice, this condition would likely
  62  * only occur when the hardware is failing.  (or so we hope :))
  63  *
  64  * Another possible performance boost may come from simply buffering
  65  * until we have 4 bytes, thus returning a u32 at a time,
  66  * instead of the current u8-at-a-time.
  67  */
  68
  69 static inline u32 xstore(u32 *addr, u32 edx_in)
  70 {
  71         u32 eax_out;
  72
  73         asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
  74                 :"=m"(*addr), "=a"(eax_out)
  75                 :"D"(addr), "d"(edx_in));
  76
  77         return eax_out;
  78 }
  79
  80 static int via_rng_data_present(struct hwrng *rng)
  81 {
  82         u32 bytes_out;
  83         u32 *via_rng_datum = (u32 *)(&rng->priv);
  84
  85         /* We choose the recommended 1-byte-per-instruction RNG rate,
  86          * for greater randomness at the expense of speed.  Larger
  87          * values 2, 4, or 8 bytes-per-instruction yield greater
  88          * speed at lesser randomness.
  89          *
  90          * If you change this to another VIA_CHUNK_n, you must also
  91          * change the ->n_bytes values in rng_vendor_ops[] tables.
  92          * VIA_CHUNK_8 requires further code changes.
  93          *
  94          * A copy of MSR_VIA_RNG is placed in eax_out when xstore
  95          * completes.
  96          */
  97
  98         *via_rng_datum = 0; /* paranoia, not really necessary */
  99         bytes_out = xstore(via_rng_datum, VIA_RNG_CHUNK_1);
 100         bytes_out &= VIA_XSTORE_CNT_MASK;
 101         if (bytes_out == 0)
 102                 return 0;
 103         return 1;
 104 }
 105
 106 static int via_rng_data_read(struct hwrng *rng, u32 *data)
 107 {
 108         u32 via_rng_datum = (u32)rng->priv;
 109
 110         *data = via_rng_datum;
 111
 112         return 1;
 113 }
 114
 115 static int via_rng_init(struct hwrng *rng)
 116 {
 117         u32 lo, hi, old_lo;
 118
 119         /* Control the RNG via MSR.  Tread lightly and pay very close
 120          * close attention to values written, as the reserved fields
 121          * are documented to be "undefined and unpredictable"; but it
 122          * does not say to write them as zero, so I make a guess that
 123          * we restore the values we find in the register.
 124          */
 125         rdmsr(MSR_VIA_RNG, lo, hi);
 126
 127         old_lo = lo;
 128         lo &= ~(0x7f << VIA_STRFILT_CNT_SHIFT);
 129         lo &= ~VIA_XSTORE_CNT_MASK;
 130         lo &= ~(VIA_STRFILT_ENABLE | VIA_STRFILT_FAIL | VIA_RAWBITS_ENABLE);
 131         lo |= VIA_RNG_ENABLE;
 132
 133         if (lo != old_lo)
 134                 wrmsr(MSR_VIA_RNG, lo, hi);
 135
 136         /* perhaps-unnecessary sanity check; remove after testing if
 137            unneeded */
 138         rdmsr(MSR_VIA_RNG, lo, hi);
 139         if ((lo & VIA_RNG_ENABLE) == 0) {
 140                 printk(KERN_ERR PFX "cannot enable VIA C3 RNG, aborting\n");
 141                 return -ENODEV;
 142         }
 143
 144         return 0;
 145 }
 146
 147
 148 static struct hwrng via_rng = {
 149         .name           = "via",
 150         .init           = via_rng_init,
 151         .data_present   = via_rng_data_present,
 152         .data_read      = via_rng_data_read,
 153 };
 154
 155
 156 static int __init mod_init(void)
 157 {
 158         int err;
 159
 160         if (!cpu_has_xstore)
 161                 return -ENODEV;
 162         printk(KERN_INFO "VIA RNG detected\n");
 163         err = hwrng_register(&via_rng);
 164         if (err) {
 165                 printk(KERN_ERR PFX "RNG registering failed (%d)\n",
 166                        err);
 167                 goto out;
 168         }
 169 out:
 170         return err;
 171 }
 172
 173 static void __exit mod_exit(void)
 174 {
 175         hwrng_unregister(&via_rng);
 176 }
 177
 178 module_init(mod_init);
 179 module_exit(mod_exit);
 180
 181 MODULE_DESCRIPTION("H/W RNG driver for VIA chipsets");
 182 MODULE_LICENSE("GPL");