drivers/char/hvc_lguest.c

   1 /*D:300
   2  * The Guest console driver
   3  *
   4  * This is a trivial console driver: we use lguest's DMA mechanism to send
   5  * bytes out, and register a DMA buffer to receive bytes in.  It is assumed to
   6  * be present and available from the very beginning of boot.
   7  *
   8  * Writing console drivers is one of the few remaining Dark Arts in Linux.
   9  * Fortunately for us, the path of virtual consoles has been well-trodden by
  10  * the PowerPC folks, who wrote "hvc_console.c" to generically support any
  11  * virtual console.  We use that infrastructure which only requires us to write
  12  * the basic put_chars and get_chars functions and call the right register
  13  * functions.
  14  :*/
  15
  16 /*M:002 The console can be flooded: while the Guest is processing input the
  17  * Host can send more.  Buffering in the Host could alleviate this, but it is a
  18  * difficult problem in general. :*/
  19 /* Copyright (C) 2006 Rusty Russell, IBM Corporation
  20  *
  21  * This program is free software; you can redistribute it and/or modify
  22  * it under the terms of the GNU General Public License as published by
  23  * the Free Software Foundation; either version 2 of the License, or
  24  * (at your option) any later version.
  25  *
  26  * This program is distributed in the hope that it will be useful,
  27  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  28  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  29  * GNU General Public License for more details.
  30  *
  31  * You should have received a copy of the GNU General Public License
  32  * along with this program; if not, write to the Free Software
  33  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  34  */
  35 #include <linux/err.h>
  36 #include <linux/init.h>
  37 #include <linux/lguest_bus.h>
  38 #include <asm/paravirt.h>
  39 #include "hvc_console.h"
  40
  41 /*D:340 This is our single console input buffer, with associated "struct
  42  * lguest_dma" referring to it.  Note the 0-terminated length array, and the
  43  * use of physical address for the buffer itself. */
  44 static char inbuf[256];
  45 static struct lguest_dma cons_input = { .used_len = 0,
  46                                         .addr[0] = __pa(inbuf),
  47                                         .len[0] = sizeof(inbuf),
  48                                         .len[1] = 0 };
  49
  50 /*D:310 The put_chars() callback is pretty straightforward.
  51  *
  52  * First we put the pointer and length in a "struct lguest_dma": we only have
  53  * one pointer, so we set the second length to 0.  Then we use SEND_DMA to send
  54  * the data to (Host) buffers attached to the console key.  Usually a device's
  55  * key is a physical address within the device's memory, but because the
  56  * console device doesn't have any associated physical memory, we use the
  57  * LGUEST_CONSOLE_DMA_KEY constant (aka 0). */
  58 static int put_chars(u32 vtermno, const char *buf, int count)
  59 {
  60         struct lguest_dma dma;
  61
  62         /* FIXME: DMA buffers in a "struct lguest_dma" are not allowed
  63          * to go over page boundaries.  This never seems to happen,
  64          * but if it did we'd need to fix this code. */
  65         dma.len[0] = count;
  66         dma.len[1] = 0;
  67         dma.addr[0] = __pa(buf);
  68
  69         lguest_send_dma(LGUEST_CONSOLE_DMA_KEY, &dma);
  70         /* We're expected to return the amount of data we wrote: all of it. */
  71         return count;
  72 }
  73
  74 /*D:350 get_chars() is the callback from the hvc_console infrastructure when
  75  * an interrupt is received.
  76  *
  77  * Firstly we see if our buffer has been filled: if not, we return.  The rest
  78  * of the code deals with the fact that the hvc_console() infrastructure only
  79  * asks us for 16 bytes at a time.  We keep a "cons_offset" variable for
  80  * partially-read buffers. */
  81 static int get_chars(u32 vtermno, char *buf, int count)
  82 {
  83         static int cons_offset;
  84
  85         /* Nothing left to see here... */
  86         if (!cons_input.used_len)
  87                 return 0;
  88
  89         /* You want more than we have to give?  Well, try wanting less! */
  90         if (cons_input.used_len - cons_offset < count)
  91                 count = cons_input.used_len - cons_offset;
  92
  93         /* Copy across to their buffer and increment offset. */
  94         memcpy(buf, inbuf + cons_offset, count);
  95         cons_offset += count;
  96
  97         /* Finished?  Zero offset, and reset cons_input so Host will use it
  98          * again. */
  99         if (cons_offset == cons_input.used_len) {
 100                 cons_offset = 0;
 101                 cons_input.used_len = 0;
 102         }
 103         return count;
 104 }
 105 /*:*/
 106
 107 static struct hv_ops lguest_cons = {
 108         .get_chars = get_chars,
 109         .put_chars = put_chars,
 110 };
 111
 112 /*D:320 Console drivers are initialized very early so boot messages can go
 113  * out.  At this stage, the console is output-only.  Our driver checks we're a
 114  * Guest, and if so hands hvc_instantiate() the console number (0), priority
 115  * (0), and the struct hv_ops containing the put_chars() function. */
 116 static int __init cons_init(void)
 117 {
 118         if (strcmp(paravirt_ops.name, "lguest") != 0)
 119                 return 0;
 120
 121         return hvc_instantiate(0, 0, &lguest_cons);
 122 }
 123 console_initcall(cons_init);
 124
 125 /*D:370 To set up and manage our virtual console, we call hvc_alloc() and
 126  * stash the result in the private pointer of the "struct lguest_device".
 127  * Since we never remove the console device we never need this pointer again,
 128  * but using ->private is considered good form, and you never know who's going
 129  * to copy your driver.
 130  *
 131  * Once the console is set up, we bind our input buffer ready for input. */
 132 static int lguestcons_probe(struct lguest_device *lgdev)
 133 {
 134         int err;
 135
 136         /* The first argument of hvc_alloc() is the virtual console number, so
 137          * we use zero.  The second argument is the interrupt number.
 138          *
 139          * The third argument is a "struct hv_ops" containing the put_chars()
 140          * and get_chars() pointers.  The final argument is the output buffer
 141          * size: we use 256 and expect the Host to have room for us to send
 142          * that much. */
 143         lgdev->private = hvc_alloc(0, lgdev_irq(lgdev), &lguest_cons, 256);
 144         if (IS_ERR(lgdev->private))
 145                 return PTR_ERR(lgdev->private);
 146
 147         /* We bind a single DMA buffer at key LGUEST_CONSOLE_DMA_KEY.
 148          * "cons_input" is that statically-initialized global DMA buffer we saw
 149          * above, and we also give the interrupt we want. */
 150         err = lguest_bind_dma(LGUEST_CONSOLE_DMA_KEY, &cons_input, 1,
 151                               lgdev_irq(lgdev));
 152         if (err)
 153                 printk("lguest console: failed to bind buffer.\n");
 154         return err;
 155 }
 156 /* Note the use of lgdev_irq() for the interrupt number.  We tell hvc_alloc()
 157  * to expect input when this interrupt is triggered, and then tell
 158  * lguest_bind_dma() that is the interrupt to send us when input comes in. */
 159
 160 /*D:360 From now on the console driver follows standard Guest driver form:
 161  * register_lguest_driver() registers the device type and probe function, and
 162  * the probe function sets up the device.
 163  *
 164  * The standard "struct lguest_driver": */
 165 static struct lguest_driver lguestcons_drv = {
 166         .name = "lguestcons",
 167         .owner = THIS_MODULE,
 168         .device_type = LGUEST_DEVICE_T_CONSOLE,
 169         .probe = lguestcons_probe,
 170 };
 171
 172 /* The standard init function */
 173 static int __init hvc_lguest_init(void)
 174 {
 175         return register_lguest_driver(&lguestcons_drv);
 176 }
 177 module_init(hvc_lguest_init);