1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * scsicam.c - SCSI CAM support functions, use for HDIO_GETGEO, etc.
4  *
5  * Copyright 1993, 1994 Drew Eckhardt
6  *      Visionary Computing
7  *      (Unix and Linux consulting and custom programming)
8  *      drew@Colorado.EDU
9  *      +1 (303) 786-7975
10  *
11  * For more information, please consult the SCSI-CAM draft.
12  */
13 
14 #include <linux/module.h>
15 #include <linux/slab.h>
16 #include <linux/fs.h>
17 #include <linux/kernel.h>
18 #include <linux/blkdev.h>
19 #include <linux/pagemap.h>
20 #include <linux/msdos_partition.h>
21 #include <asm/unaligned.h>
22 
23 #include <scsi/scsicam.h>
24 
25 /**
26  * scsi_bios_ptable - Read PC partition table out of first sector of device.
27  * @dev: from this device
28  *
29  * Description: Reads the first sector from the device and returns %0x42 bytes
30  *              starting at offset %0x1be.
31  * Returns: partition table in kmalloc(GFP_KERNEL) memory, or NULL on error.
32  */
scsi_bios_ptable(struct block_device * dev)33 unsigned char *scsi_bios_ptable(struct block_device *dev)
34 {
35 	struct address_space *mapping = bdev_whole(dev)->bd_inode->i_mapping;
36 	unsigned char *res = NULL;
37 	struct folio *folio;
38 
39 	folio = read_mapping_folio(mapping, 0, NULL);
40 	if (IS_ERR(folio))
41 		return NULL;
42 
43 	res = kmemdup(folio_address(folio) + 0x1be, 66, GFP_KERNEL);
44 	folio_put(folio);
45 	return res;
46 }
47 EXPORT_SYMBOL(scsi_bios_ptable);
48 
49 /**
50  * scsi_partsize - Parse cylinders/heads/sectors from PC partition table
51  * @bdev: block device to parse
52  * @capacity: size of the disk in sectors
53  * @geom: output in form of [hds, cylinders, sectors]
54  *
55  * Determine the BIOS mapping/geometry used to create the partition
56  * table, storing the results in @geom.
57  *
58  * Returns: %false on failure, %true on success.
59  */
scsi_partsize(struct block_device * bdev,sector_t capacity,int geom[3])60 bool scsi_partsize(struct block_device *bdev, sector_t capacity, int geom[3])
61 {
62 	int cyl, ext_cyl, end_head, end_cyl, end_sector;
63 	unsigned int logical_end, physical_end, ext_physical_end;
64 	struct msdos_partition *p, *largest = NULL;
65 	void *buf;
66 	int ret = false;
67 
68 	buf = scsi_bios_ptable(bdev);
69 	if (!buf)
70 		return false;
71 
72 	if (*(unsigned short *) (buf + 64) == 0xAA55) {
73 		int largest_cyl = -1, i;
74 
75 		for (i = 0, p = buf; i < 4; i++, p++) {
76 			if (!p->sys_ind)
77 				continue;
78 #ifdef DEBUG
79 			printk("scsicam_bios_param : partition %d has system \n",
80 			       i);
81 #endif
82 			cyl = p->cyl + ((p->sector & 0xc0) << 2);
83 			if (cyl > largest_cyl) {
84 				largest_cyl = cyl;
85 				largest = p;
86 			}
87 		}
88 	}
89 	if (largest) {
90 		end_cyl = largest->end_cyl + ((largest->end_sector & 0xc0) << 2);
91 		end_head = largest->end_head;
92 		end_sector = largest->end_sector & 0x3f;
93 
94 		if (end_head + 1 == 0 || end_sector == 0)
95 			goto out_free_buf;
96 
97 #ifdef DEBUG
98 		printk("scsicam_bios_param : end at h = %d, c = %d, s = %d\n",
99 		       end_head, end_cyl, end_sector);
100 #endif
101 
102 		physical_end = end_cyl * (end_head + 1) * end_sector +
103 		    end_head * end_sector + end_sector;
104 
105 		/* This is the actual _sector_ number at the end */
106 		logical_end = get_unaligned_le32(&largest->start_sect)
107 		    + get_unaligned_le32(&largest->nr_sects);
108 
109 		/* This is for >1023 cylinders */
110 		ext_cyl = (logical_end - (end_head * end_sector + end_sector))
111 		    / (end_head + 1) / end_sector;
112 		ext_physical_end = ext_cyl * (end_head + 1) * end_sector +
113 		    end_head * end_sector + end_sector;
114 
115 #ifdef DEBUG
116 		printk("scsicam_bios_param : logical_end=%d physical_end=%d ext_physical_end=%d ext_cyl=%d\n"
117 		  ,logical_end, physical_end, ext_physical_end, ext_cyl);
118 #endif
119 
120 		if (logical_end == physical_end ||
121 		    (end_cyl == 1023 && ext_physical_end == logical_end)) {
122 			geom[0] = end_head + 1;
123 			geom[1] = end_sector;
124 			geom[2] = (unsigned long)capacity /
125 				((end_head + 1) * end_sector);
126 			ret = true;
127 			goto out_free_buf;
128 		}
129 #ifdef DEBUG
130 		printk("scsicam_bios_param : logical (%u) != physical (%u)\n",
131 		       logical_end, physical_end);
132 #endif
133 	}
134 
135 out_free_buf:
136 	kfree(buf);
137 	return ret;
138 }
139 EXPORT_SYMBOL(scsi_partsize);
140 
141 /*
142  * Function : static int setsize(unsigned long capacity,unsigned int *cyls,
143  *      unsigned int *hds, unsigned int *secs);
144  *
145  * Purpose : to determine a near-optimal int 0x13 mapping for a
146  *      SCSI disk in terms of lost space of size capacity, storing
147  *      the results in *cyls, *hds, and *secs.
148  *
149  * Returns : -1 on failure, 0 on success.
150  *
151  * Extracted from
152  *
153  * WORKING                                                    X3T9.2
154  * DRAFT                                                        792D
155  * see http://www.t10.org/ftp/t10/drafts/cam/cam-r12b.pdf
156  *
157  *                                                        Revision 6
158  *                                                         10-MAR-94
159  * Information technology -
160  * SCSI-2 Common access method
161  * transport and SCSI interface module
162  *
163  * ANNEX A :
164  *
165  * setsize() converts a read capacity value to int 13h
166  * head-cylinder-sector requirements. It minimizes the value for
167  * number of heads and maximizes the number of cylinders. This
168  * will support rather large disks before the number of heads
169  * will not fit in 4 bits (or 6 bits). This algorithm also
170  * minimizes the number of sectors that will be unused at the end
171  * of the disk while allowing for very large disks to be
172  * accommodated. This algorithm does not use physical geometry.
173  */
174 
setsize(unsigned long capacity,unsigned int * cyls,unsigned int * hds,unsigned int * secs)175 static int setsize(unsigned long capacity, unsigned int *cyls, unsigned int *hds,
176 		   unsigned int *secs)
177 {
178 	unsigned int rv = 0;
179 	unsigned long heads, sectors, cylinders, temp;
180 
181 	cylinders = 1024L;	/* Set number of cylinders to max */
182 	sectors = 62L;		/* Maximize sectors per track */
183 
184 	temp = cylinders * sectors;	/* Compute divisor for heads */
185 	heads = capacity / temp;	/* Compute value for number of heads */
186 	if (capacity % temp) {	/* If no remainder, done! */
187 		heads++;	/* Else, increment number of heads */
188 		temp = cylinders * heads;	/* Compute divisor for sectors */
189 		sectors = capacity / temp;	/* Compute value for sectors per
190 						   track */
191 		if (capacity % temp) {	/* If no remainder, done! */
192 			sectors++;	/* Else, increment number of sectors */
193 			temp = heads * sectors;		/* Compute divisor for cylinders */
194 			cylinders = capacity / temp;	/* Compute number of cylinders */
195 		}
196 	}
197 	if (cylinders == 0)
198 		rv = (unsigned) -1;	/* Give error if 0 cylinders */
199 
200 	*cyls = (unsigned int) cylinders;	/* Stuff return values */
201 	*secs = (unsigned int) sectors;
202 	*hds = (unsigned int) heads;
203 	return (rv);
204 }
205 
206 /**
207  * scsicam_bios_param - Determine geometry of a disk in cylinders/heads/sectors.
208  * @bdev: which device
209  * @capacity: size of the disk in sectors
210  * @ip: return value: ip[0]=heads, ip[1]=sectors, ip[2]=cylinders
211  *
212  * Description : determine the BIOS mapping/geometry used for a drive in a
213  *      SCSI-CAM system, storing the results in ip as required
214  *      by the HDIO_GETGEO ioctl().
215  *
216  * Returns : -1 on failure, 0 on success.
217  */
scsicam_bios_param(struct block_device * bdev,sector_t capacity,int * ip)218 int scsicam_bios_param(struct block_device *bdev, sector_t capacity, int *ip)
219 {
220 	u64 capacity64 = capacity;	/* Suppress gcc warning */
221 	int ret = 0;
222 
223 	/* try to infer mapping from partition table */
224 	if (scsi_partsize(bdev, capacity, ip))
225 		return 0;
226 
227 	if (capacity64 < (1ULL << 32)) {
228 		/*
229 		 * Pick some standard mapping with at most 1024 cylinders, and
230 		 * at most 62 sectors per track - this works up to 7905 MB.
231 		 */
232 		ret = setsize((unsigned long)capacity, (unsigned int *)ip + 2,
233 		       (unsigned int *)ip + 0, (unsigned int *)ip + 1);
234 	}
235 
236 	/*
237 	 * If something went wrong, then apparently we have to return a geometry
238 	 * with more than 1024 cylinders.
239 	 */
240 	if (ret || ip[0] > 255 || ip[1] > 63) {
241 		if ((capacity >> 11) > 65534) {
242 			ip[0] = 255;
243 			ip[1] = 63;
244 		} else {
245 			ip[0] = 64;
246 			ip[1] = 32;
247 		}
248 
249 		if (capacity > 65535*63*255)
250 			ip[2] = 65535;
251 		else
252 			ip[2] = (unsigned long)capacity / (ip[0] * ip[1]);
253 	}
254 
255 	return 0;
256 }
257 EXPORT_SYMBOL(scsicam_bios_param);
258