Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

This chapter is from the book

This chapter is from the book

Map Drivers

To MTD-enable your device, your first task is to tell MTD how to access the flash device. For this, you have to map your flash memory range for CPU access and provide methods to operate on the flash. The next task is to inform MTD about the different storage partitions residing on your flash. Unlike hard disks on PC-compatible systems, flash-based storage does not contain a standard partition table on the media. Because of this, disk-partitioning tools such as fdisk and cfdisk2 cannot be used to partition flash devices. Instead, partitioning information has to be implemented as part of kernel code.3 These tasks are accomplished with the help of an MTD map driver.

To better understand the function of map drivers, let's look at an example.

Device Example: Handheld

Consider the Linux handheld shown in Figure 17.2. The flash has a size of 32MB and is mapped to 0xC0000000 in the processor's address space. It contains three partitions, one each for the bootloader, the kernel, and the root filesystem. The bootloader partition starts from the top of the flash, the kernel partition begins at offset MY_KERNEL_START, and the root filesystem starts at offset MY_FS_START.4 The bootloader and the kernel reside on read-only partitions to avoid unexpected damage, while the filesystem partition is flagged read-write.

Figure 17.2

Figure 17.2 Flash Memory on a sample Linux handheld.

Let's first create the flash map and then proceed with the driver initialization. The map driver has to translate the flash layout shown in the figure to an mtd_partition structure. Listing 17.1 contains the mtd_partition definition corresponding to Figure 17.2. Note that the mask_flags field holds the permissions to be masked, so MTD_WRITEABLE implies a read-only partition.

Listing 17.1. Creating an MTD Partition Map

#define FLASH_START          0x00000000
#define MY_KERNEL_START      0x00080000 /* 512K for bootloader */
#define MY_FS_START          0x00280000 /* 2MB for kernel */
#define FLASH_END            0x02000000 /* 32MB */
static struct mtd_partition pda_partitions[] = {
  {
    .name       = "pda_btldr",        /* This string is used by
                                         /proc/mtd to identify
                                         the bootloader partition */
    .size:      = (MY_KERNEL_START-FLASH_START),
    .offset     = FLASH_START,        /* Start from top of flash */
    .mask_flags = MTD_WRITEABLE       /* Read-only partition */
  },
  {
    .name       = "pda_krnl",         /* Kernel partition */
    .size:      = (MY_FS_START-MY_KERNEL_START),
    .offset     = MTDPART_OFS_APPEND, /* Start immediately after
                                         the bootloader partition */
    .mask_flags = MTD_WRITEABLE       /* Read-only partition */
  },
  {
    .name:      = "pda_fs",           /* Filesystem partition */
    .size:      = MTDPART_SIZ_FULL,   /* Use up the rest of the
                                         flash */
    .offset     = MTDPART_OFS_NEXTBLK,/* Align this partition with
                                         the erase size */
  }
};

Listing 17.1 uses MTDPART_OFS_APPEND to start a partition adjacent to the previous one. The start addresses of writeable partitions, however, need to be aligned with the erase/sector size of the flash chip. To achieve this, the filesystem partition uses MTD_OFS_NEXTBLK rather than MTD_OFS_APPEND.

Now that you have populated the mtd_partition structure, let's proceed and complete a basic map driver for the example handheld. Listing 17.2 registers the map driver with the MTD core. It's implemented as a platform driver, assuming that your architecture-specific code registers an associated platform device having the same name. Rewind to the section "Device Example: Cell Phone" in Chapter 6, "Serial Drivers," for a discussion on platform devices and platform drivers. The platform_device is defined by the associated architecture-specific code as follows:

struct resource pda_flash_resource = { /* Used by Listing 17.3 */
  .start = 0xC0000000,                 /* Physical start of the
                                          flash in Figure 17.2 */
  .end   = 0xC0000000+0x02000000-1,    /* Physical end of flash */
  .flags = IORESOURCE_MEM,             /* Memory resource */
};
struct platform_device pda_platform_device = {
  .name = "pda",                   /* Platform device name */
  .id   = 0,                       /* Instance number */
  /* ... */
  .resource = &pda_flash_resource, /* See above */
};
platform_device_register(&pda_platform_device);

Listing 17.2. Registering the Map Driver

static struct platform_driver pda_map_driver = {
  .driver = {
    .name     =  "pda",         /* ID */
   },
  .probe      =  pda_mtd_probe, /* Probe */
  .remove     =  NULL,          /* Release */
  .suspend    =  NULL,          /* Power management */
  .resume     =  NULL,          /* Power management */
};

/* Driver/module Initialization */
static int __init pda_mtd_init(void)
{
  return platform_driver_register(&pda_map_driver);
}

/* Module Exit */
static int __init pda_mtd_exit(void)
{
  return platform_driver_uregister(&pda_map_driver);
}

Because the kernel finds that the name of the platform driver registered in Listing 17.2 matches with that of an already-registered platform device, it invokes the probe method, pda_mtd_probe(), shown in Listing 17.3. This routine

  • Reserves the flash memory address range using request_mem_region(), and obtains CPU access to that memory using ioremap_nocache(). You learned how to do this in Chapter 10, "Peripheral Component Interconnect."
  • Populates a map_info structure (discussed next) with information such as the start address and size of flash memory. The information in this structure is used while performing the probing in the next step.
  • Probes the flash via a suitable MTD chip driver (discussed in the next section). Only the chip driver knows how to query the chip and elicit the command-set required to access it. The chip layer tries different permutations of bus widths and interleaves while querying. In Figure 17.2, two 16-bit flash banks are connected in parallel to fill the 32-bit processor bus width, so you have a two-way interleave.
  • Registers the mtd_partition structure that you populated earlier, with the MTD core.

Before looking at Listing 17.3, let's meet the map_info structure. It contains the address, size, and width of the flash memory and routines to access it:

struct map_info {
  char * name;               /* Name */
  unsigned long size;        /* Flash size */
  int bankwidth;             /* In bytes */
  /* ... */
  /* You need to implement custom routines for the following methods
     only if you have special needs. Else populate them with built-
     in methods using simple_map_init() as done in Listing 17.3 */
  map_word (*read)(struct map_info *, unsigned long);
  void     (*write)(struct map_info *, const map_word,
                    unsigned long);
  /* ... */
};

While we are in the topic of accessing flash chips, let's briefly revisit memory barriers that we discussed in Chapter 4, "Laying the Groundwork." An instruction reordering that appears semantically unchanged to the compiler (or the processor) may not be so in reality, so the ordering of data operations on flash memory is best left alone. You don't want to, for example, end up erasing a flash sector after writing to it, instead of doing the reverse. Also, the same flash chips, and hence their device drivers, are used on diverse embedded processors having different instruction reordering algorithms. For these reasons, MTD drivers are notable users of hardware memory barriers. simple_map_write(), a generic routine available to map drivers for use as the write() method in the map_info structure previously listed, inserts a call to mb() before returning. This ensures that the processor does not reorder flash reads or writes across the barrier.

Listing 17.3. Map Driver Probe Method

#include <linux/mtd/mtd.h>
#include <linux/mtd/map.h>
#include <linux/ioport.h>

static int
pda_mtd_probe(struct platform_device *pdev)
{
  struct map_info *pda_map;
  struct mtd_info *pda_mtd;
  struct resource *res = pdev->resource;

  /* Populate pda_map with information obtained
     from the associated platform device */
  pda_map->virt = ioremap_nocache(res->start,
                                  (res->end – res->start + 1));
  pda_map->name = pdev->dev.bus_id;
  pda_map->phys = res->start;
  pda_map->size = res->end – res->start + 1;
  pda_map->bankwidth = 2;     /* Two 16-bit banks sitting
                                 on a 32-bit bus */
  simple_map_init(&pda_map);  /* Fill in default access methods */

  /* Probe via the CFI chip driver */
  pda_mtd = do_map_probe("cfi_probe", &pda_map);
  /* Register the mtd_partition structure */
  add_mtd_partitions(pda_mtd, pda_partitions, 3); /* Three Partitions */

  /* ... */
}

Don't worry if the CFI probing done in Listing 17.3 seems esoteric. It's discussed in the next section when we look at NOR chip drivers.

MTD now knows how your flash device is organized and how to access it. When you boot the kernel with your map driver compiled in, user-space applications can respectively see your bootloader, kernel, and filesystem partitions as /dev/mtd/0, /dev/mtd/1, and /dev/mtd/2. So, to test drive a new kernel image on the handheld, you can do this:

bash> dd if=zImage.new of=/dev/mtd/1

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020