Chapter 11 Mapping Device or Kernel Memory

Some device drivers allow applications to access device or kernel memory using mmap(2). Examples are a frame buffer driver that allows the frame buffer to be mapped into a user thread or a pseudo driver that communicates with an application using a shared kernel memory pool. This chapter describes how to associate device or kernel memory with user mappings.

Memory Mapping Operations

In general, the steps for exporting device or kernel memory are:

1. Set the D_DEVMAP flag in the cb_flag flag of the cb_ops(9S) structure.

2. Define a devmap(9E) driver entry point to export the mapping.

3. To set up user mappings to the device, use devmap_devmem_setup(9F). To set up user mappings to kernel memory, use devmap_umem_setup(9F).

Exporting the Mapping

The devmap(9E) entry point is called as a result of an mmap(2) system call. devmap(9E) is used to:

• Validate the user mapping to the device or kernel memory.

• Translate the logical offset within the application mapping to the corresponding offset within the device or kernel memory.

• Pass the mapping information to the system for setting up the mapping.

devmap()()

int xxdevmap(dev_t dev, devmap_cookie_t handle, offset_t off,
 	size_t len, size_t *maplen, u_int model);

dev is the device whose memory is to be mapped. handle is a device-mapping handle that the system creates and uses to describe a mapping to contiguous device or kernel memory. The system may create multiple mapping handles in one mmap(2) system call (for example, if the mapping contains multiple physically discontiguous memory regions). off is the logical offset within the application mapping which has to be translated by the driver to the corresponding offset within the device or kernel memory. len is the length (in bytes) of the memory being mapped.

Initially devmap(9E) is called with parameters off and len, which were passed by the application to mmap(2). devmap(9E) sets *maplen to the length from off to the end of a contiguous memory region. *maplen must be rounded up to a multiple of a page size. If *maplen is set to less than the original mapping length len, the system will repeatedly call devmap(9E) with a new mapping handle and adjusted off and len parameters until the initial mapping length is satisfied. Setting *maplen to less then len allows the driver to associate different kernel memory regions or multiple physically discontiguous memory regions with one contiguous user application mapping.

model is the data model type of the current thread. If a driver supports multiple application data models, model has to be passed to ddi_model_convert_from(9F) to determine whether there is a data model mismatch between the current thread and the device driver. The device driver might have to adjust the shape of data structures before exporting them to a user thread which supports a different data model. See Appendix F, Making a Device Driver 64-Bit Ready for more details.

devmap(9E) must return ENXIO if the logical offset, off, is out of the range of memory exported by the driver.

Associating Device Memory With User Mappings

devmap_devmem_setup(9F) is provided to export device memory to user applications. devmap_devmem_setup(9F) has to be called from the driver's devmap(9E) entry point:

int devmap_devmem_setup(devmap_cookie_t handle,
 		dev_info_t *dip,
 		struct devmap_callback_ctl *callbackops,
 		u_int rnumber, offset_t roff,
 		size_t len, u_int maxprot, u_int flags,
 		ddi_device_acc_attr_t *accattrp);

handle is an opaque device-mapping handle that the system uses to identify the mapping. It is passed in by the devmap(9E) entry point. dip is a pointer to the device's dev_info structure. dip is stored by the driver in its private data structure during attach(9E). callbackops is a pointer to a devmap_callback_ctl(9S) structure that allows the driver to be notified of user events on the mapping. See Chapter 12, Device Context Managementfor a complete description of devmap_callback_ctl(9S).

roff and len describe a range within the device memory specified by the register set rnumber. roff is the offset into the device memory, and len is the length in bytes that is exported. The register specifications referred to by rnumber are described by the reg property (see driver.conf(4), isa(4), eisa(4), mca(4), sysbus(4), vme(4), sbus(4) and pci(4)). For devices with only one register set, pass zero for rnumber. The range described by roff and len are made accessible to the user's application mapping at the offset passed in by the devmap(9E) entry point. Usually the driver will pass the devmap(9E) offset directly to devmap_devmem_setup(9F). The return address of mmap(2) will then map to the beginning of the register set.

maxprot allows the driver to specify different protections for different regions within the exported device memory. For example, one region might not allow write access by only setting PROT_READ and PROT_USER.

flags must be set to DEVMAP_DEFAULTS. accattrp is a pointer to a ddi_device_acc_attr(9S) structure.

Example 11-1 shows how to export device memory to an application. The driver first determines whether the requested mapping falls within the device memory region. The size of the device memory is determined using ddi_dev_regsize(9F). The length of the mapping is rounded up to a multiple of a page size using ptob(9F) and btopr(9F), and devmap_devmem_setup(9F) is called to export the device memory to the application.

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Example 11-1 devmap_devmem_setup(9F) Routine

static int
xxdevmap(dev_t dev, devmap_cookie_t handle, offset_t off,
	size_t len, size_t *maplen, uint_t model)
{
	struct xxstate *xsp;
	int	error, rnumber;
	off_t regsize;
	
	/* Set up data access attribute structure */
	struct ddi_device_acc_attr xx_acc_attr = {
		    DDI_DEVICE_ATTR_V0,
	    	DDI_NEVERSWAP_ACC,
	    	DDI_STRICTORDER_ACC
	};
	xsp = ddi_get_soft_state(statep, getminor(dev));
	if (xsp == NULL)
	    	return (ENXIO);
	/* use register set 0 */
	rnumber = 0;
	/* get size of register set */
	if (ddi_dev_regsize(xsp->dip, rnumber, &regsize) !=
DDI_SUCCESS)
	    	return (ENXIO);
	/* round up len to a multiple of a page size */
	len = ptob(btopr(len));
	if (off + len > regsize)
	    	return (ENXIO);
	/* Set up the device mapping */
	error = devmap_devmem_setup(handle, xsp->dip, NULL, rnumber
				off, len, PROT_ALL, DEVMAP_DEFAULTS, &xx_acc_attr);
	/* acknowledge the entire range */
	*maplen = len;
	return (error);
}

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Associating Kernel Memory With User Mappings

Some device drivers may need to allocate kernel memory that is made accessible to user programs by using mmap(2). Examples of this are setting up shared memory for communication between two applications or between driver and application.

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Caution -

MAP_FIXED must not be set in mmap(2) if kernel memory is mapped. Setting MAP_FIXED makes the application non-portable.

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In general, the steps for exporting kernel memory to user applications are:

1. Allocate kernel memory using ddi_umem_alloc(9F).

2. Export the memory using devmap_umem_setup(9F).

3. If not needed, free the memory using ddi_umem_free(9F).

Device Mapping Additions to the State Structure

This section adds the following fields to the state structure. See "Software State Structure" for more information.

	void 					*umem;			/* exported kernel memory */
 	ddi_umem_cookie_t					ucookie;			/* kernel memory cookie */

Allocating Kernel Memory for User Access

ddi_umem_alloc(9F) is provided to allocate kernel memory that is exported to applications:

	void *ddi_umem_alloc(size_t size, int flag,
 			ddi_umem_cookie_t *cookiep);			

size is the number of bytes to allocate.

flag is used to determine the sleep conditions and the memory type.

cookiep is a pointer to a kernel memory cookie.

ddi_umem_alloc(9F) allocates page-aligned kernel memory and returns a pointer to the allocated memory. The initial contents of the memory is zero-filled. The number of bytes allocated is a multiple of the system page size (roundup of size). The allocated memory can be used in the kernel and can be exported to applications. cookiep is a pointer to the kernel memory cookie that describes the kernel memory being allocated. It is used in devmap_umem_setup(9F) when the driver exports the kernel memory to a user application.

The flag argument indicates whether ddi_umem_alloc(9F) will block or return immediately, and whether the allocated kernel memory is pageable. Table 11-1 lists the values for flag.

Table 11-1 ddi_umem_alloc(9F) flag Values

Values	Indicated Action
DDI_UMEM_NOSLEEP	Driver does not need to wait for memory to become available. Return NULL if memory unavailable.
DDI_UMEM_SLEEP	Driver can wait indefinitely for memory to become available.
DDI_UMEM_PAGEABLE	Driver allows memory to be paged out. If not set, the memory will be locked down.

Example 11-2 shows how to allocate kernel memory for application access. The driver exports one page of kernel memory, which is used by multiple applications as a shared memory area. The memory is allocated in segmap(9E) when an application maps the shared page the first time. An additional page is allocated if the driver has to support multiple application data models (for example a 64-bit driver exporting memory to 64-bit and 32-bit applications). 64-bit applications share the first page, and 32-bit applications share the second page.

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Example 11-2 ddi_umem_alloc(9F) Routine

static int
xxsegmap(dev_t dev, off_t off, struct as *asp, caddr_t *addrp,
	off_t len, unsigned int prot, unsigned int maxprot,
	unsigned int flags, cred_t *credp)
{
	int error;
	minor_t instance = getminor(dev);
	struct xxstate *xsp = ddi_get_soft_state(statep, instance);

	mutex_enter(&xsp->mu);
	if (xsp->umem == NULL) {
	   	size_t mem_size;
#ifdef  _MULTI_DATAMODEL
		/* 64-bit driver supports 64-bit and 32-bit applications */
	   	mem_size = ptob(2);
#else
	   	mem_size = ptob(1);
#endif /* _MULTI_DATAMODEL */

	   	/* allocate the shared area as kernel pageable memory */
	   	xsp->umem = ddi_umem_alloc(mem_size,
						DDI_UMEM_SLEEP | DDI_UMEM_PAGEABLE,
						&xsp->ucookie);
	}
	mutex_exit(&xsp->mu);
	/* Set up the user mapping */
	error = devmap_setup(dev, (offset_t)off, asp, addrp, len,
				prot, maxprot, flags, credp);
	
	return (error);
}

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Exporting Kernel Memory to Applications

devmap_umem_setup(9F) is provided to export kernel memory to user applications. devmap_umem_setup(9F) must be called from the driver's devmap(9E) entry point:

	int devmap_umem_setup(devmap_cookie_t handle, dev_info_t *dip,
 			struct devmap_callback_ctl *callbackops,
 			ddi_umem_cookie_t cookie, offset_t koff,
 			size_t len, u_int maxprot, u_int flags,
 			ddi_device_acc_attr_t *accattrp);

handle is an opaque structure that the system uses to describe the mapping.

dip is a pointer to the device's dev_info structure.

callbackops is a pointer to a devmap_callback_ctl(9S) structure.

cookie is a kernel memory cookie returned by ddi_umem_alloc(9F).

koff is the offset into the kernel memory specified by cookie.

len is the length in bytes that is exported.

maxprot specifies the maximum protection possible for the exported mapping.

flags must be set to DEVMAP_DEFAULTS.

accattrp is a pointer to a ddi_device_acc_attr(9S) structure.

handle is a device-mapping handle that the system uses to identify the mapping. It is passed in by the devmap(9E) entry point. dip is a pointer to the device's dev_info structure. dip is stored by the driver in its private data structure during attach(9E). callbackops allows the driver to be notified of user events on the mapping. Most drivers will set callbackops to NULL when kernel memory is exported.

koff and len specify a range within the kernel memory allocated by ddi_umem_alloc(9F). This range will be made accessible to the user's application mapping at the offset passed in by the devmap(9E) entry point. Usually the driver will pass the devmap(9E) offset directly to devmap_umem_setup(9F). The return address of mmap(2) will then map to the kernel address returned by ddi_umem_alloc(9F). koff and len must be page aligned.

maxprot enables the driver to specify different protections for different regions within the exported kernel memory. For example, one region might not allow write access by only setting PROT_READ and PROT_USER.

Example 11-3 shows how to export kernel memory to an application. The driver first checks if the requested mapping falls within the allocated kernel memory region. If a 64-bit driver receives a mapping request from a 32-bit application, the request is redirected to the second page of the kernel memory area. This ensures that only applications compiled to the same data model will share the same page.

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Example 11-3 devmap_umem_setup(9F) Routine

static int
xxdevmap(dev_t dev, devmap_cookie_t handle, offset_t off,
	size_t len, size_t *maplen, uint_t model)
{
	struct xxstate *xsp;
	int	error;

	/* round up len to a multiple of a page size */
	len = ptob(btopr(len));
	/* check if the requested range is ok */
	if (off + len > ptob(1))
	   	return (ENXIO);
	xsp = ddi_get_soft_state(statep, getminor(dev));
	if (xsp == NULL)
	   	return (ENXIO);
	
#ifdef  _MULTI_DATAMODEL
	if (ddi_model_convert_from(model) == DDI_MODEL_ILP32) {
	   	/* request from 32-bit application. Skip first page */
	   	off += ptob(1);
	}
#endif  /* _MULTI_DATAMODEL */
	/* export the memory to the application */
	error = devmap_umem_setup(handle, xsp->dip, NULL, xsp-
>ucookie,
				off, len, PROT_ALL, DEVMAP_DEFAULTS, NULL);
	*maplen = len;
	return (error);
}

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Freeing Kernel Memory Exported for User Access

When the driver is unloaded, the memory must be freed. ddi_umem_free(9F) frees memory that was allocated by ddi_umem_alloc(9F).

	void ddi_umem_free(ddi_umem_cookie_t cookie);

cookie is the kernel memory cookie returned by ddi_umem_alloc(9F).