5 This chapter documents DRM internals relevant to driver authors and
6 developers working to add support for the latest features to existing
9 First, we go over some typical driver initialization requirements, like
10 setting up command buffers, creating an initial output configuration,
11 and initializing core services. Subsequent sections cover core internals
12 in more detail, providing implementation notes and examples.
14 The DRM layer provides several services to graphics drivers, many of
15 them driven by the application interfaces it provides through libdrm,
16 the library that wraps most of the DRM ioctls. These include vblank
17 event handling, memory management, output management, framebuffer
18 management, command submission & fencing, suspend/resume support, and
24 At the core of every DRM driver is a :c:type:`struct drm_driver
25 <drm_driver>` structure. Drivers typically statically initialize
26 a drm_driver structure, and then pass it to
27 :c:func:`drm_dev_alloc()` to allocate a device instance. After the
28 device instance is fully initialized it can be registered (which makes
29 it accessible from userspace) using :c:func:`drm_dev_register()`.
31 The :c:type:`struct drm_driver <drm_driver>` structure
32 contains static information that describes the driver and features it
33 supports, and pointers to methods that the DRM core will call to
34 implement the DRM API. We will first go through the :c:type:`struct
35 drm_driver <drm_driver>` static information fields, and will
36 then describe individual operations in details as they get used in later
45 Drivers inform the DRM core about their requirements and supported
46 features by setting appropriate flags in the driver_features field.
47 Since those flags influence the DRM core behaviour since registration
48 time, most of them must be set to registering the :c:type:`struct
49 drm_driver <drm_driver>` instance.
54 Driver uses AGP interface, the DRM core will manage AGP resources.
57 Denote a legacy driver using shadow attach. Don't use.
59 DRIVER_KMS_LEGACY_CONTEXT
60 Used only by nouveau for backwards compatibility with existing userspace.
64 Driver is capable of PCI DMA, mapping of PCI DMA buffers to
65 userspace will be enabled. Deprecated.
68 Driver can perform scatter/gather DMA, allocation and mapping of
69 scatter/gather buffers will be enabled. Deprecated.
72 Driver supports DMA, the userspace DMA API will be supported.
75 DRIVER_HAVE_IRQ; DRIVER_IRQ_SHARED
76 DRIVER_HAVE_IRQ indicates whether the driver has an IRQ handler
77 managed by the DRM Core. The core will support simple IRQ handler
78 installation when the flag is set. The installation process is
81 DRIVER_IRQ_SHARED indicates whether the device & handler support
82 shared IRQs (note that this is required of PCI drivers).
85 Driver use the GEM memory manager.
88 Driver supports mode setting interfaces (KMS).
91 Driver implements DRM PRIME buffer sharing.
94 Driver supports dedicated render nodes.
97 Driver supports atomic properties. In this case the driver must
98 implement appropriate obj->atomic_get_property() vfuncs for any
99 modeset objects with driver specific properties.
101 Major, Minor and Patchlevel
102 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
104 int major; int minor; int patchlevel;
105 The DRM core identifies driver versions by a major, minor and patch
106 level triplet. The information is printed to the kernel log at
107 initialization time and passed to userspace through the
108 DRM_IOCTL_VERSION ioctl.
110 The major and minor numbers are also used to verify the requested driver
111 API version passed to DRM_IOCTL_SET_VERSION. When the driver API
112 changes between minor versions, applications can call
113 DRM_IOCTL_SET_VERSION to select a specific version of the API. If the
114 requested major isn't equal to the driver major, or the requested minor
115 is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will
116 return an error. Otherwise the driver's set_version() method will be
117 called with the requested version.
119 Name, Description and Date
120 ~~~~~~~~~~~~~~~~~~~~~~~~~~
122 char \*name; char \*desc; char \*date;
123 The driver name is printed to the kernel log at initialization time,
124 used for IRQ registration and passed to userspace through
127 The driver description is a purely informative string passed to
128 userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by
131 The driver date, formatted as YYYYMMDD, is meant to identify the date of
132 the latest modification to the driver. However, as most drivers fail to
133 update it, its value is mostly useless. The DRM core prints it to the
134 kernel log at initialization time and passes it to userspace through the
135 DRM_IOCTL_VERSION ioctl.
137 Device Instance and Driver Handling
138 -----------------------------------
140 .. kernel-doc:: drivers/gpu/drm/drm_drv.c
141 :doc: driver instance overview
143 .. kernel-doc:: drivers/gpu/drm/drm_drv.c
146 .. kernel-doc:: include/drm/drm_drv.h
155 The DRM core tries to facilitate IRQ handler registration and
156 unregistration by providing :c:func:`drm_irq_install()` and
157 :c:func:`drm_irq_uninstall()` functions. Those functions only
158 support a single interrupt per device, devices that use more than one
159 IRQs need to be handled manually.
161 Managed IRQ Registration
162 ''''''''''''''''''''''''
164 :c:func:`drm_irq_install()` starts by calling the irq_preinstall
165 driver operation. The operation is optional and must make sure that the
166 interrupt will not get fired by clearing all pending interrupt flags or
167 disabling the interrupt.
169 The passed-in IRQ will then be requested by a call to
170 :c:func:`request_irq()`. If the DRIVER_IRQ_SHARED driver feature
171 flag is set, a shared (IRQF_SHARED) IRQ handler will be requested.
173 The IRQ handler function must be provided as the mandatory irq_handler
174 driver operation. It will get passed directly to
175 :c:func:`request_irq()` and thus has the same prototype as all IRQ
176 handlers. It will get called with a pointer to the DRM device as the
179 Finally the function calls the optional irq_postinstall driver
180 operation. The operation usually enables interrupts (excluding the
181 vblank interrupt, which is enabled separately), but drivers may choose
182 to enable/disable interrupts at a different time.
184 :c:func:`drm_irq_uninstall()` is similarly used to uninstall an
185 IRQ handler. It starts by waking up all processes waiting on a vblank
186 interrupt to make sure they don't hang, and then calls the optional
187 irq_uninstall driver operation. The operation must disable all hardware
188 interrupts. Finally the function frees the IRQ by calling
189 :c:func:`free_irq()`.
191 Manual IRQ Registration
192 '''''''''''''''''''''''
194 Drivers that require multiple interrupt handlers can't use the managed
195 IRQ registration functions. In that case IRQs must be registered and
196 unregistered manually (usually with the :c:func:`request_irq()` and
197 :c:func:`free_irq()` functions, or their :c:func:`devm_request_irq()` and
198 :c:func:`devm_free_irq()` equivalents).
200 When manually registering IRQs, drivers must not set the
201 DRIVER_HAVE_IRQ driver feature flag, and must not provide the
202 irq_handler driver operation. They must set the :c:type:`struct
203 drm_device <drm_device>` irq_enabled field to 1 upon
204 registration of the IRQs, and clear it to 0 after unregistering the
207 Memory Manager Initialization
208 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
210 Every DRM driver requires a memory manager which must be initialized at
211 load time. DRM currently contains two memory managers, the Translation
212 Table Manager (TTM) and the Graphics Execution Manager (GEM). This
213 document describes the use of the GEM memory manager only. See ? for
216 Miscellaneous Device Configuration
217 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
219 Another task that may be necessary for PCI devices during configuration
220 is mapping the video BIOS. On many devices, the VBIOS describes device
221 configuration, LCD panel timings (if any), and contains flags indicating
222 device state. Mapping the BIOS can be done using the pci_map_rom()
223 call, a convenience function that takes care of mapping the actual ROM,
224 whether it has been shadowed into memory (typically at address 0xc0000)
225 or exists on the PCI device in the ROM BAR. Note that after the ROM has
226 been mapped and any necessary information has been extracted, it should
227 be unmapped; on many devices, the ROM address decoder is shared with
228 other BARs, so leaving it mapped could cause undesired behaviour like
229 hangs or memory corruption.
231 Bus-specific Device Registration and PCI Support
232 ------------------------------------------------
234 A number of functions are provided to help with device registration. The
235 functions deal with PCI and platform devices respectively and are only
236 provided for historical reasons. These are all deprecated and shouldn't
237 be used in new drivers. Besides that there's a few helpers for pci
240 .. kernel-doc:: drivers/gpu/drm/drm_pci.c
243 .. kernel-doc:: drivers/gpu/drm/drm_platform.c
246 Open/Close, File Operations and IOCTLs
247 ======================================
252 Open and close handlers. None of those methods are mandatory::
254 int (*firstopen) (struct drm_device *);
255 void (*lastclose) (struct drm_device *);
256 int (*open) (struct drm_device *, struct drm_file *);
257 void (*preclose) (struct drm_device *, struct drm_file *);
258 void (*postclose) (struct drm_device *, struct drm_file *);
260 The firstopen method is called by the DRM core for legacy UMS (User Mode
261 Setting) drivers only when an application opens a device that has no
262 other opened file handle. UMS drivers can implement it to acquire device
263 resources. KMS drivers can't use the method and must acquire resources
264 in the load method instead.
266 Similarly the lastclose method is called when the last application
267 holding a file handle opened on the device closes it, for both UMS and
268 KMS drivers. Additionally, the method is also called at module unload
269 time or, for hot-pluggable devices, when the device is unplugged. The
270 firstopen and lastclose calls can thus be unbalanced.
272 The open method is called every time the device is opened by an
273 application. Drivers can allocate per-file private data in this method
274 and store them in the struct :c:type:`struct drm_file
275 <drm_file>` driver_priv field. Note that the open method is
276 called before firstopen.
278 The close operation is split into preclose and postclose methods.
279 Drivers must stop and cleanup all per-file operations in the preclose
280 method. For instance pending vertical blanking and page flip events must
281 be cancelled. No per-file operation is allowed on the file handle after
282 returning from the preclose method.
284 Finally the postclose method is called as the last step of the close
285 operation, right before calling the lastclose method if no other open
286 file handle exists for the device. Drivers that have allocated per-file
287 private data in the open method should free it here.
289 The lastclose method should restore CRTC and plane properties to default
290 value, so that a subsequent open of the device will not inherit state
291 from the previous user. It can also be used to execute delayed power
292 switching state changes, e.g. in conjunction with the :ref:`vga_switcheroo`
293 infrastructure. Beyond that KMS drivers should not do any
294 further cleanup. Only legacy UMS drivers might need to clean up device
295 state so that the vga console or an independent fbdev driver could take
301 .. kernel-doc:: drivers/gpu/drm/drm_fops.c
302 :doc: file operations
304 .. kernel-doc:: drivers/gpu/drm/drm_fops.c
310 struct drm_ioctl_desc \*ioctls; int num_ioctls;
311 Driver-specific ioctls descriptors table.
313 Driver-specific ioctls numbers start at DRM_COMMAND_BASE. The ioctls
314 descriptors table is indexed by the ioctl number offset from the base
315 value. Drivers can use the DRM_IOCTL_DEF_DRV() macro to initialize
320 DRM_IOCTL_DEF_DRV(ioctl, func, flags)
322 ``ioctl`` is the ioctl name. Drivers must define the DRM_##ioctl and
323 DRM_IOCTL_##ioctl macros to the ioctl number offset from
324 DRM_COMMAND_BASE and the ioctl number respectively. The first macro is
325 private to the device while the second must be exposed to userspace in a
328 ``func`` is a pointer to the ioctl handler function compatible with the
329 ``drm_ioctl_t`` type.
333 typedef int drm_ioctl_t(struct drm_device *dev, void *data,
334 struct drm_file *file_priv);
336 ``flags`` is a bitmask combination of the following values. It restricts
337 how the ioctl is allowed to be called.
339 - DRM_AUTH - Only authenticated callers allowed
341 - DRM_MASTER - The ioctl can only be called on the master file handle
343 - DRM_ROOT_ONLY - Only callers with the SYSADMIN capability allowed
345 - DRM_CONTROL_ALLOW - The ioctl can only be called on a control
348 - DRM_UNLOCKED - The ioctl handler will be called without locking the
349 DRM global mutex. This is the enforced default for kms drivers (i.e.
350 using the DRIVER_MODESET flag) and hence shouldn't be used any more
353 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
363 .. kernel-doc:: include/drm/drm_print.h
366 .. kernel-doc:: include/drm/drm_print.h
369 .. kernel-doc:: drivers/gpu/drm/drm_print.c
376 The section very briefly covers some of the old legacy support code
377 which is only used by old DRM drivers which have done a so-called
378 shadow-attach to the underlying device instead of registering as a real
379 driver. This also includes some of the old generic buffer management and
380 command submission code. Do not use any of this in new and modern
383 Legacy Suspend/Resume
384 ---------------------
386 The DRM core provides some suspend/resume code, but drivers wanting full
387 suspend/resume support should provide save() and restore() functions.
388 These are called at suspend, hibernate, or resume time, and should
389 perform any state save or restore required by your device across suspend
392 int (\*suspend) (struct drm_device \*, pm_message_t state); int
393 (\*resume) (struct drm_device \*);
394 Those are legacy suspend and resume methods which *only* work with the
395 legacy shadow-attach driver registration functions. New driver should
396 use the power management interface provided by their bus type (usually
397 through the :c:type:`struct device_driver <device_driver>`
398 dev_pm_ops) and set these methods to NULL.
403 This should cover how DMA mapping etc. is supported by the core. These
404 functions are deprecated and should not be used.