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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="drmDevelopersGuide">
<bookinfo>
<title>Linux DRM Developer's Guide</title>
<copyright>
<year>2008-2009</year>
<holder>
Intel Corporation (Jesse Barnes <jesse.barnes@intel.com>)
</holder>
</copyright>
<legalnotice>
<para>
The contents of this file may be used under the terms of the GNU
General Public License version 2 (the "GPL") as distributed in
the kernel source COPYING file.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<!-- Introduction -->
<chapter id="drmIntroduction">
<title>Introduction</title>
<para>
The Linux DRM layer contains code intended to support the needs
of complex graphics devices, usually containing programmable
pipelines well suited to 3D graphics acceleration. Graphics
drivers in the kernel can make use of DRM functions to make
tasks like memory management, interrupt handling and DMA easier,
and provide a uniform interface to applications.
</para>
<para>
A note on versions: this guide covers features found in the DRM
tree, including the TTM memory manager, output configuration and
mode setting, and the new vblank internals, in addition to all
the regular features found in current kernels.
</para>
<para>
[Insert diagram of typical DRM stack here]
</para>
</chapter>
<!-- Internals -->
<chapter id="drmInternals">
<title>DRM Internals</title>
<para>
This chapter documents DRM internals relevant to driver authors
and developers working to add support for the latest features to
existing drivers.
</para>
<para>
First, we'll go over some typical driver initialization
requirements, like setting up command buffers, creating an
initial output configuration, and initializing core services.
Subsequent sections will cover core internals in more detail,
providing implementation notes and examples.
</para>
<para>
The DRM layer provides several services to graphics drivers,
many of them driven by the application interfaces it provides
through libdrm, the library that wraps most of the DRM ioctls.
These include vblank event handling, memory
management, output management, framebuffer management, command
submission & fencing, suspend/resume support, and DMA
services.
</para>
<para>
The core of every DRM driver is struct drm_device. Drivers
will typically statically initialize a drm_device structure,
then pass it to drm_init() at load time.
</para>
<!-- Internals: driver init -->
<sect1>
<title>Driver initialization</title>
<para>
Before calling the DRM initialization routines, the driver must
first create and fill out a struct drm_device structure.
</para>
<programlisting>
static struct drm_driver driver = {
/* don't use mtrr's here, the Xserver or user space app should
* deal with them for intel hardware.
*/
.driver_features =
DRIVER_USE_AGP | DRIVER_REQUIRE_AGP |
DRIVER_HAVE_IRQ | DRIVER_IRQ_SHARED | DRIVER_MODESET,
.load = i915_driver_load,
.unload = i915_driver_unload,
.firstopen = i915_driver_firstopen,
.lastclose = i915_driver_lastclose,
.preclose = i915_driver_preclose,
.save = i915_save,
.restore = i915_restore,
.device_is_agp = i915_driver_device_is_agp,
.get_vblank_counter = i915_get_vblank_counter,
.enable_vblank = i915_enable_vblank,
.disable_vblank = i915_disable_vblank,
.irq_preinstall = i915_driver_irq_preinstall,
.irq_postinstall = i915_driver_irq_postinstall,
.irq_uninstall = i915_driver_irq_uninstall,
.irq_handler = i915_driver_irq_handler,
.reclaim_buffers = drm_core_reclaim_buffers,
.get_map_ofs = drm_core_get_map_ofs,
.get_reg_ofs = drm_core_get_reg_ofs,
.fb_probe = intelfb_probe,
.fb_remove = intelfb_remove,
.fb_resize = intelfb_resize,
.master_create = i915_master_create,
.master_destroy = i915_master_destroy,
#if defined(CONFIG_DEBUG_FS)
.debugfs_init = i915_debugfs_init,
.debugfs_cleanup = i915_debugfs_cleanup,
#endif
.gem_init_object = i915_gem_init_object,
.gem_free_object = i915_gem_free_object,
.gem_vm_ops = &i915_gem_vm_ops,
.ioctls = i915_ioctls,
.fops = {
.owner = THIS_MODULE,
.open = drm_open,
.release = drm_release,
.ioctl = drm_ioctl,
.mmap = drm_mmap,
.poll = drm_poll,
.fasync = drm_fasync,
#ifdef CONFIG_COMPAT
.compat_ioctl = i915_compat_ioctl,
#endif
},
.pci_driver = {
.name = DRIVER_NAME,
.id_table = pciidlist,
.probe = probe,
.remove = __devexit_p(drm_cleanup_pci),
},
.name = DRIVER_NAME,
.desc = DRIVER_DESC,
.date = DRIVER_DATE,
.major = DRIVER_MAJOR,
.minor = DRIVER_MINOR,
.patchlevel = DRIVER_PATCHLEVEL,
};
</programlisting>
<para>
In the example above, taken from the i915 DRM driver, the driver
sets several flags indicating what core features it supports.
We'll go over the individual callbacks in later sections. Since
flags indicate which features your driver supports to the DRM
core, you need to set most of them prior to calling drm_init(). Some,
like DRIVER_MODESET can be set later based on user supplied parameters,
but that's the exception rather than the rule.
</para>
<variablelist>
<title>Driver flags</title>
<varlistentry>
<term>DRIVER_USE_AGP</term>
<listitem><para>
Driver uses AGP interface
</para></listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_REQUIRE_AGP</term>
<listitem><para>
Driver needs AGP interface to function.
</para></listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_USE_MTRR</term>
<listitem>
<para>
Driver uses MTRR interface for mapping memory. Deprecated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_PCI_DMA</term>
<listitem><para>
Driver is capable of PCI DMA. Deprecated.
</para></listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_SG</term>
<listitem><para>
Driver can perform scatter/gather DMA. Deprecated.
</para></listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_HAVE_DMA</term>
<listitem><para>Driver supports DMA. Deprecated.</para></listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_HAVE_IRQ</term><term>DRIVER_IRQ_SHARED</term>
<listitem>
<para>
DRIVER_HAVE_IRQ indicates whether the driver has a IRQ
handler, DRIVER_IRQ_SHARED indicates whether the device &
handler support shared IRQs (note that this is required of
PCI drivers).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_DMA_QUEUE</term>
<listitem>
<para>
If the driver queues DMA requests and completes them
asynchronously, this flag should be set. Deprecated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_FB_DMA</term>
<listitem>
<para>
Driver supports DMA to/from the framebuffer. Deprecated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>DRIVER_MODESET</term>
<listitem>
<para>
Driver supports mode setting interfaces.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
In this specific case, the driver requires AGP and supports
IRQs. DMA, as we'll see, is handled by device specific ioctls
in this case. It also supports the kernel mode setting APIs, though
unlike in the actual i915 driver source, this example unconditionally
exports KMS capability.
</para>
</sect1>
<!-- Internals: driver load -->
<sect1>
<title>Driver load</title>
<para>
In the previous section, we saw what a typical drm_driver
structure might look like. One of the more important fields in
the structure is the hook for the load function.
</para>
<programlisting>
static struct drm_driver driver = {
...
.load = i915_driver_load,
...
};
</programlisting>
<para>
The load function has many responsibilities: allocating a driver
private structure, specifying supported performance counters,
configuring the device (e.g. mapping registers & command
buffers), initializing the memory manager, and setting up the
initial output configuration.
</para>
<para>
Note that the tasks performed at driver load time must not
conflict with DRM client requirements. For instance, if user
level mode setting drivers are in use, it would be problematic
to perform output discovery & configuration at load time.
Likewise, if pre-memory management aware user level drivers are
in use, memory management and command buffer setup may need to
be omitted. These requirements are driver specific, and care
needs to be taken to keep both old and new applications and
libraries working. The i915 driver supports the "modeset"
module parameter to control whether advanced features are
enabled at load time or in legacy fashion. If compatibility is
a concern (e.g. with drivers converted over to the new interfaces
from the old ones), care must be taken to prevent incompatible
device initialization and control with the currently active
userspace drivers.
</para>
<sect2>
<title>Driver private & performance counters</title>
<para>
The driver private hangs off the main drm_device structure and
can be used for tracking various device specific bits of
information, like register offsets, command buffer status,
register state for suspend/resume, etc. At load time, a
driver can simply allocate one and set drm_device.dev_priv
appropriately; at unload the driver can free it and set
drm_device.dev_priv to NULL.
</para>
<para>
The DRM supports several counters which can be used for rough
performance characterization. Note that the DRM stat counter
system is not often used by applications, and supporting
additional counters is completely optional.
</para>
<para>
These interfaces are deprecated and should not be used. If performance
monitoring is desired, the developer should investigate and
potentially enhance the kernel perf and tracing infrastructure to export
GPU related performance information to performance monitoring
tools and applications.
</para>
</sect2>
<sect2>
<title>Configuring the device</title>
<para>
Obviously, device configuration will be device specific.
However, there are several common operations: finding a
device's PCI resources, mapping them, and potentially setting
up an IRQ handler.
</para>
<para>
Finding & mapping resources is fairly straightforward. The
DRM wrapper functions, drm_get_resource_start() and
drm_get_resource_len() can be used to find BARs on the given
drm_device struct. Once those values have been retrieved, the
driver load function can call drm_addmap() to create a new
mapping for the BAR in question. Note you'll probably want a
drm_local_map_t in your driver private structure to track any
mappings you create.
<!-- !Fdrivers/gpu/drm/drm_bufs.c drm_get_resource_* -->
<!-- !Finclude/drm/drmP.h drm_local_map_t -->
</para>
<para>
if compatibility with other operating systems isn't a concern
(DRM drivers can run under various BSD variants and OpenSolaris),
native Linux calls can be used for the above, e.g. pci_resource_*
and iomap*/iounmap. See the Linux device driver book for more
info.
</para>
<para>
Once you have a register map, you can use the DRM_READn() and
DRM_WRITEn() macros to access the registers on your device, or
use driver specific versions to offset into your MMIO space
relative to a driver specific base pointer (see I915_READ for
example).
</para>
<para>
If your device supports interrupt generation, you may want to
setup an interrupt handler at driver load time as well. This
is done using the drm_irq_install() function. If your device
supports vertical blank interrupts, it should call
drm_vblank_init() to initialize the core vblank handling code before
enabling interrupts on your device. This ensures the vblank related
structures are allocated and allows the core to handle vblank events.
</para>
<!--!Fdrivers/char/drm/drm_irq.c drm_irq_install-->
<para>
Once your interrupt handler is registered (it'll use your
drm_driver.irq_handler as the actual interrupt handling
function), you can safely enable interrupts on your device,
assuming any other state your interrupt handler uses is also
initialized.
</para>
<para>
Another task that may be necessary during configuration is
mapping the video BIOS. On many devices, the VBIOS describes
device configuration, LCD panel timings (if any), and contains
flags indicating device state. Mapping the BIOS can be done
using the pci_map_rom() call, a convenience function that
takes care of mapping the actual ROM, whether it has been
shadowed into memory (typically at address 0xc0000) or exists
on the PCI device in the ROM BAR. Note that once you've
mapped the ROM and extracted any necessary information, be
sure to unmap it; on many devices the ROM address decoder is
shared with other BARs, so leaving it mapped can cause
undesired behavior like hangs or memory corruption.
<!--!Fdrivers/pci/rom.c pci_map_rom-->
</para>
</sect2>
<sect2>
<title>Memory manager initialization</title>
<para>
In order to allocate command buffers, cursor memory, scanout
buffers, etc., as well as support the latest features provided
by packages like Mesa and the X.Org X server, your driver
should support a memory manager.
</para>
<para>
If your driver supports memory management (it should!), you'll
need to set that up at load time as well. How you initialize
it depends on which memory manager you're using, TTM or GEM.
</para>
<sect3>
<title>TTM initialization</title>
<para>
TTM (for Translation Table Manager) manages video memory and
aperture space for graphics devices. TTM supports both UMA devices
and devices with dedicated video RAM (VRAM), i.e. most discrete
graphics devices. If your device has dedicated RAM, supporting
TTM is desirable. TTM also integrates tightly with your
driver specific buffer execution function. See the radeon
driver for examples.
</para>
<para>
The core TTM structure is the ttm_bo_driver struct. It contains
several fields with function pointers for initializing the TTM,
allocating and freeing memory, waiting for command completion
and fence synchronization, and memory migration. See the
radeon_ttm.c file for an example of usage.
</para>
<para>
The ttm_global_reference structure is made up of several fields:
</para>
<programlisting>
struct ttm_global_reference {
enum ttm_global_types global_type;
size_t size;
void *object;
int (*init) (struct ttm_global_reference *);
void (*release) (struct ttm_global_reference *);
};
</programlisting>
<para>
There should be one global reference structure for your memory
manager as a whole, and there will be others for each object
created by the memory manager at runtime. Your global TTM should
have a type of TTM_GLOBAL_TTM_MEM. The size field for the global
object should be sizeof(struct ttm_mem_global), and the init and
release hooks should point at your driver specific init and
release routines, which will probably eventually call
ttm_mem_global_init and ttm_mem_global_release respectively.
</para>
<para>
Once your global TTM accounting structure is set up and initialized
(done by calling ttm_global_item_ref on the global object you
just created), you'll need to create a buffer object TTM to
provide a pool for buffer object allocation by clients and the
kernel itself. The type of this object should be TTM_GLOBAL_TTM_BO,
and its size should be sizeof(struct ttm_bo_global). Again,
driver specific init and release functions can be provided,
likely eventually calling ttm_bo_global_init and
ttm_bo_global_release, respectively. Also like the previous
object, ttm_global_item_ref is used to create an initial reference
count for the TTM, which will call your initialization function.
</para>
</sect3>
<sect3>
<title>GEM initialization</title>
<para>
GEM is an alternative to TTM, designed specifically for UMA
devices. It ha
|