Overview of Intel hardware platforms

ACPI platforms (introduced before and up to 2015)

On Baytrail, Cherrytrail, Braswell, and Broadwell devices (also referred to as legacy devices), the DSP enumeration is handled by the ACPI subsystem.

1. Local audio accessories (mics, speakers, headset)

On Baytrail, Cherrytrail, Braswell, and Broadwell, the BIOS can either enable or disable the DSP:

  • Enable the DSP. In this case, a DSP driver is required. This mode is selected on platforms where the audio interface for 3rd-party codecs is based on the I2C/I2S/TDM interfaces.

  • Disable the DSP. In this case, an HDaudio controller is exposed and the snd-intel-hda driver will take care of all audio usages. SOF cannot be used in this case.

2. HDMI/DP interfaces

On Broadwell, HDMI/DP is handled by an HDaudio controller.

On Baytrail/Cherrytrail and Braswell, the BIOS can enable two modes:

  • HDAudio-based solution (similar to Broadwell).

  • LPE HDMI Audio. This mode is used by the majority of tablets and low-cost devices. It provides functionality similar to HDaudio, but with a different interface. This mode is enabled in Linux via the CONFIG_HDMI_LPE_AUDIO option.

The DSP cannot control any of these interfaces because SOF does not support HDMI/DP on those devices.

On all of these legacy platforms, HDMI support is exposed in Linux as a separate card.

PCI devices (introduced after 2016)

In newer devices, the same HDAudio controller can handle both local accessories and HDMI/DP interfaces. However, SOF is not always supported on those platforms.

When the Intel DSP is not enabled in the BIOS (OEM choice), audio interfaces are handled by the snd-hda-intel driver. The platform only exposes PCM devices and no audio processing capabilities.

When OEM platforms integrate digital microphones attached directly to the Intel chipset (aka DMIC), or they use I2C/I2S or SoundWire interfaces, the DSP must be enabled by the BIOS. There is, however, one more option. On Skylake and Kabylake platforms, the Intel DSP is handled by the snd-soc-skl module which relies on closed-source firmware.

SOF is available on Intel PCI devices starting with GeminiLake, and has since been the only solution provided by Intel for the following platforms: CometLake, IceLake, and TigerLake.

Since multiple drivers can register for the same PCI ID, it was (until recently) common for users and distributions to use the wrong driver, which could only be resolved by changing the Linux .config file or deselecting drivers in the /etc/modprobe.d configuration files.

The snd-intel-dspcfg module introduced in early 2020 exposes an API used by all drivers, and the user can now override default choices by setting the dsp_driver parameter. For example, setting

options snd-intel-dspcfg dsp_driver=1

will allow for the HDaudio legacy driver to be used. This will typically work for speakers and headphones/headsets, but will not allow DMIC capture.

Conversely, when a platform does not require a DSP-based platform, but the DSP is still enabled by the OEM, the user or integration can force the SOF Linux driver to be used.

options snd-intel-dspcfg dsp_driver=3

User space and filesystem requirements

Selecting the SOF driver is not enough. Audio is properly configured only if the following elements are present on the file system.

1. Firmware binary

The firmware file, /lib/firmware/intel/sof/sof-tgl.ri, contains all DSP code and tables. On PCI devices, the firmware can only be signed by an Intel production key which prevents community users from installing their own firmware. Notable exceptions include Google Chromebooks and Up2/Up-Extreme boards, where the community key is used.

The Intel ME (Management Engine) is responsible for authentication of the firmware, whether it is signed by an Intel production key (consumer products), a community key (open development systems and Chromebooks since GeminiLake) or an OEM key. If the Intel ME is disabled by an OEM, or disabled by user-accessible BIOS options, the firmware authentication will fail and the firmware boot will not complete. If the ME is disabled by the OEM, the only solution is to fall-back to the legacy HDAudio driver. If the ME is disabled by the user, the user must re-enable it. Unfortunately, no documented mechanism exists for the Linux kernel to query whether or not the firmware authentication is enabled, which means dmesg logs cannot be provided to alert the user to an ME configuration issue.

2. Topology file

The topology file, such as /lib/firmware/intel/sof-tplg/sof-hda-generic-2ch.tplg, describes the processing graph and controls to be instantiated by the SOF driver. The topology can be regenerated and reconfigured with tools but requires expert knowledge of the ALSA/ASoC/topology frameworks.

3. UCM file

The UCM file, such as /usr/share/alsa/ucm2/sof-hda-dsp/, configures the controls exposed by the topology file and the external audio chips. UCM can be used in a terminal via the alsaucm command but will typically be used by audio servers such as PulseAudio or PipeWire. UCM files released by Intel are compatible with different drivers and should work when changing the dsp_driver parameter.

The selection of firmware, topology, and UCM files is based on platform capabilities, codec names, and DMI options. While the SOF team and the community try to cover all possible cases, errors will happen when the wrong file is selected at any of the three layers.

4. Chromebooks and SOF

As stated above, starting from 2019/2020, Intel Chromeboooks have been configured with the community key. It means that Chromebooks can run audio firmware signed by anyone. The entire filesystem is locked by default instead, but there are several options to disable security for development purposes. In all cases the first step is to switch the Chromebook to (non-secure) Developer Mode. Developer Mode is the only required step if you only want to install and run your own SOF firmware and are not interested in changing anything else in Chrome OS.

If you need the flexibility to make more changes, Chromebooks can run Linux in several non-mutually exclusive ways. All the options listed below let you run any SOF firmware. One of the biggest differences between them is how to install and run your own Linux kernel.

  • Chrome OS has direct hardware access, but Chrome OS development cannot happen on Chrome OS itself. It requires a separate workstation similar to how most embedded development typically does. For information about setting up the cros_sdk, see the Chromium OS Developer Guide. The cros_sdk is a complete environment that lets you modify anything in Chrome OS and even build an entire system image. The cros_sdk requires significant disk space and some learning effort if you are not already familiar with Portage, a build system in Gentoo, and especially with building the Linux kernel in Portage.

  • Crostini is a secure Linux Virtual Machine that does not have direct access to the hardware and cannot be used for SOF. It does not require Developer Mode. Crostini is listed here for completeness. You might use Crostini as your pseudo-separate cros_sdk workstation, but a different, more powerful system that you never have to reboot is a much better cros_sdk option.

  • Crouton is a non-secure chroot that does allow direct hardware access and can be used for SOF. It lets you install a choice of popular Linux distributions, which you can use for development on the device itself. Make regular backups! The Zephyr project has very detailed specific instructions on how to use Crouton for SOF. Most of these instructions are not Zephyr-specific. With Crouton, you can configure and compile a Linux kernel as usual. However, the kernel installation process is similar to the cros_sdk process with a couple of small twists.

  • Finally, it is possible to dual-boot or completely replace Chrome OS with a regular Linux distribution on some Chromebooks and forget it is a Chromebook entirely. However, this comes at a price: it is the least secure option and the more likely to make your device permanently unusable (“brick”). That level of risk is highly dependent on your particular Chromebook model. If that does not scare you, then https://chrx.org/ is a good starting point. Pay special attention to the note on security. This is the only option that lets you manage kernel installations as a typical Linux distribution does.