Re: [PATCH libdrm 3/3] xf86drm: Add platform and host1x bus support

[Thierry Reding <thierry.reding@gmail.com>]

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On Sat, Dec 24, 2016 at 05:00:27PM +0000, Emil Velikov wrote:
> On 23 December 2016 at 17:49, Thierry Reding <thierry.reding@gmail.com> wrote:
> > From: Thierry Reding <treding@nvidia.com>
> >
> > ARM SoCs usually have their DRM/KMS devices on the platform bus, so add
> > support for that to enable these devices to be used with the drmDevice
> > infrastructure.
> >
> > NVIDIA Tegra SoCs have an additional level in the hierarchy and DRM/KMS
> > devices can also be on the host1x bus. This is mostly equivalent to the
> > platform bus.
> >
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> > Note that we could probably get away with treating host1x as platform
> > bus. However, they are technically two different busses in the kernel
> > and hence we may want to make use of that differentiation later on.
> >
> Admittedly there's no clear cut either way, but I'm inclined to have
> host1x as platform.
> I'm leaning towards that since there is no /sys/bus/host1x (afaict),

Actually there is:

	# find /sys/bus/host1x
	/sys/bus/host1x
	/sys/bus/host1x/drivers_probe
	/sys/bus/host1x/devices
	/sys/bus/host1x/devices/drm
	/sys/bus/host1x/uevent
	/sys/bus/host1x/drivers
	/sys/bus/host1x/drivers/drm
	/sys/bus/host1x/drivers/drm/bind
	/sys/bus/host1x/drivers/drm/unbind
	/sys/bus/host1x/drivers/drm/module
	/sys/bus/host1x/drivers/drm/uevent
	/sys/bus/host1x/drivers/drm/drm
	/sys/bus/host1x/drivers_autoprobe

> plus otherwise devices (like imx-vpu/vc4) will end with their own bus
> type.

I don't think that would happen. Tegra is somewhat special in this case
because of the host1x device that is a parent for both display and
capture devices. The host1x bus is architected to allow binding each of
the DRM/KMS and V4L2 devices (the V4L2 driver hasn't been merged yet)
to the host1x device.

The reason for this is mostly historical. Back when Tegra support was
added there was a strict rule against adding dummy device nodes to the
device tree. For Tegra this wasn't much of a problem because the host1x
device is the logical choice for a parent, so I just had to write some
glue (the host1x bus) to instantiate a dummy device for each composite
driver.

Later on the component/master infrastructure was added and the rules
regarding dummy device nodes were relaxed, so as far as I know all other
devices end up anchored to a dummy device on the platform bus.

> > +static int drmParsePlatformBusInfo(int maj, int min, drmPlatformBusInfoPtr info)
> > +{
> > +    char path[PATH_MAX + 1], *name;
> > +
> > +    snprintf(path, PATH_MAX, "/sys/dev/char/%d:%d/device", maj, min);
> > +
> > +    name = sysfs_uevent_get(path, "OF_FULLNAME");
> > +    strcpy(info->fullname, name);
> > +    free(name);
> > +
> Let's be extra careful and ensure that we don't overrun (and properly
> terminate) the fixed size fullname[].

Okay, will do.

> > +    return 0;
> > +}
> > +
> > +static int drmParsePlatformDeviceInfo(int maj, int min,
> > +                                      drmPlatformDeviceInfoPtr info)
> > +{
> > +    /* XXX fill in platform device info */
> > +
> Not 100% sure what exactly we should consider bus and what device
> info, either way an empty struct is not going to go well.
> As a food for thought, here is some info for imx/etna and vc4.
> 
> cat /sys/dev/char/226\:*/device/uevent
> 
> DRIVER=etnaviv
> OF_NAME=gpu-subsystem
> OF_FULLNAME=/gpu-subsystem
> OF_COMPATIBLE_0=fsl,imx-gpu-subsystem
> OF_COMPATIBLE_N=1
> MODALIAS=of:Ngpu-subsystemT<NULL>Cfsl,imx-gpu-subsystem
> DRIVER=imx-drm
> OF_NAME=display-subsystem
> OF_FULLNAME=/display-subsystem
> OF_COMPATIBLE_0=fsl,imx-display-subsystem
> OF_COMPATIBLE_N=1
> 
> DRIVER=vc4-drm
> OF_NAME=gpu
> OF_FULLNAME=/soc/gpu
> OF_COMPATIBLE_0=brcm,bcm2835-vc4
> OF_COMPATIBLE_N=1
> MODALIAS=of:NgpuT<NULL>Cbrcm,bcm2835-vc4

I think the full name is appropriate for bus info because it's about the
closest thing we have to address the device (the equivalent to domain,
bus, device and function numbers).

The list of compatible strings might be suitable for device information
and might be useful for drivers to determine capabilities if the kernel
doesn't have another means of communicating that to userspace.

Thierry

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