linux-kernel - Re: [PATCH v2 1/2] drm: Add NVIDIA Tegra20 support

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Message-ID: <20121113093725.GB8122@avionic-0098.mockup.avionic-design.de>
Date:	Tue, 13 Nov 2012 10:37:26 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Mark Zhang <markz@...dia.com>
Cc:	Dave Airlie <airlied@...hat.com>, Rob Clark <robdclark@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] drm: Add NVIDIA Tegra20 support

On Tue, Nov 13, 2012 at 04:49:24PM +0800, Mark Zhang wrote:
> On 11/13/2012 03:48 PM, Thierry Reding wrote:
> > * PGP Signed by an unknown key
> > 
> > On Tue, Nov 13, 2012 at 03:15:47PM +0800, Mark Zhang wrote:
> >> On 11/13/2012 05:55 AM, Thierry Reding wrote:
> >>> This commit adds a KMS driver for the Tegra20 SoC. This includes basic
> >>> support for host1x and the two display controllers found on the Tegra20
> >>> SoC. Each display controller can drive a separate RGB/LVDS output.
> >>>
> >>> Signed-off-by: Thierry Reding <thierry.reding@...onic-design.de>
> >>> ---
> >>> Changes in v2:
> >>> - drop Linux-specific drm subdirectory for DT bindings documentation
> >>> - remove display helper leftovers that belong in a later patch
> >>> - reuse debugfs infrastructure provided by the DRM core
> >>> - move vblank syncpoint defines to dc.h
> >>> - use drm_compat_ioctl()
> >>>
> >> [...]
> >>> diff --git a/drivers/gpu/drm/tegra/Kconfig b/drivers/gpu/drm/tegra/Kconfig
> >>> new file mode 100644
> >>> index 0000000..be1daf7
> >>> --- /dev/null
> >>> +++ b/drivers/gpu/drm/tegra/Kconfig
> >>> @@ -0,0 +1,23 @@
> >>> +config DRM_TEGRA
> >>> +       tristate "NVIDIA Tegra DRM"
> >>> +       depends on DRM && OF && ARCH_TEGRA
> >>> +       select DRM_KMS_HELPER
> >>> +       select DRM_GEM_CMA_HELPER
> >>> +       select DRM_KMS_CMA_HELPER
> >>
> >> Just for curious, according to my testing, why the "CONFIG_CMA" is not
> >> enabled while DRM_GEM_CMA_HELPER & DRM_KMS_CMA_HELPER are enabled here?
> > 
> > The reason is that CMA doesn't actually provide any API for drivers to
> > use and in fact unless you use very large buffers you could indeed run
> > this code on top of a non-CMA kernel and it will likely even work.
> > 
> 
> Okay. But I think it's better to turn on CMA defaultly. During my
> testing, it's hard to allocate more 2MB without CMA...

CMA is enabled by default in one of the Tegra default configuration
patches in my tegra/next branch. I will submit that patch to Stephen
when the 3.8 cycle starts, so that it'll be automatically enabled along
with the DRM driver.

But I don't think it makes sense to couple it to the DRM_TEGRA symbol as
it isn't strictly required.

> >>> +static struct of_device_id tegra_dc_of_match[] = {
> >>> +       { .compatible = "nvidia,tegra20-dc", },
> >>> +       { .compatible = "nvidia,tegra30-dc", },
> >>
> >> If you don't want add Tegra 3 support in this patch set, remove
> >> { .compatible = "nvidia,tegra30-dc", } here.
> > 
> > Good catch! I'll move that into the Tegra30 support patch.
> > 
> >>> +static int host1x_activate_drm_client(struct host1x *host1x,
> >>> +                                     struct host1x_drm_client *drm,
> >>> +                                     struct host1x_client *client)
> >>> +{
> >>> +       mutex_lock(&host1x->drm_clients_lock);
> >>> +       list_del_init(&drm->list);
> >>> +       list_add_tail(&drm->list, &host1x->drm_active);
> >>
> >> Why we need this "drm_active" list? We can combine this function and
> >> function "host1x_remove_drm_client" and free the drm client just here.
> >> It's useless after host1x clients registered themselves.
> > 
> > The list is used to properly remove all clients and resources when the
> > module is unloaded. Granted, this code isn't executed if you don't build
> > the driver as a loadable module, but it should still be a supported use-
> > case.
> > 
> 
> My opinion is, after registration is completed, host1x_drm_client is
> useless, host1x_client is enough for follow-up operations.
> I still don't get how this is related with building the driver into the
> kernel or as a kernel module, so if something I misunderstood, please
> let me know it. Thanks.

I can take another look at this and see if it can be further simplified.
This was actually a rather tricky part to get right, so I'm naturally a
bit hesitant to touch it.

Thierry

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