Hi Laurent,
On Tue, Oct 13, 2026 at 01:11:58PM +0200, Laurent Pinchart wrote:
> Hi Francesco,
>
> On Tue, Oct 13, 2026 at 11:06:31AM +0200, Francesco Dolcini wrote:
> > On Mon, Oct 12, 2026 at 06:40:54PM +0200, Laurent Pinchart wrote:
> > > On Mon, Oct 12, 2026 at 04:25:22AM -0300, Leonardo Costa wrote:
> > > > This series adds a device tree overlay that enables the VGA output on
> > > > Apalis
> > > > iMX6, which is driven from a parallel RGB interface passing through a
> > > > non-programmable video DAC.
> > > >
> > > > The video DAC is a simple resistor network gated by an active-low
> > > > data-enable signal. For the bridge to work properly, this series also
> > > > changes the following in the dt-bindings and simple-bridge driver:
> > > >
> > > > - dt-bindings: Add a new compatible for the bridge in the simple-bridge
> > > > binding. This is necessary because the existing dumb-vga-dac
> > > > compatible
> > > > doesn't describe a data-enable signal polarity.
> > > >
> > > > - simple-bridge: Add the VGA bridge match-table entry and timings
> > > > struct, setting the DRM_BUS_FLAG_DE_LOW flag for the bridge.
> > >
> > > Is there schematics available anywhere ? That would help reviewing the
> > > series ?
> >
> > This circuit is on the SoM, and the SoM schematics are
> > confidential and not generally available.
> >
> > I tried to extract as ascii art the relevant circuit, hope it helps
>
> Thank you, it makes things clearer.
>
> The circuit seems fairly generic. Wouldn't it be better to add an
> optional property ot the dumb-vga-dac instead of a new compatible string
> specific to Apalis ?
See the diff below, this was the initial idea we had when working on
this. On one hand, at least to me it seems like something that other
bridges could use, and a new property seems reasonable. On the other,
since we don't know exactly how much configuration is expected from a
bridge in the simple-bridge driver, we weren't sure if this would be
accepted, and went with the simpler solution of creating a new binding
for our bridge, which also seemed less intrusive.
What are your thoughts about a change like this?
diff --git a/drivers/gpu/drm/bridge/simple-bridge.c
b/drivers/gpu/drm/bridge/simple-bridge.c
index 3fba7002322e..41b00a44983a 100644
--- a/drivers/gpu/drm/bridge/simple-bridge.c
+++ b/drivers/gpu/drm/bridge/simple-bridge.c
@@ -171,6 +171,7 @@ static int simple_bridge_probe(struct platform_device *pdev)
{
struct simple_bridge *sbridge;
struct device_node *remote;
+ u32 de;
sbridge = devm_drm_bridge_alloc(&pdev->dev, struct simple_bridge,
bridge, &simple_bridge_bridge_funcs);
@@ -212,6 +213,19 @@ static int simple_bridge_probe(struct platform_device
*pdev)
sbridge->bridge.of_node = pdev->dev.of_node;
sbridge->bridge.timings = sbridge->info->timings;
+ if (!sbridge->bridge.timings &&
+ !of_property_read_u32(pdev->dev.of_node, "data-enable-active",
&de)) {
+ struct drm_bridge_timings *timings;
+
+ timings = devm_kzalloc(&pdev->dev, sizeof(*timings),
GFP_KERNEL);
+ if (!timings)
+ return -ENOMEM;
+
+ timings->input_bus_flags = de ? DRM_BUS_FLAG_DE_HIGH :
+ DRM_BUS_FLAG_DE_LOW;
+ sbridge->bridge.timings = timings;
+ }
+
return devm_drm_bridge_add(&pdev->dev, &sbridge->bridge);
}
