Re: [PATCH 6/8] drm: Allow DSI devices to be registered before the host registers.

[Archit Taneja]

On 07/15/2017 04:31 AM, Eric Anholt wrote:
> Archit Taneja <architt@xxxxxxxxxxxxxx> writes:
>
> > On 06/29/2017 04:09 PM, Andrzej Hajda wrote:
> > > On 29.06.2017 07:03, Archit Taneja wrote:
> > > > On 06/28/2017 01:28 AM, Eric Anholt wrote:
> > > > > When a mipi_dsi_host is registered, the DT is walked to find any child
> > > > > nodes with compatible strings.  Those get registered as DSI devices,
> > > > > and most DSI panel drivers are mipi_dsi_drivers that attach to those nodes.
> > > > >
> > > > > There is one special case currently, the adv7533 bridge, where the
> > > > > bridge probes on I2C, and during the bridge attach step it looks up
> > > > > the mipi_dsi_host and registers the mipi_dsi_device (for its own stub
> > > > > mipi_dsi_driver).
> > > > >
> > > > > For the Raspberry Pi panel, though, we also need to attach on I2C (our
> > > > > control bus), but don't have a bridge driver.  The lack of a bridge's
> > > > > attach() step like adv7533 uses means that we aren't able to delay the
> > > > > mipi_dsi_device creation until the mipi_dsi_host is present.
> > > > >
> > > > > To fix this, we extend mipi_dsi_device_register_full() to allow being
> > > > > called with a NULL host, which puts the device on a queue waiting for
> > > > > a host to appear.  When a new host is registered, we fill in the host
> > > > > value and finish the device creation process.
> > > > This is quite a nice idea. The only bothering thing is the info.of_node usage
> > > > varies between child nodes (mipi_dsi_devs) and non-child nodes (i2c control
> > > > bus).
> > > >
> > > > For DSI children expressed in DT, the of_node in info holds the DT node
> > > > corresponding to the DSI child itself. For non-DT ones, this patch assumes
> > > > that info.of_node stores the DSI host DT node. I think it should be okay as
> > > > long as we mention the usage in a comment somewhere. The other option is to
> > > > have a new info.host_node field to keep a track of the host DT node.
> > >
> > > Field abuse is not a biggest issue.
> > >
> > > This patch changes totally semantic of mipi_dsi_device_register_full.
> > > Currently semantic of *_device_register* function is to create and add
> > > device to existing bus, ie after return we have device attached to bus,
> > > so it can be instantly used. With this change function can return some
> > > unattached device, without warranty it will be ever attached - kind of
> > > hidden deferring. Such change looks for me quite dangerous, even if it
> > > looks convenient in this case.
> > >
> > > As discussed in other thread more appealing solution for me would be:
> > > 1. host creates dsi bus, but doesn't call component_add as it does not
> > > have all required resources.
> > > 2. host waits for all required dsi devs attached, gets registered panels
> > > or bridges and calls component_add after that.
> > > 3. in bind phase it has all it needs, hasn't it?
> >
> > This seems like it would work, but would require KMS drivers to restructure
> > themselves around this approach. For KMS drivers that don't even use the
> > component stuff, it might be asking too much.
> >
> > We could maybe consider Eric's patch as an intermediate solution, we should
> > definitely put WARN_ON(!dsi->host) like checks for all the existing
> > mipi_dsi_*() API.
>
> Could you clarify which entrypoints you'd like a warning on?  Is it just
> "everything that gets the host ops"?

Sorry, all API was a bad suggestion.

I think a warning and early bail in mipi_dsi_attach() should be sufficient.
A panel/bridge DSI device shouldn't use any other API if it hasn't called
mipi_dsi_attach().

Thanks,
Archit

>

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