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

From: Archit Taneja
Date: Mon Jul 03 2017 - 06:56:26 EST

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

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

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.

About the solution that you and Boris discussed in the other thread, I'll give
this a try on the msm driver and see how it does.


I did not spent much time on this idea, so I cannot guarantee it has not
fundamental issues :)



Signed-off-by: Eric Anholt <eric@xxxxxxxxxx>
drivers/gpu/drm/drm_mipi_dsi.c | 49 ++++++++++++++++++++++++++++++++----------
include/drm/drm_mipi_dsi.h | 3 +++
2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
index 1160a579e0dc..9cdd68a7dc0d 100644
--- a/drivers/gpu/drm/drm_mipi_dsi.c
+++ b/drivers/gpu/drm/drm_mipi_dsi.c
@@ -45,6 +45,13 @@
* subset of the MIPI DCS command set.
+static DEFINE_MUTEX(host_lock);
+static LIST_HEAD(host_list);
+/* List of struct mipi_dsi_device which were registered while no host
+ * was available.
+ */
+static LIST_HEAD(unattached_device_list);
static int mipi_dsi_device_match(struct device *dev, struct device_driver *drv)
struct mipi_dsi_device *dsi = to_mipi_dsi_device(dev);
@@ -138,10 +145,12 @@ static struct mipi_dsi_device *mipi_dsi_device_alloc(struct mipi_dsi_host *host)
dsi->host = host;
dsi->dev.bus = &mipi_dsi_bus_type;
- dsi->dev.parent = host->dev;
dsi->dev.type = &mipi_dsi_device_type;
- device_initialize(&dsi->dev);
+ if (dsi->host) {
+ dsi->dev.parent = host->dev;
+ device_initialize(&dsi->dev);
+ }
return dsi;
@@ -206,7 +215,7 @@ mipi_dsi_device_register_full(struct mipi_dsi_host *host,
const struct mipi_dsi_device_info *info)
struct mipi_dsi_device *dsi;
- struct device *dev = host->dev;
+ struct device *dev = host ? host->dev : NULL;
int ret;
if (!info) {
@@ -230,11 +239,17 @@ mipi_dsi_device_register_full(struct mipi_dsi_host *host,
dsi->channel = info->channel;
strlcpy(dsi->name, info->type, sizeof(dsi->name));
- ret = mipi_dsi_device_add(dsi);
- if (ret) {
- dev_err(dev, "failed to add DSI device %d\n", ret);
- kfree(dsi);
- return ERR_PTR(ret);
+ if (!dsi->host) {
+ mutex_lock(&host_lock);
+ list_add(&dsi->list, &unattached_device_list);
+ mutex_unlock(&host_lock);
+ } else {
+ ret = mipi_dsi_device_add(dsi);
+ if (ret) {
+ dev_err(dev, "failed to add DSI device %d\n", ret);
+ kfree(dsi);
+ return ERR_PTR(ret);
+ }
return dsi;
@@ -251,9 +266,6 @@ void mipi_dsi_device_unregister(struct mipi_dsi_device *dsi)
-static DEFINE_MUTEX(host_lock);
-static LIST_HEAD(host_list);
* of_find_mipi_dsi_host_by_node() - find the MIPI DSI host matching a
* device tree node
@@ -285,6 +297,7 @@ EXPORT_SYMBOL(of_find_mipi_dsi_host_by_node);
int mipi_dsi_host_register(struct mipi_dsi_host *host)
struct device_node *node;
+ struct mipi_dsi_device *dsi, *temp;
for_each_available_child_of_node(host->dev->of_node, node) {
/* skip nodes without reg property */
@@ -295,6 +308,20 @@ int mipi_dsi_host_register(struct mipi_dsi_host *host)
list_add_tail(&host->list, &host_list);
+ /* If any DSI devices were registered under our OF node, then
+ * connect our host to it and probe them now.
+ */
+ list_for_each_entry_safe(dsi, temp, &unattached_device_list, list) {
+ if (of_get_parent(dsi->dev.of_node) == host->dev->of_node) {
+ dsi->host = host;
+ dsi->dev.parent = host->dev;
+ device_initialize(&dsi->dev);
+ mipi_dsi_device_add(dsi);
+ list_del_init(&dsi->list);
+ }
+ }
return 0;
diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
index 4fef19064b0f..699ea4acd5b6 100644
--- a/include/drm/drm_mipi_dsi.h
+++ b/include/drm/drm_mipi_dsi.h
@@ -178,6 +178,9 @@ struct mipi_dsi_device {
unsigned int lanes;
enum mipi_dsi_pixel_format format;
unsigned long mode_flags;
+ /* Entry on the unattached_device_list */
+ struct list_head list;
#define MIPI_DSI_MODULE_PREFIX "mipi-dsi:"

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