On 26.04.2018 10:07, Jyri Sarha wrote:
> Add device_link from panel device (supplier) to drm device (consumer)
> when drm_panel_attach() is called. This patch should protect the
> master drm driver if an attached panel driver unbinds while it is in
> use. The device_link should make sure the drm device is unbound before
> the panel driver becomes unavailable.
>
> The device_link is removed when drm_panel_detach() is called. The
> drm_panel_detach() should be called by the consumer DRM driver, not the
> panel driver, otherwise both drivers are racing to delete the same link.
>
> Signed-off-by: Jyri Sarha <jsarha@xxxxxx>
> Reviewed-by: Eric Anholt <eric@xxxxxxxxxx>
Hi Jyri,
This patch breaks few platforms: tm2, tm2e, trats2 - ie all platforms
using exynos_drm_dsi and dsi panels.
Exynos-DSI properly handles panels unbind - ie references to panel are
properly removed on panels removal and respective drm_connector enters
disconnected state, without destroying whole drm device.
And again on panel driver re-bind drm_panel is properly attached to
exynos-dsi and panel pipeline is working again.
This patch will break this behavior, ie it will destroy whole drm device.
Making device_links for panels optional seems to me the best solution,
what do you think?
The funny thing is that due to bug in device link code, your patch has
no effect on these platforms. So it does not hurt these platform yet,
but the bug will be fixed sooner or later.
The trick in case of exynos-dsi is to use mipi_dsi attach/detach
callbacks to get notifications about drm_panel's
appearance/disappearance, I guess ultimately such callbacks should be a
part of drm_panel
framework.
Below description how all the code works:
Initialization:
1. exynos_dsi (component of exynos_drm) at the end of bind callback
registers mipi_dsi bus.
2. during mipi_dsi bus registration or later (if panel driver was not
yet registered), panel driver is bound ie - its probe callback is called.
3. during panel's probe drm_panel_add is called, and later mipi_dsi_attach.
4. mipi_dsi_attach calls dsi .attach callback of exynos_dsi.
5. exynos_dsi.attach callback looks for drm_panel, calls
drm_panel_attach and changes connector's status to connected.
....
Panel's unbind:
1. device_release_driver calls panel's remove callback.
2. panel's remove callback calls mipi_dsi_detach.
3. mipi_dsi_detach calls exynos_dsi.detach callback.
4. exynos_dsi.detach calls drm_panel_detach and changes connector's
state to disconnected.
Panel's rebind is equivalent of steps 2-4 of Initialization.
I have wrote the code few years ago, and it seemed to me the only safe
way to handle dynamic panel bind/unbind.
For example please note that step 5 of initialization and steps 2-4 of
unbind are executed always under panel's device_lock so it makes it
safe. Otherwise you risk that between panel lookup and panel attach
panel's driver can be unbind and consumer is left with dangling panel's
pointer, so even with your patch most of the drivers are still buggy.
And few words about the bug(?) in device_link code: If you call
device_link_add from probe of the supplier(as it is in this case) its
status is set to DL_STATE_DORMANT, and after successful bind it is
changed to DL_STATE_AVAILABLE, which is incorrect (it should be
DL_STATE_ACTIVE), as a result on supplier's unbind consumer is not unbound.
Regards
Andrzej
> ---
> drivers/gpu/drm/drm_panel.c | 10 ++++++++++
> include/drm/drm_panel.h | 1 +
> 2 files changed, 11 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
> index 71e4075..965530a 100644
> --- a/drivers/gpu/drm/drm_panel.c
> +++ b/drivers/gpu/drm/drm_panel.c
> @@ -24,6 +24,7 @@
> #include <linux/err.h>
> #include <linux/module.h>
>
> +#include <drm/drm_device.h>
> #include <drm/drm_crtc.h>
> #include <drm/drm_panel.h>
>
> @@ -104,6 +105,13 @@ int drm_panel_attach(struct drm_panel *panel, struct drm_connector *connector)
> if (panel->connector)
> return -EBUSY;
>
> + panel->link = device_link_add(connector->dev->dev, panel->dev, 0);
> + if (!panel->link) {
> + dev_err(panel->dev, "failed to link panel to %s\n",
> + dev_name(connector->dev->dev));
> + return -EINVAL;
> + }
> +
> panel->connector = connector;
> panel->drm = connector->dev;
>
> @@ -125,6 +133,8 @@ EXPORT_SYMBOL(drm_panel_attach);
> */
> int drm_panel_detach(struct drm_panel *panel)
> {
> + device_link_del(panel->link);
> +
> panel->connector = NULL;
> panel->drm = NULL;
>
> diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
> index 14ac240..26a1b5f 100644
> --- a/include/drm/drm_panel.h
> +++ b/include/drm/drm_panel.h
> @@ -89,6 +89,7 @@ struct drm_panel {
> struct drm_device *drm;
> struct drm_connector *connector;
> struct device *dev;
> + struct device_link *link;
>
> const struct drm_panel_funcs *funcs;
>
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