Re: [PATCH v10 5/5] drm/rockchip: Add dmc notifier in vop driver

[Sean Paul <seanpaul@google.com>]

On Wed, Sep 7, 2016 at 2:13 PM, dbasehore . <dbasehore@chromium.org> wrote:
> On Tue, Sep 6, 2016 at 10:18 AM, Sean Paul <seanpaul@google.com> wrote:
>> On Mon, Sep 5, 2016 at 1:06 AM, Lin Huang <hl@rock-chips.com> wrote:
>>> when in ddr frequency scaling process, vop can not do enable or
>>> disable operation, since in dcf we check vop clock to see whether
>>> vop work. If vop work, dcf do ddr frequency scaling when vop
>>> in vblank status, and we need to read vop register to check whether
>>> vop go into vblank status. If vop not work, dcf can do ddr frequency
>>> any time. So when do ddr frequency scaling, you disabled or enable
>>> vop, there may two bad thing happen: 1, the panel flicker(when vop from
>>> disable status change to enable). 2, kernel hang (when vop from enable
>>> status change to disable, dcf need to read vblank status, but if you disable
>>> vop clock, it can not get the status, it will lead soc dead) So we need
>>> register to devfreq notifier, and we can get the dmc status. Also, when
>>> there have two vop enabled, we need to disable dmc, since dcf only base
>>> on one vop vblank time, so the other panel will flicker when do ddr
>>> frequency scaling.
>>>
>>> Signed-off-by: Lin Huang <hl@rock-chips.com>
>>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>>> ---
>>> Changes in v10:
>>> - None
>>>
>>> Changes in v9:
>>> - None
>>>
>>> Changes in v8:
>>> - None
>>>
>>> Changes in v7:
>>> - None
>>>
>>> Changes in v6:
>>> - fix a build error
>>>
>>> Changes in v5:
>>> - improve some nits
>>>
>>> Changes in v4:
>>> - register notifier to devfreq_register_notifier
>>> - use DEVFREQ_PRECHANGE and DEVFREQ_POSTCHANGE to get dmc status
>>> - when two vop enable, disable dmc
>>> - when two vop back to one vop, enable dmc
>>>
>>> Changes in v3:
>>> - when do vop eanble/disable, dmc will wait until it finish
>>>
>>> Changes in v2:
>>> - None
>>>
>>> Changes in v1:
>>> - use wait_event instead usleep
>>>
>>>  drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 116 ++++++++++++++++++++++++++++
>>>  1 file changed, 116 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>> index efbc41a..a73f3aa 100644
>>> --- a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>> @@ -19,6 +19,8 @@
>>>  #include <drm/drm_crtc_helper.h>
>>>  #include <drm/drm_plane_helper.h>
>>>
>>> +#include <linux/devfreq.h>
>>> +#include <linux/devfreq-event.h>
>>>  #include <linux/kernel.h>
>>>  #include <linux/module.h>
>>>  #include <linux/platform_device.h>
>>> @@ -118,6 +120,13 @@ struct vop {
>>>
>>>         const struct vop_data *data;
>>>
>>> +       struct devfreq *devfreq;
>>> +       struct devfreq_event_dev *devfreq_event_dev;
>>> +       struct notifier_block dmc_nb;
>>> +       int dmc_in_process;
>>> +       int vop_switch_status;
>>> +       wait_queue_head_t wait_dmc_queue;
>>> +       wait_queue_head_t wait_vop_switch_queue;
>>>         uint32_t *regsbak;
>>>         void __iomem *regs;
>>>
>>> @@ -428,11 +437,47 @@ static void vop_dsp_hold_valid_irq_disable(struct vop *vop)
>>>         spin_unlock_irqrestore(&vop->irq_lock, flags);
>>>  }
>>>
>>> +static int dmc_notify(struct notifier_block *nb, unsigned long event,
>>> +                     void *data)
>>> +{
>>> +       struct vop *vop = container_of(nb, struct vop, dmc_nb);
>>> +
>>> +       if (event == DEVFREQ_PRECHANGE) {
>>> +               /*
>>> +                * check if vop in enable or disable process,
>>> +                * if yes, wait until it finishes, use 200ms as
>>> +                * timeout.
>>> +                */
>>> +               if (!wait_event_timeout(vop->wait_vop_switch_queue,
>>> +                                       !vop->vop_switch_status, HZ / 5))
>>> +                       dev_warn(vop->dev,
>>> +                                "Timeout waiting for vop swtich status\n");
>>> +               vop->dmc_in_process = 1;
>>> +       } else if (event == DEVFREQ_POSTCHANGE) {
>>> +               vop->dmc_in_process = 0;
>>> +               wake_up(&vop->wait_dmc_queue);
>>> +       }
>>> +
>>> +       return NOTIFY_OK;
>>> +}
>>> +
>>>  static int vop_enable(struct drm_crtc *crtc)
>>>  {
>>>         struct vop *vop = to_vop(crtc);
>>> +       int num_enabled_crtc = 0;
>>>         int ret;
>>>
>>> +       /*
>>> +        * if in dmc scaling frequency process, wait until it finishes
>>> +        * use 200ms as timeout time.
>>> +        */
>>> +       if (!wait_event_timeout(vop->wait_dmc_queue,
>>> +                               !vop->dmc_in_process, HZ / 5))
>>> +               dev_warn(vop->dev,
>>> +                        "Timeout waiting for dmc when vop enable\n");
>>> +
>>
>>
>> This wait_event_timeout code is terribly racey (same goes above and below).
>>
>>
>>> +       vop->vop_switch_status = 1;
>>> +
>>>         ret = pm_runtime_get_sync(vop->dev);
>>>         if (ret < 0) {
>>>                 dev_err(vop->dev, "failed to get pm runtime: %d\n", ret);
>>> @@ -479,6 +524,21 @@ static int vop_enable(struct drm_crtc *crtc)
>>>
>>>         drm_crtc_vblank_on(crtc);
>>>
>>> +       vop->vop_switch_status = 0;
>>> +       wake_up(&vop->wait_vop_switch_queue);
>>> +
>>> +       /* check how many VOPs in use now */
>>> +       drm_for_each_crtc(crtc, vop->drm_dev) {
>>> +               if (crtc->state->enable)
>>
>> I think you really want to check active, instead of enable.
>>
>>> +                       num_enabled_crtc++;
>>> +       }
>>> +
>>> +       /* if enable two vop, need to disable dmc */
>>> +       if ((num_enabled_crtc > 1) && vop->devfreq) {
>>> +               if (vop->devfreq_event_dev)
>>> +                       devfreq_event_disable_edev(vop->devfreq_event_dev);
>>> +               devfreq_suspend_device(vop->devfreq);
>>> +       }
>>
>> This really feels like something that should be handled somewhere
>> else. I don't fully understand how this works, but it seems like this
>> dependency should be handled where it actually matters, rather than
>> building in a seemingly arbitrary restriction in the vop driver.
>>
>> Even if this is the best place for it, this needs to be refactored to
>> eliminate the races that exist now.
>>
>> Sean
>
> DMC transitions can't happen when there is more than one display. You
> would have to synchronize with more than one vblank on the DCF (which
> actually changes the DDR frequency), and that's probably not going to
> happen.
>
> If this code should be done somewhere else, it needs to still be able
> to check the number of displays and avoid race conditions with
> displays getting enabled or disabled. I don't know enough about DRM to
> know if that's possible outside of this code.
>

Yeah, so I suppose that's another race, since we aren't necessarily
guaranteed to be enabling/disabling the crtcs in the order this patch
assumes.

I think we can probably handle this better by doing this up a level
where we have a more global view of which vops are enabled/disabled
and the order in which they will be processed.

I'm working on this now, stay tuned.

Sean



>>
>>>         return 0;
>>>
>>>  err_disable_aclk:
>>> @@ -489,17 +549,31 @@ err_disable_hclk:
>>>         clk_disable(vop->hclk);
>>>  err_put_pm_runtime:
>>>         pm_runtime_put_sync(vop->dev);
>>> +       vop->vop_switch_status = 0;
>>> +       wake_up(&vop->wait_vop_switch_queue);
>>>         return ret;
>>>  }
>>>
>>>  static void vop_crtc_disable(struct drm_crtc *crtc)
>>>  {
>>>         struct vop *vop = to_vop(crtc);
>>> +       int num_enabled_crtc = 0;
>>>         int i;
>>>
>>>         WARN_ON(vop->event);
>>>
>>>         /*
>>> +        * if in dmc scaling frequency process, wait until it finish
>>> +        * use 200ms as timeout time.
>>> +        */
>>> +       if (!wait_event_timeout(vop->wait_dmc_queue,
>>> +                               !vop->dmc_in_process, HZ / 5))
>>> +               dev_warn(vop->dev,
>>> +                        "Timeout waiting for dmc when vop disable\n");
>>> +
>>> +       vop->vop_switch_status = 1;
>>> +
>>> +       /*
>>>          * We need to make sure that all windows are disabled before we
>>>          * disable that crtc. Otherwise we might try to scan from a destroyed
>>>          * buffer later.
>>> @@ -555,6 +629,24 @@ static void vop_crtc_disable(struct drm_crtc *crtc)
>>>                 spin_unlock_irq(&crtc->dev->event_lock);
>>>
>>>                 crtc->state->event = NULL;
>>> +
>>> +       vop->vop_switch_status = 0;
>>> +       wake_up(&vop->wait_vop_switch_queue);
>>> +
>>> +       /* check how many VOPs in use now */
>>> +       drm_for_each_crtc(crtc, vop->drm_dev) {
>>> +               if (crtc->state->enable)
>>> +                       num_enabled_crtc++;
>>> +       }
>>> +
>>> +       /*
>>> +        * if num_enabled_crtc = 1 now, it means 2 vop enabled
>>> +        * change to 1 vop enabled  need to enable dmc again.
>>> +        */
>>> +       if ((num_enabled_crtc == 1) && vop->devfreq) {
>>> +               if (vop->devfreq_event_dev)
>>> +                       devfreq_event_enable_edev(vop->devfreq_event_dev);
>>> +               devfreq_resume_device(vop->devfreq);
>>>         }
>>>  }
>>>
>>> @@ -1413,6 +1505,8 @@ static int vop_bind(struct device *dev, struct device *master, void *data)
>>>         struct drm_device *drm_dev = data;
>>>         struct vop *vop;
>>>         struct resource *res;
>>> +       struct devfreq *devfreq;
>>> +       struct devfreq_event_dev *event_dev;
>>>         size_t alloc_size;
>>>         int ret, irq;
>>>
>>> @@ -1474,6 +1568,28 @@ static int vop_bind(struct device *dev, struct device *master, void *data)
>>>                 return ret;
>>>
>>>         pm_runtime_enable(&pdev->dev);
>>> +
>>> +       init_waitqueue_head(&vop->wait_vop_switch_queue);
>>> +       vop->vop_switch_status = 0;
>>> +       init_waitqueue_head(&vop->wait_dmc_queue);
>>> +       vop->dmc_in_process = 0;
>>> +
>>> +       devfreq = devfreq_get_devfreq_by_phandle(dev, 0);
>>> +       if (IS_ERR(devfreq))
>>> +               goto out;
>>> +
>>> +       vop->devfreq = devfreq;
>>> +       vop->dmc_nb.notifier_call = dmc_notify;
>>> +       devfreq_register_notifier(vop->devfreq, &vop->dmc_nb,
>>> +                                 DEVFREQ_TRANSITION_NOTIFIER);
>>> +
>>> +       event_dev = devfreq_event_get_edev_by_phandle(vop->devfreq->dev.parent,
>>> +                                                     0);
>>> +       if (IS_ERR(event_dev))
>>> +               goto out;
>>> +
>>> +       vop->devfreq_event_dev = event_dev;
>>> +out:
>>>         return 0;
>>>  }
>>>
>>> --
>>> 2.6.6
>>>