Re: [PATCH 1/5] drm/i915: add and enable DP AUX CH mutex

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 14 Feb 2018, Rodrigo Vivi <rodrigo.vivi@intel.com> wrote:
> On Wed, Feb 14, 2018 at 09:47:00AM +0000, Jani Nikula wrote:
>> On Tue, 13 Feb 2018, José Roberto de Souza <jose.souza@intel.com> wrote:
>> > When PSR/PSR2 is enabled hardware can do AUX transactions by it self, so
>> > lets use the mutex register that is available in gen9+ to avoid concurrent
>> > access by hardware and driver.
>> >
>> > Reference: https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-skl-vol12-display.pdf
>> > Page 198 - AUX programming sequence
>> >
>> > As gen < 9 don't have the mutex PSR needs to be disabled, that is why the
>> > 'else TODO' was added. This is also backed by spec:
>> > "DDI A AUX channel transactions must not be sent while SRD is enabled.
>> > SRD must be completely disabled before a DDI A AUX channel transaction can
>> > be sent." BSpec: 7530
>> 
>> We explicitly rejected the use of the hw mutex way back when Skylake was
>> enabled. At that time the understanding was that the hw will try to do
>> the arbitration itself. Has something changed?
>
> By that time spec wasn't clear about the actual need or its reliability.
> Nowdays spec is clear that that is absolutely needed for PSR/PSR2/GTC.
>
> Also we were using a lot of SW tracking and nowadays we are moving PSR
> to use more HW tracking.
>
> So I believe we should add this to get a stable PSR.

Fair enough. The patch at hand needs a bunch of work though.

BR,
Jani.


>
>> 
>> Some comments inline.
>> 
>> >
>> > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
>> > Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
>> > ---
>> >  drivers/gpu/drm/i915/i915_reg.h  |  9 ++++++++
>> >  drivers/gpu/drm/i915/intel_dp.c  | 48 ++++++++++++++++++++++++++++++++++++++++
>> >  drivers/gpu/drm/i915/intel_drv.h |  1 +
>> >  drivers/gpu/drm/i915/intel_psr.c |  6 +++++
>> >  4 files changed, 64 insertions(+)
>> >
>> > diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
>> > index f6afa5e5e7c1..5d7736117cb9 100644
>> > --- a/drivers/gpu/drm/i915/i915_reg.h
>> > +++ b/drivers/gpu/drm/i915/i915_reg.h
>> > @@ -5318,6 +5318,7 @@ enum {
>> >  #define _DPA_AUX_CH_DATA3	(dev_priv->info.display_mmio_offset + 0x6401c)
>> >  #define _DPA_AUX_CH_DATA4	(dev_priv->info.display_mmio_offset + 0x64020)
>> >  #define _DPA_AUX_CH_DATA5	(dev_priv->info.display_mmio_offset + 0x64024)
>> > +#define _DPA_AUX_CH_MUTEX	(dev_priv->info.display_mmio_offset + 0x6402C)
>> >  
>> >  #define _DPB_AUX_CH_CTL		(dev_priv->info.display_mmio_offset + 0x64110)
>> >  #define _DPB_AUX_CH_DATA1	(dev_priv->info.display_mmio_offset + 0x64114)
>> > @@ -5325,6 +5326,7 @@ enum {
>> >  #define _DPB_AUX_CH_DATA3	(dev_priv->info.display_mmio_offset + 0x6411c)
>> >  #define _DPB_AUX_CH_DATA4	(dev_priv->info.display_mmio_offset + 0x64120)
>> >  #define _DPB_AUX_CH_DATA5	(dev_priv->info.display_mmio_offset + 0x64124)
>> > +#define _DPB_AUX_CH_MUTEX	(dev_priv->info.display_mmio_offset + 0x6412C)
>> >  
>> >  #define _DPC_AUX_CH_CTL		(dev_priv->info.display_mmio_offset + 0x64210)
>> >  #define _DPC_AUX_CH_DATA1	(dev_priv->info.display_mmio_offset + 0x64214)
>> > @@ -5332,6 +5334,7 @@ enum {
>> >  #define _DPC_AUX_CH_DATA3	(dev_priv->info.display_mmio_offset + 0x6421c)
>> >  #define _DPC_AUX_CH_DATA4	(dev_priv->info.display_mmio_offset + 0x64220)
>> >  #define _DPC_AUX_CH_DATA5	(dev_priv->info.display_mmio_offset + 0x64224)
>> > +#define _DPC_AUX_CH_MUTEX	(dev_priv->info.display_mmio_offset + 0x6422C)
>> >  
>> >  #define _DPD_AUX_CH_CTL		(dev_priv->info.display_mmio_offset + 0x64310)
>> >  #define _DPD_AUX_CH_DATA1	(dev_priv->info.display_mmio_offset + 0x64314)
>> > @@ -5339,6 +5342,7 @@ enum {
>> >  #define _DPD_AUX_CH_DATA3	(dev_priv->info.display_mmio_offset + 0x6431c)
>> >  #define _DPD_AUX_CH_DATA4	(dev_priv->info.display_mmio_offset + 0x64320)
>> >  #define _DPD_AUX_CH_DATA5	(dev_priv->info.display_mmio_offset + 0x64324)
>> > +#define _DPD_AUX_CH_MUTEX	(dev_priv->info.display_mmio_offset + 0x6432C)
>> >  
>> >  #define _DPF_AUX_CH_CTL		(dev_priv->info.display_mmio_offset + 0x64510)
>> >  #define _DPF_AUX_CH_DATA1	(dev_priv->info.display_mmio_offset + 0x64514)
>> > @@ -5346,9 +5350,11 @@ enum {
>> >  #define _DPF_AUX_CH_DATA3	(dev_priv->info.display_mmio_offset + 0x6451c)
>> >  #define _DPF_AUX_CH_DATA4	(dev_priv->info.display_mmio_offset + 0x64520)
>> >  #define _DPF_AUX_CH_DATA5	(dev_priv->info.display_mmio_offset + 0x64524)
>> > +#define _DPF_AUX_CH_MUTEX	(dev_priv->info.display_mmio_offset + 0x6452C)
>> >  
>> >  #define DP_AUX_CH_CTL(port)	_MMIO_PORT(port, _DPA_AUX_CH_CTL, _DPB_AUX_CH_CTL)
>> >  #define DP_AUX_CH_DATA(port, i)	_MMIO(_PORT(port, _DPA_AUX_CH_DATA1, _DPB_AUX_CH_DATA1) + (i) * 4) /* 5 registers */
>> > +#define DP_AUX_CH_MUTEX(port)	_MMIO_PORT(port, _DPA_AUX_CH_MUTEX, _DPB_AUX_CH_MUTEX)
>> >  
>> >  #define   DP_AUX_CH_CTL_SEND_BUSY	    (1 << 31)
>> >  #define   DP_AUX_CH_CTL_DONE		    (1 << 30)
>> > @@ -5378,6 +5384,9 @@ enum {
>> >  #define   DP_AUX_CH_CTL_FW_SYNC_PULSE_SKL(c) (((c) - 1) << 5)
>> >  #define   DP_AUX_CH_CTL_SYNC_PULSE_SKL(c)   ((c) - 1)
>> >  
>> > +#define   DP_AUX_CH_MUTEX_ENABLE		(1 << 31)
>> > +#define   DP_AUX_CH_MUTEX_STATUS		(1 << 30)
>> 
>> The bit definitions should immediately follow the register offset
>> definition. I.e. DP_AUX_CH_MUTEX() should be right above these.
>> 
>> > +
>> >  /*
>> >   * Computing GMCH M and N values for the Display Port link
>> >   *
>> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> > index f20b25f98e5a..bd5f1bb730ff 100644
>> > --- a/drivers/gpu/drm/i915/intel_dp.c
>> > +++ b/drivers/gpu/drm/i915/intel_dp.c
>> > @@ -1081,6 +1081,28 @@ static uint32_t intel_dp_get_aux_send_ctl(struct intel_dp *intel_dp,
>> >  						aux_clock_divider);
>> >  }
>> >  
>> > +static bool intel_dp_aux_get(struct drm_i915_private *dev_priv,
>> > +			     struct intel_dp *intel_dp)
>> 
>> You can get at dev_priv from intel_dp, please don't pass both.
>> 
>> The names _get/_put imply the calls can be nested, which is not the case
>> here. Please use _lock/_unlock instead.
>> 
>> If the locking can fail, call it _trylock.
>> 
>> > +{
>> > +	int try, ret;
>> > +
>> > +	for (try = 0, ret = -1; try < 3 && ret; try++)
>> > +		ret = wait_for(!(I915_READ_NOTRACE(intel_dp->aux_ch_mutex_reg)
>> > +				 & DP_AUX_CH_MUTEX_STATUS), 0.5);
>> 
>> 0.5? Really?
>> 
>> What's the point of the loop around the wait_for? wait_for already loops
>> with increasing delays in between the condition check.
>> 
>> > +
>> > +	return ret == 0;
>> > +}
>> > +
>> > +static void intel_dp_aux_put(struct drm_i915_private *dev_priv,
>> > +			     struct intel_dp *intel_dp)
>> > +{
>> > +	/* setting the status bit releases the mutex + keep set the bit
>> > +	 * to keep the mutex enabled.
>> > +	 */
>> > +	I915_WRITE(intel_dp->aux_ch_mutex_reg, DP_AUX_CH_MUTEX_ENABLE |
>> > +		   DP_AUX_CH_MUTEX_STATUS);
>> > +}
>> > +
>> >  static int
>> >  intel_dp_aux_ch(struct intel_dp *intel_dp,
>> >  		const uint8_t *send, int send_bytes,
>> > @@ -1115,6 +1137,21 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>> >  
>> >  	intel_dp_check_edp(intel_dp);
>> >  
>> > +	/* Get hardware mutex when PSR is enabled */
>> > +	if (dev_priv->psr.enabled == intel_dp) {
>> > +		if (intel_dp->aux_ch_mutex_reg.reg) {
>> 
>> See i915_mmio_reg_valid(). But I'd rather the check was within the
>> locking primitives.
>> 
>> Overall I think mixing all this PSR conditional stuff in the aux channel
>> primitives is very, very wrong.
>> 
>> At the very least this is racy with PSR enabling/disabling.
>> 
>> > +			if (intel_dp_aux_get(dev_priv, intel_dp)) {
>> > +				DRM_WARN("dp_aux_ch port locked for too long");
>> > +				ret = -EBUSY;
>> > +				goto out_locked;
>> > +			}
>> > +		} else {
>> > +			/* TODO: go to PSR exit state waiting for change before
>> > +			 * do a aux ch transaction.
>> > +			 */
>> > +		}
>> > +	}
>> > +
>> >  	/* Try to wait for any previous AUX channel activity */
>> >  	for (try = 0; try < 3; try++) {
>> >  		status = I915_READ_NOTRACE(ch_ctl);
>> > @@ -1243,7 +1280,16 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>> >  				    recv + i, recv_bytes - i);
>> >  
>> >  	ret = recv_bytes;
>> > +
>> >  out:
>> > +	if (dev_priv->psr.enabled == intel_dp) {
>> > +		if (intel_dp->aux_ch_mutex_reg.reg)
>> > +			intel_dp_aux_put(dev_priv, intel_dp);
>> > +		else
>> > +			/* TODO: schedule PSR work to active PSR again */
>> > +	}
>> > +
>> > +out_locked:
>> >  	pm_qos_update_request(&dev_priv->pm_qos, PM_QOS_DEFAULT_VALUE);
>> >  
>> >  	if (vdd)
>> > @@ -1496,6 +1542,8 @@ static void intel_aux_reg_init(struct intel_dp *intel_dp)
>> >  	intel_dp->aux_ch_ctl_reg = intel_aux_ctl_reg(dev_priv, port);
>> >  	for (i = 0; i < ARRAY_SIZE(intel_dp->aux_ch_data_reg); i++)
>> >  		intel_dp->aux_ch_data_reg[i] = intel_aux_data_reg(dev_priv, port, i);
>> > +	if (INTEL_INFO(dev_priv)->gen >= 9)
>> > +		intel_dp->aux_ch_mutex_reg = DP_AUX_CH_MUTEX(port);
>> >  }
>> >  
>> >  static void
>> > diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
>> > index 898064e8bea7..0d235b7dab21 100644
>> > --- a/drivers/gpu/drm/i915/intel_drv.h
>> > +++ b/drivers/gpu/drm/i915/intel_drv.h
>> > @@ -1040,6 +1040,7 @@ struct intel_dp {
>> >  	i915_reg_t output_reg;
>> >  	i915_reg_t aux_ch_ctl_reg;
>> >  	i915_reg_t aux_ch_data_reg[5];
>> > +	i915_reg_t aux_ch_mutex_reg;
>> >  	uint32_t DP;
>> >  	int link_rate;
>> >  	uint8_t lane_count;
>> > diff --git a/drivers/gpu/drm/i915/intel_psr.c b/drivers/gpu/drm/i915/intel_psr.c
>> > index 2ef374f936b9..8b456e4e1b47 100644
>> > --- a/drivers/gpu/drm/i915/intel_psr.c
>> > +++ b/drivers/gpu/drm/i915/intel_psr.c
>> > @@ -484,6 +484,9 @@ static void hsw_psr_enable_source(struct intel_dp *intel_dp,
>> >  			   EDP_PSR_DEBUG_MASK_HPD |
>> >  			   EDP_PSR_DEBUG_MASK_LPSP);
>> >  	}
>> > +
>> > +	if (intel_dp->aux_ch_mutex_reg.reg)
>> > +		I915_WRITE(intel_dp->aux_ch_mutex_reg, DP_AUX_CH_MUTEX_ENABLE);
>> 
>> You can't just go stomping on the register without locking. Racy.
>> 
>> >  }
>> >  
>> >  /**
>> > @@ -617,6 +620,9 @@ static void hsw_psr_disable(struct intel_dp *intel_dp,
>> >  		else
>> >  			WARN_ON(I915_READ(EDP_PSR_CTL) & EDP_PSR_ENABLE);
>> >  	}
>> > +
>> > +	if (intel_dp->aux_ch_mutex_reg.reg)
>> > +		I915_WRITE(intel_dp->aux_ch_mutex_reg, ~DP_AUX_CH_MUTEX_ENABLE);
>> 
>> Are you sure you want to set all other bits than enable?
>> 
>> 
>> BR,
>> Jani.
>> 
>> >  }
>> >  
>> >  /**
>> 
>> -- 
>> Jani Nikula, Intel Open Source Technology Center

-- 
Jani Nikula, Intel Open Source Technology Center
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