[PATCH 1/3] drm/tilcdc: Take mode config lock while updating the crtc clock rate

[Jyri Sarha]

On 09/06/16 12:07, Tomi Valkeinen wrote:
> 
> 
> On 06/09/16 11:19, Jyri Sarha wrote:
>> Take mode config lock while updating the crtc clock rate. To avoid a
>> race in tilcdc_crtc_update_clk(), we do not want the mode to change
>> while we update crtc clock.
>>
>> Signed-off-by: Jyri Sarha <jsarha at ti.com>
>> ---
>>  drivers/gpu/drm/tilcdc/tilcdc_drv.c | 5 +++++
>>  drivers/gpu/drm/tilcdc/tilcdc_drv.h | 2 ++
>>  2 files changed, 7 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_drv.c b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
>> index f8892e9..882d9b5 100644
>> --- a/drivers/gpu/drm/tilcdc/tilcdc_drv.c
>> +++ b/drivers/gpu/drm/tilcdc/tilcdc_drv.c
>> @@ -184,10 +184,14 @@ static int cpufreq_transition(struct notifier_block *nb,
>>  {
>>  	struct tilcdc_drm_private *priv = container_of(nb,
>>  			struct tilcdc_drm_private, freq_transition);
>> +	struct drm_mode_config *config = &priv->dev->mode_config;
>> +
>>  	if (val == CPUFREQ_POSTCHANGE) {
>>  		if (priv->lcd_fck_rate != clk_get_rate(priv->clk)) {
>> +			mutex_lock(&config->mutex);
> 
> drm_modeset_lock_crtc()? Or drm_modeset_lock_all() if per-crtc is not
> suitable.
> 

I guess that should work too, all I need is just to make sure no one
calls mode_set_nofb() while the clock is updated.

I just thought that since I am not actually touching drm state at all
the back off mechanisms etc add just unnecessary complexity. But now
after reading a little bit drm locking code, for sure the mode config
mutex is not the light weight lock I was looking for.

One solution would be adding tilcdc internal mutex to synchronize
mode_set_nofb() and cpufreq_transition(), but after all it is probably
better to use the mechanisms that are already there and just use
drm_modeset_lock_crtc() despite its apparent complexity.

BR,
Jyri