Re: [PATCH] drm/fsl-dcu: use flat regmap cache

[Stefan Agner <stefan@agner.ch>]

On 2016-06-03 16:00, Daniel Vetter wrote:
> On Fri, Jun 03, 2016 at 03:43:19PM -0700, Stefan Agner wrote:
>> Using flat regmap cache instead of RB-tree to avoid the following
>> lockdep warning on driver load:
>> WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:2755 lockdep_trace_alloc+0x15c/0x160()
>> DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))
>>
>> The RB-tree regmap cache needs to allocate new space on first
>> writes. However, allocations in an atomic context (e.g. when a
>> spinlock is held) are not allowed. The function regmap_write
>> calls map->lock, which acquires a spinlock in the fast_io case.
>> Since the FSL DCU driver uses MMIO, the regmap bus of type
>> regmap_mmio is being used which has fast_io set to true.
>>
>> Use flat regmap cache and specify max register to be large
>> enouth to cover all registers available in LS1021a and Vybrids
>> register space.
>>
>> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> Cc: Mark Brown <broonie@kernel.org>
>> Cc: stable@vger.kernel.org
>> ---
>> While regmap cache is used for suspend/resume only (which is
>> broken in its current state) Mark noted that using the RB regmap
>> cache can also cause issues during initialization of the driver.
>> This patch migrates to flat regmap cache (which we can also use
>> to fix the issue in stable kernels), and yet another patchset
>> moves to the atomic suspend/resume helpers (which will not go
>> into stable...)
> 
> While talking suspend/resume, I highly recommend
> drm_atomic_helper_suspend and drm_atomic_helper_resume. In my experience
> dumb safe/restore of registers for restoring modeset state leads to tears.
> -Daniel

Since you commented to the patchset implementing exactly that I assume
you have seen it.

I still would like to have this patch applied to fix the regmap cache
warning also for older kernels.

--
Stefan

> 
>>
>> Dave, I saw that you just sent out the pull for rc2, will send
>> a pull request for rc3 early next week...
>>
>> --
>> Stefan
>>
>>  drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c b/drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c
>> index 0ec1ad9..dc723f7 100644
>> --- a/drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c
>> +++ b/drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c
>> @@ -42,9 +42,10 @@ static const struct regmap_config fsl_dcu_regmap_config = {
>>  	.reg_bits = 32,
>>  	.reg_stride = 4,
>>  	.val_bits = 32,
>> -	.cache_type = REGCACHE_RBTREE,
>> +	.cache_type = REGCACHE_FLAT,
>>
>>  	.volatile_reg = fsl_dcu_drm_is_volatile_reg,
>> +	.max_register = 0x11fc,
>>  };
>>
>>  static int fsl_dcu_drm_irq_init(struct drm_device *dev)
>> --
>> 2.8.2
>>
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