[RFC PATCH 2/2] regmap: Reject fast_io regmap configurations with RBTREE and MAPLE caches

[Guenter Roeck]

On 7/21/23 07:53, Marek Szyprowski wrote:
> Hi,
> 
> On 20.07.2023 05:28, Guenter Roeck wrote:
>> REGCACHE_RBTREE and REGCACHE_MAPLE dynamically allocate memory for regmap
>> operations. This is incompatible with spinlock based locking which is used
>> for fast_io operations. Reject affected configurations.
>>
>> Signed-off-by: Guenter Roeck <linux at roeck-us.net>
>> ---
>> This seems prudent, given that accesses will be protected by spinlock
>> but may allocate memory with GFP_KERNEL. Another option might be to use
>> WARN_ON instead of rejecting the configuration to avoid hard regressions
>> (and I think both drivers/net/ieee802154/mcr20a.c and
>> sound/soc/codecs/sti-sas.c may be affected, though I can not test it).
> 
> This patch, which landed in today's linux-next, breaks operation of the
> RockChip's VOP2 DRM driver
> (drivers/gpu/drm/rockchip/rockchip_drm_vop2.c). I'm not sure what is the
> proper fix in this case. Should one change the cache type to REGCACHE_FLAT?
> 

Ah, I missed regcache_init_mmio() when looking for affected drivers.
This affects a larger number of drivers than I thought. In addition
to the drivers mentioned above,

  drivers/soc/qcom/icc-bwmon.c
  sound/soc/bcm/bcm2835-i2s.c
  sound/soc/codecs/jz4740.c
  sound/soc/fsl/fsl_aud2htx.c
  sound/soc/fsl/fsl_easrc.c
  sound/soc/fsl/fsl_micfil.c

all use unsafe locking (spinlock with REGCACHE_RBTREE).

Thanks,
Guenter

> 
>>    drivers/base/regmap/regmap.c | 9 +++++++++
>>    1 file changed, 9 insertions(+)
>>
>> diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
>> index 89a7f1c459c1..b4640285c0b9 100644
>> --- a/drivers/base/regmap/regmap.c
>> +++ b/drivers/base/regmap/regmap.c
>> @@ -777,6 +777,15 @@ struct regmap *__regmap_init(struct device *dev,
>>    	} else {
>>    		if ((bus && bus->fast_io) ||
>>    		    config->fast_io) {
>> +			/*
>> +			 * fast_io is incompatible with REGCACHE_RBTREE and REGCACHE_MAPLE
>> +			 * since both need to dynamically allocate memory.
>> +			 */
>> +			if (config->cache_type == REGCACHE_RBTREE ||
>> +			    config->cache_type == REGCACHE_MAPLE) {
>> +				ret = -EINVAL;
>> +				goto err_name;
>> +			}
>>    			if (config->use_raw_spinlock) {
>>    				raw_spin_lock_init(&map->raw_spinlock);
>>    				map->lock = regmap_lock_raw_spinlock;
> 
> Best regards