Re: [RFC PATCH 1/2] regmap: Add cache_default_is_zero flag for flat cache

["Sheetal ." <sheetal@nvidia.com>]

On 06-01-2026 20:42, Mark Brown wrote:
> On Tue, Jan 06, 2026 at 07:38:26PM +0530, Sheetal . wrote:
> 
>> 2. No functional benefit: Entries like { REG, 0x0 } only set the
>>     validity bit; the cache value is already zero.
> 
> For sparse caches specifying the register also allocates the cache
> entry.

ACK

> 
>> Add a cache_default_is_zero flag to struct regmap_config. When set,
>> the flat cache marks registers as valid on first read instead of
>> warning. This ensures only accessed registers are marked valid,
> 
> Why do this on first read rather than than just fill the valid flags
> during initialisation?
> 


Setting valid bits on first read rather than bitmap_fill() at init ensures:
- Only accessed registers are marked valid
- regcache_sync() only syncs registers that were actually used
- Avoids writes to holes or unused registers during sync
- Safer for drivers without writeable_reg callback


>> index b0b9be750d93..bf918f88bfd3 100644
>> --- a/include/linux/regmap.h
>> +++ b/include/linux/regmap.h
>> @@ -452,6 +452,7 @@ struct regmap_config {
>>   	enum regcache_type cache_type;
>>   	const void *reg_defaults_raw;
>>   	unsigned int num_reg_defaults_raw;
>> +	bool cache_default_is_zero;
> 
> It would be better if this were something specific to the flat cache
> since otherwise we have to consider what this means for the other cache
> types.

I can address this by rename to flat_cache_default_is_zero.