linux-kernel - Re: [RFC PATCH 1/2] regmap: Add cache_default_is

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Message-ID: <b0032054-4eea-44f4-bf70-204fa414f297@nvidia.com>
Date: Wed, 7 Jan 2026 12:25:01 +0530
From: "Sheetal ." <sheetal@...dia.com>
To: Mark Brown <broonie@...nel.org>
Cc: Sander Vanheule <sander@...nheule.net>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J . Wysocki" <rafael@...nel.org>, Danilo Krummrich
 <dakr@...nel.org>, Liam Girdwood <lgirdwood@...il.com>,
 Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
 Thierry Reding <thierry.reding@...il.com>,
 Jonathan Hunter <jonathanh@...dia.com>, linux-kernel@...r.kernel.org,
 linux-sound@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] regmap: Add cache_default_is_zero flag for flat
 cache

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.