linux-kernel - Re: [RFC] regmap: allow volatile register writes with cached only read maps

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Date:   Thu, 17 May 2018 09:12:49 +0200
From:   Jorge Ramirez-Ortiz <jramirez@...libre.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC] regmap: allow volatile register writes with cached only
 read maps

On 05/13/2018 04:22 AM, Mark Brown wrote:
> On Fri, May 11, 2018 at 12:29:42PM +0200, Jorge Ramirez-Ortiz wrote:
>> On 05/11/2018 04:00 AM, Mark Brown wrote:
>>> We don't currently suppress writes except when regmap_update_bits()
>>> notices that the modification was a noop.  You probably want to be using
>>> regmap_write_bits() here instead of regmap_update_bits(), that will
>>> always do the write.
>> but isnt that interface at a different level?
> It's at the level where we suppress writes - the write suppression isn't
> a feature of the caching, it's something that regmap_update_bits() does
> if it notices that it won't change anything.  It'll happen even if
> there's no cache at all.
>
>> I am not sure if you are asking me to review my patch or just discarding the
>> RFC and highlighting that I have a configuration problem.
> I don't understand your patch as-is.
>
>> In my use case and what triggered this RFC (config below), an 'amixer set'
>> might never reach the driver's .reg_write interface even though the register
>> is configured as volatile (to me this is not consistent since volatile_reg
>> is being silently ignored).
> I'm not seeing any inconsistency there.  Volatility means the register
> can't be cached as it might change underneath us, it doesn't have any
> impact on writes and it's happening at a lower level.  Like I say if you
> absolutely need a write to happen you should be explicitly doing a
> write, though if you need a write to happen for a noop control change it
> sounds like there's something weird with that control that's possibly a
> problem anyway.
>
>> So I dont see where/how your recommendation fits; maybe you could clarify a
>> bit more please?
> As I've been saying if you explicitly need a write to happen don't use
> regmap_update_bits(), do something that guarantees you'll get a write
> like regmap_write() or regmap_write_bits().

I do understand your point but Mark, that interface you mention sits 
above the user request (as a client the user does not call 
regmap_update_bits or regmap_write_bits or regmap_write(): none of those 
functions mean anything to the foo_regmap definition below - that is why 
we have an interface).

The client just uses this request:

static const struct regmap_config foo_regmap = {
     .reg_write           = foo_write_reg,

     .reg_bits            = 32,
     .val_bits            = 32,
     .reg_stride          = 1,

     .volatile_reg        = foo_volatile_reg,

     .max_register        = CODEC_ENABLE_DEBUG_CTRL_REG,
     .reg_defaults        = foo_reg_defaults,
     .num_reg_defaults    = ARRAY_SIZE(foo_reg_defaults),
     .cache_type          = REGCACHE_RBTREE,
};


and all this request means is that it expects foo_volatile_regs to be 
taken into consideration when accessing the reg_write callback: so 
whoever is calling the interface reg_write (be it regmap_update_bits or 
regmap_write_bits or whoever it is, I dont know) must make sure the 
volatile request applies.

the RFC patch that I submitted achieves exactly that.

does this make more sense now?