Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC] regmap: allow volatile register writes with cached only
 read maps
To:     Mark Brown <broonie@kernel.org>
Cc:     linux-kernel@vger.kernel.org
References: <1525817169-29233-1-git-send-email-jramirez@baylibre.com>
 <20180509083919.GU13402@sirena.org.uk>
From:   Jorge Ramirez-Ortiz <jramirez@baylibre.com>
Message-ID: <65607fde-d0e9-0f08-3042-f6a58b760896@baylibre.com>
Date:   Wed, 9 May 2018 13:49:21 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.7.0
MIME-Version: 1.0
In-Reply-To: <20180509083919.GU13402@sirena.org.uk>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 05/09/2018 10:39 AM, Mark Brown wrote:
> On Wed, May 09, 2018 at 12:06:09AM +0200, Jorge Ramirez-Ortiz wrote:
>> Regmap only allows volatile access to registers when the client
>> supports both reads and writes.
>>
>> This commit bypasses that limitation and enables volatile writes to
>> selected registers while maintaining cached accesses on all reads. For
>> this, the client does not need to configure the reg_read callback.
> I don't understand what voltile access means for write only devices.
> Volatile means that we don't read the cache but go direct to the
> hardware so if we can't read the hardware that's pretty redundant, a
> volatile read that goes to the cache is just a default read.

oops, sorry will try to be a bit more clear with an example.

This patch tries to support a map that provides:

1. only cached reads: (as a consequence every regmap write must succeed).
2. cached writes: do not access the hardware unless the value differs 
from what is in the cache already or (3) applies.
3. support for selectable volatile writes: those that will always access 
the device no matter what the cache holds.

Something like this:

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,
};


I dont think - I could be wrong- that this is something that we can 
support today since the current code seems to require that the regmap is 
readable (ie, that it implements reg_read).
But it could also be that I am missing something in my config? This is 
why I sent an RFC instead of a PATCH, because I am not 100% sure that I 
am not missing something.