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

[Jorge Ramirez-Ortiz]

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.