Re: [PATCH 4/5] ASoC: fsl_ssi: use flat regmap cache

[Marek Vasut <marex@denx.de>]

On 09/21/2016 06:28 PM, Maciej S. Szmigiero wrote:
> Hi Marek,

Hi!

> On 19.09.2016 21:30, Marek Vasut wrote:
>> Same as commit ce492b3b8f99cf9d2f807ec22d8805c996a09503
>> Subject: drm/fsl-dcu: use flat regmap cache
>>
>> Using flat regmap cache instead of RB-tree to avoid the following
>> lockdep warning on driver load:
>> WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:2871 lockdep_trace_alloc+0x104/0x128
>> DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))
>>
>> The RB-tree regmap cache needs to allocate new space on first
>> writes.
> 
> That's why SSI driver had .num_reg_defaults_raw set in regmap config.
> With this set and .reg_defaults_raw unset regcache_hw_init() will read
> existing register content which then will be used to populate cache
> (and so allocate RB tree nodes) at regcache initialization time.

OK

> This means that allocation of new cache RB tree nodes should never happen
> during actual write.
> If you get a warning that it happened then maybe there is a write to
> some register not covered by .num_reg_defaults_raw - this should be
> fixed.

I might be blind, but I really don't see that. I just discussed it with
Mark and he thinks the same. I will revisit this one more time.

> The problem with flat cache is that it is zero-initialized, that is,
> all registers are assumed to contain zeros by default.
> This is generally not true in case of SSI so reads from and bit updates
> in non-volatile registers will corrupt their value.

This will let the driver init the hardware into a well-defined state.
Are there any bits in the SSI config registers which we must preserve?

> You can also refer to discussion when .num_reg_defaults_raw was first
> added here:
> http://www.gossamer-threads.com/lists/linux/kernel/2330573

Thanks!

> Maciej
> 


-- 
Best regards,
Marek Vasut