[PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

[PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

[Alexander Stein]

Having no cache defaults is not a mistake, especially for devices
which do have some kind of strapping values which can't be provided
statically. For this case a warning is the wrong log level, the initial
cache values do need to be read form hardware. Reduce level to info.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
---
 drivers/base/regmap/regcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/regmap/regcache.c b/drivers/base/regmap/regcache.c
index 362e043e26d8..89b92eda8b02 100644
--- a/drivers/base/regmap/regcache.c
+++ b/drivers/base/regmap/regcache.c
@@ -54,7 +54,7 @@ static int regcache_hw_init(struct regmap *map)
 
 	if (!map->reg_defaults_raw) {
 		bool cache_bypass = map->cache_bypass;
-		dev_warn(map->dev, "No cache defaults, reading back from HW\n");
+		dev_info(map->dev, "No cache defaults, reading back from HW\n");
 
 		/* Bypass the cache access till data read from HW */
 		map->cache_bypass = true;
-- 
2.25.1

Re: [PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 1236 bytes --]

On Wed, Sep 21, 2022 at 09:10:54AM +0200, Alexander Stein wrote:

> Having no cache defaults is not a mistake, especially for devices
> which do have some kind of strapping values which can't be provided
> statically. For this case a warning is the wrong log level, the initial
> cache values do need to be read form hardware. Reduce level to info.

>  	if (!map->reg_defaults_raw) {
>  		bool cache_bypass = map->cache_bypass;
> -		dev_warn(map->dev, "No cache defaults, reading back from HW\n");
> +		dev_info(map->dev, "No cache defaults, reading back from HW\n");

This is a warning which is only generated in the case where we have a
non-sparse cache where we won't read from the hardware in the csae of
missing values and will instead just substitute zero as a default value.
It's warning that we might end up changing values for the device
unexpectedly in the case of a read/modify/write cycle that doesn't
actually check what the device currently has set.

In this case if you can't use a rbtree cache I would recommend
bootstrapping by initially reading without a cache first to get the
default values, then replacing the regmap with one that uses the
defaults you just read.  You'll need some care over suspend/resume
though.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

[Alexander Stein]

Hello Mark,

Am Freitag, 25. November 2022, 20:30:52 CET schrieb Mark Brown:
> * PGP Signed by an unknown key
> 
> On Wed, Sep 21, 2022 at 09:10:54AM +0200, Alexander Stein wrote:
> > Having no cache defaults is not a mistake, especially for devices
> > which do have some kind of strapping values which can't be provided
> > statically. For this case a warning is the wrong log level, the initial
> > cache values do need to be read form hardware. Reduce level to info.
> > 
> >  	if (!map->reg_defaults_raw) {
> >  	
> >  		bool cache_bypass = map->cache_bypass;
> > 
> > -		dev_warn(map->dev, "No cache defaults, reading back from 
HW\n");
> > +		dev_info(map->dev, "No cache defaults, reading back from 
HW\n");
> 
> This is a warning which is only generated in the case where we have a
> non-sparse cache where we won't read from the hardware in the csae of
> missing values and will instead just substitute zero as a default value.
> It's warning that we might end up changing values for the device
> unexpectedly in the case of a read/modify/write cycle that doesn't
> actually check what the device currently has set.
> 
> In this case if you can't use a rbtree cache I would recommend
> bootstrapping by initially reading without a cache first to get the
> default values, then replacing the regmap with one that uses the
> defaults you just read.  You'll need some care over suspend/resume
> though.

The driver I'm working on clk-renesas-pcie.c only has 8 registers, so there is 
no benefit to use a different cache than REGCACHE_FLAT.

But I currently fail to see why this is only for non-sparse caches. This is 
printed even before the cache_ops init() is called.

Before the call to regcache_hw_init() there is this comment:
> /* Some devices such as PMICs don't have cache defaults,
>  * we cope with this by reading back the HW registers and
>  * crafting the cache defaults by hand.
>  */

This is exactly what I want to do: Read back HW registers and create a cache.
Actually regcache_hw_init() is exactly doing what you are suggesting:
1. reading HW registers into map->reg_defaults_raw
2.
 a) copy them into map->reg_defaults
 b) read HW register individually if 1. failed

So thinking more about this, this warning should be removed altogether IMHO.

Best regards,
Alexander

Re: [PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 2120 bytes --]

On Mon, Nov 28, 2022 at 03:58:40PM +0100, Alexander Stein wrote:
> Am Freitag, 25. November 2022, 20:30:52 CET schrieb Mark Brown:

> > In this case if you can't use a rbtree cache I would recommend
> > bootstrapping by initially reading without a cache first to get the
> > default values, then replacing the regmap with one that uses the
> > defaults you just read.  You'll need some care over suspend/resume
> > though.

> The driver I'm working on clk-renesas-pcie.c only has 8 registers, so there is 
> no benefit to use a different cache than REGCACHE_FLAT.

There's a benefit when it comes to bootstrapping (though also a slight
performance cost, and non-atomicity).

> But I currently fail to see why this is only for non-sparse caches. This is 
> printed even before the cache_ops init() is called.

We only go into hw_init() in the case where the map has specified
num_reg_defaults_raw, as you say you could trigger that for any cache
type but if you're using a sparse cache you're either going to provide
no defaults or provide defaults in the standard, sparse format rather
than the raw format so in practice it won't trigger.

> Before the call to regcache_hw_init() there is this comment:
> > /* Some devices such as PMICs don't have cache defaults,
> >  * we cope with this by reading back the HW registers and
> >  * crafting the cache defaults by hand.
> >  */

> This is exactly what I want to do: Read back HW registers and create a cache.
> Actually regcache_hw_init() is exactly doing what you are suggesting:
> 1. reading HW registers into map->reg_defaults_raw
> 2.
>  a) copy them into map->reg_defaults
>  b) read HW register individually if 1. failed

The other thing with doing a readback to fill the cache is that it's a
potentially slow operation (depending on how big the cache is and the
bus in use) and also a potentially destructive one for things like clear
on read registers.  If regmaps are doing this deliberately (which does
seem valid) we probably want a clearer mechanism for specifying it - I
think an explicit flag would do the trick.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]