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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 28 Nov 2022 15:58:40 +0100
From:   Alexander Stein <alexander.stein@...tq-group.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] regmap: cache: downgrade log level for no cache defaults message

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