linux-kernel - Re: [RFC PATCH 0/4] Drop custom logging

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Message-ID: <Y8nnwIqah3O9QfIU@pendragon.ideasonboard.com>
Date:   Fri, 20 Jan 2023 03:00:48 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Dan Carpenter <error27@...il.com>
Cc:     Phil Elwell <phil@...pberrypi.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Umang Jain <umang.jain@...asonboard.com>,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Adrien Thierry <athierry@...hat.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] Drop custom logging

On Fri, Jan 20, 2023 at 02:53:17AM +0200, Laurent Pinchart wrote:
> On Thu, Jan 19, 2023 at 05:37:48PM +0300, Dan Carpenter wrote:
> > On Thu, Jan 19, 2023 at 02:31:50PM +0000, Phil Elwell wrote:
> > > On Thu, 19 Jan 2023 at 14:25, Dan Carpenter wrote:
> > > > On Thu, Jan 19, 2023 at 01:47:44PM +0000, Phil Elwell wrote:
> > > > > > > I understand the desire to remove the custom logging. I don't welcome
> > > > > > > the loss of flexibility that comes with such a strategy
> > > > > >
> > > > > > What "loss of flexibility"?  You now have access to the full dynamic
> > > > > > debugging facilities that all of the rest of the kernel has.  What is
> > > > > > lacking?
> > > > >
> > > > > Perhaps I've missed something, either in this patch set or the kernel
> > > > > as a whole, but how is one supposed to set different logging levels on
> > > > > different facilities within a driver/module, or even for the module as
> > > > > a whole?
> > > >
> > > > Yeah.  You will be still able to do that and more besides after the
> > > > transition.  Cleaning this up makes the code better in every way.
> > > >
> > > > Documentation/admin-guide/dynamic-debug-howto.rst
> > > 
> > > Are you saying this patch set gets us to that point?
> > 
> > Yes.  The patch has some issues, but yes.
> 
> I think I'm missing something too then. Dynamic debug provides the
> ability to easily switch dev_dbg() messages on and off at runtime, but
> it doesn't provide, as far as I'm aware, log levels or log categories.
> 
> Log levels are currently used by the vchiq code to suppress messages
> below a certain level. Kernel log levels are not an exact replacement,
> as the messages still end up in the kernel log (except for debug
> messages).
> 
> Log categories are used to group messages in categories and control
> their log level per category. As far as I know, dynamic debug doesn't
> provide any such feature.

After a bit more research (which I should have done before replying,
sorry), it looks like dynamic debug has support for classes, which are
used by, for instance, the DRM logging infrastructure (see
include/drm/drm_print.h). I don't see that being wired up to dev_*()
print macros though, am I missing something, or would vchiq need to keep
using custom logging macros (with dynamic debug used as a backend,
replacing the current custom implementation) to make use of this feature
?

-- 
Regards,

Laurent Pinchart