Re: [PATCH v3 3/4] driver core: add dev_print_hex_dump() logging function.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Apr 01, 2019 at 07:47:14PM -0700, Life is hard, and then you die wrote:
> 
> On Thu, Mar 28, 2019 at 12:29:52PM +0100, Greg Kroah-Hartman wrote:
> > On Thu, Mar 28, 2019 at 03:27:55AM -0700, Life is hard, and then you die wrote:
> > > 
> > > On Thu, Mar 28, 2019 at 06:29:17AM +0100, Greg Kroah-Hartman wrote:
> > > > On Wed, Mar 27, 2019 at 05:28:17PM -0700, Life is hard, and then you die wrote:
> > > > > 
> > > > > On Wed, Mar 27, 2019 at 11:37:57AM +0900, Greg Kroah-Hartman wrote:
> > > > > > On Tue, Mar 26, 2019 at 06:48:06PM -0700, Ronald Tschalär wrote:
> > > > > > > This is the dev_xxx() analog to print_hex_dump(), using dev_printk()
> > > > > > > instead of straight printk() to match other dev_xxx() logging functions.
> > > > > > > ---
> > > > > > >  drivers/base/core.c    | 43 ++++++++++++++++++++++++++++++++++++++++++
> > > > > > >  include/linux/device.h | 15 +++++++++++++++
> > > > > > >  2 files changed, 58 insertions(+)
> [snip]
> > > > > > Anyway, no, please do not do this.  Please do not dump large hex values
> > > > > > like this to the kernel log, it does not help anyone.
> > > > > > 
> > > > > > You can do this while debugging, sure, but not for "real" kernel code.
> > > > > 
> > > > > As used by this driver, it is definitely called for debugging only and
> > > > > must be explicitly enabled via a module param. But having the ability
> > > > > for folks to easily generate and print out debugging info has proven
> > > > > quite valuable.
> > > > 
> > > > We have dynamic debugging, no need for module parameters at all.  This
> > > > isn't the 1990's anymore :)
> > > 
> > > I am aware of dynamic debugging, but there are several issues with it
> > > from the perspective of the logging I'm doing here (I mentioned these
> > > in response to an earlier review already):
> > > 
> > >   1. Dynamic debugging can't be enabled at a function or line level on
> > >      the kernel command line, so this means that to debug issues
> > >      during boot you have to enable everything, which can be way too
> > >      verbose
> > 
> > You can, and should enable it at a function or line level by writing to
> > the proper kernel file in debugfs.
> > 
> > You have read Documentation/admin-guide/dynamic-debug-howto.rst, right?
> 
> Yes, and I've played with the parameters quite a bit.
> 
> > No need to do anything on the command line, that's so old-school :)
> 
> Sorry if I'm being unduly dense, but then how to enable debugging
> during early boot? The only other alternative I see is modifying the
> initrd, and asking folks to do that is noticeably more complicated
> than having them add something to the command line in grub. So from my
> perspective I find kernel params far from old-school :-)

You can do dynamic debugging from the kernel command line, if your code
is built into the kernel (but why would a tiny driver under testing like
this, not be built into the kernel?) what specifically did not work for you?

> > >   2. The expressions to enable the individual logging statements are
> > >      quite brittle (in particular the line-based ones) since they
> > >      (may) change any time the code is changed - having stable
> > >      commands to give to users and put in documentation (e.g.
> > >      "echo 0x200 > /sys/module/applespi/parameters/debug") is
> > >      quite valuable.
> > > 
> > >      One way to work around this would be to put every single logging
> > >      statement in a function of its own, thereby ensuring a stable
> > >      name is associated with each one.
> > 
> > Again, read the documentation, this works today as-is.
> 
> I have read it (we're talking about the dynamic debug docs here), but
> I just don't see how it addresses this in any way.

You can enable/disable logging per-function, which is what you want,
right?


Anyway, I'm glad tracepoints work for you, that should be all that is
now needed.

good luck with your driver!

greg k-h