Re: [RFC PATCH v2] media: renesas: vsp1: Add VSPD underrun detection & tracing

[Eugeniu Rosca <erosca@de.adit-jv.com>]

Dear Laurent, dear Geert,

On Di, Jun 28, 2022 at 11:11:51 +0300, Laurent Pinchart wrote:
> On Tue, Jun 28, 2022 at 10:08:28PM +0200, Geert Uytterhoeven wrote:
> > On Tue, Jun 28, 2022 at 9:53 PM Laurent Pinchart wrote:
> > > On Tue, Jun 28, 2022 at 09:05:34PM +0200, Eugeniu Rosca wrote:
> > > > On So, Jun 26, 2022 at 09:46:42 +0300, Laurent Pinchart wrote:
> > > > > On Tue, May 03, 2022 at 03:20:10PM +0200, Eugeniu Rosca wrote:
> > > > > >
> > > > > > Troubleshooting the above without the right tools becomes a nightmare.
> > > > >
> > > > > Having spent lots of time working in userspace recently, I can't agree
> > > > > more.
> > > >
> > > > Thanks for the feedback and for endorsing the utility of this patch.
> > > >
> > > > > > +static int vspd_underrun[VSPD_MAX_NUM];
> > > > > > +module_param_array(vspd_underrun, int, NULL, 0444);
> > > > > > +MODULE_PARM_DESC(vspd_underrun, "VSPD underrun counter");
> > > > >
> > > > > Module parameters are not meant to convey information back to userspace.
> > > > > This should be done through either a debugfs file or a sysfs file. Given
> > > > > the debugging nature of this feature, I'd recommend the former.
> > > >
> > > > It is a bit unfortunate that we have to go the debugFS route, since I
> > > > recall at least one Customer in the past, who disabled the debugFS in
> > > > the end product, since it was the only available means to meet the
> > > > stringent automotive requirements (w.r.t. KNL binary size). Anybody
> > > > who has no choice but to disable debugFS will consequently not be able
> > > > to take advantage of this patch in the production/release software.
> > >
> > > debugfs isn't meant to be enabled in production, so if you need a
> > > solution for production environment, it's not an option indeed.
> > >
> > > > If there is no alternative, then for sure I can go this way.
> > > >
> > > > However, before submitting PATCH v3, would you consider SYSFS viable
> > > > too, if keeping the module param is totally unacceptable?
> > > >
> > > > I was hoping to keep the number of external dependencies to the bare
> > > > minimum, hence the initial choice of module param. Looking forward to
> > > > your final suggestion/preference.
> > >
> > > sysfs would be my next recommendation. I don't think a Linux system can
> > > meaningfully run without sysfs, so it shouldn't be an issue
> > > dependency-wise.
> > 
> > Indeed, you can add a device attribute.
> > But as that is not a debug feature, the attribute must be documented,
> > and becomes ABI.
> 
> Thanks for the comment, that's correct

Thanks for the precious and insightful comments.
I will try to get them resolved in PATCH v3, to the best of my ability.

BR, Eugeniu