Re: [PATCH v2 0/9] drm: Switch from dev_err to dev_err_probe for missing DSI host error path

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Fri, Mar 01, 2024 at 11:19:27AM -0500, Nícolas F. R. A. Prado wrote:
> On Fri, Mar 01, 2024 at 08:34:31AM +0200, Laurent Pinchart wrote:
> > Hi Nícolas,
> > 
> > On Thu, Feb 29, 2024 at 07:12:06PM -0500, Nícolas F. R. A. Prado wrote:
> > > This series changes every occurence of the following pattern: 
> > > 
> > > 	dsi_host = of_find_mipi_dsi_host_by_node(dsi);
> > > 	if (!dsi_host) {
> > > 		dev_err(dev, "failed to find dsi host\n");
> > > 		return -EPROBE_DEFER;
> > > 	}
> > > 
> > > into
> > > 
> > > 	dsi_host = of_find_mipi_dsi_host_by_node(dsi);
> > > 	if (!dsi_host)
> > > 		return dev_err_probe(dev, -EPROBE_DEFER, "failed to find dsi host\n");
> > > 
> > > This registers the defer probe reason (so it can later be printed by the
> > > driver core or checked on demand through the devices_deferred file in
> > > debugfs) and prevents errors to be spammed in the kernel log every time
> > > the driver retries to probe, unnecessarily alerting userspace about
> > > something that is a normal part of the boot process.
> > 
> > The idea is good, but I have a small issue with patches 1/9 to 7/9. They
> > all patch a function that is called by the probe function. Calling
> > dev_err_probe() in such functions is error-prone. I had to manually
> > check when reviewing the patches that those functions were indeed called
> > at probe time, and not through other code paths, and I also had to check
> > that no callers were using dev_err_probe() in the error handling path,
> > as that would have overridden the error message.
> > 
> > Would there be a way to move the dev_err_probe() to the top-level ? I
> > understand it's not always possible or convenient, but if it was doable
> > in at least some of the drivers, I think it would be better. I'll let
> > you be the judge.
> 
> Hey Laurent, thanks for the review.
> 
> I get where you're coming from, as I checked those things myself while writing
> the patch. That said, I don't think moving dev_err_probe() to the top-level is a
> good move for a few reasons:
> * Keeping the log message as close to the source of the error makes it more
>   specific, and consequently, more useful.
> * The original code already returned -EPROBE_DEFER, implying the function is
>   expected to be called only from the probe function.
> 
> With those points in mind, the only way I see to guarantee
> dev_err_probe(...,-EPROBE_DEFER...) would only be called by probe, and that the
> reason wouldn't be overriden, would be to move the entire code path of that
> function that calls into dev_err_probe() up into the probe function. But if we
> adopt this pattern consistently across the drivers in the tree, I think it would
> drastically worsen readability and cancel out the benefits.
> 
> IMO the way forward with the API we have, is to make use of warnings and static
> checkers to catch cases where dev_err_probe() is overriding a defer probe
> reason, and where it's called outside of the probe function scope.
> 
> So I'm inclined to leave the patches as they are, but am happy to discuss this
> further or other ideas.

Thanks for checking and having taken the time to explain your rationale.
For the whole series,

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

-- 
Regards,

Laurent Pinchart