Re: [PATCH 1/2] iio: adc: ad7124: Don't create more channels than the hardware is capable of

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 11 Nov 2024 08:21:45 -0600
David Lechner <dlechner@baylibre.com> wrote:

> On 11/11/24 6:08 AM, Uwe Kleine-König wrote:
> > [dropped Mircea Caprioru from Cc: as their address bounces.]
> > 
> > Hello David,
> > 
> > On Fri, Nov 08, 2024 at 12:52:35PM -0600, David Lechner wrote:  
> >> On 11/8/24 12:18 PM, Uwe Kleine-König wrote:  
> >>> The ad7124-4 and ad7124-8 both support 16 channel registers. Don't
> >>> accept more (logical) channels from dt than that.  
> >>
> >> Why should the devicetree be limited by the number of channel
> >> registers? Channel registers are a resource than can be
> >> dynamically assigned, so it doesn't seem like the devicetree
> >> should be specifying that assignment.  
> > 
> > Note the device tree isn't limited as I didn't adapt the binding. It's
> > just that the driver doesn't bind if too many channels are specified.
> > And while your statement about the channels being a dynamic resource is
> > right, currently the driver doesn't cope and allocates resources
> > statically, and happily assumes there is a CHANNEL_16 register if the
> > device tree specifies 17 (or more) logical channels and writes to
> > CONFIG_0 then which very likely results in strange effects.
> > 
> > So as long as the driver doesn't implement this (possible) dynamic
> > mapping to the CHANNEL registers, it's IMHO right to refuse to bind (or
> > alternatively only use the 16 first logical channels).
> > 
> > Best regards
> > Uwe  
> 
> Understood. It would be nice to implement such dynamic allocation
> in the future but as a fix to backport to stable kernels, this makes
> sense.

Agreed.  We do have other drivers that have internal allocators
for constrained sequencer resources, but they are complex so not
fix material!

Jonathan