linux-kernel - Re: [PATCH 3/4] uio: introduce devicetree bindings for uio_dmem

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Date:	Thu, 19 May 2016 10:45:56 +0200
From:	Jan Viktorin <viktorin@...ivetech.com>
To:	Rob Herring <robh@...nel.org>
Cc:	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"Hans J. Koch" <hjk@...sjkoch.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/4] uio: introduce devicetree bindings for
 uio_dmem_genirq

Hello Rob,

thank you for your opinion...

On Wed, 18 May 2016 12:01:05 -0500
Rob Herring <robh@...nel.org> wrote:

> On Tue, May 17, 2016 at 11:22:19AM +0200, Jan Viktorin wrote:
> > Signed-off-by: Jan Viktorin <viktorin@...ivetech.com>
> > ---
> >  .../devicetree/bindings/uio/uio_dmem_genirq.txt          | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/uio/uio_dmem_genirq.txt  
> 
> DT describes h/w. UIO is not a h/w block, so this does not belong in DT. 
> A UIO vs. kernel driver is purely a kernel decision which shouldn't 
> require a DT change.

I mostly agree and I don't say this is the very best solution... however,
it is quite a straightforward one.

> 
> The properties should be part of match data for a compatible string that 

True. But in case of probing from DT, where can I obtain those match data
from? I have to patch the kernel... and that is what I tried to avoid.

With this patch set, you only modify your current device-tree (extend the
appropriate devices by "uio,...") and reboot with this new one.

> needs them set. Or if they can be defined in a way that is actually a 
> property of the h/w, then it would be acceptible. You'd still need to 
> define compatible strings that the properties apply to.

If you look at uio_pdrv_genirq you can see that it has already been extended by
the module param "of_id". I.e. it is possible to specify the compatible property
it would match when doing insmod. So, it is possible to bind it to any platform
device described by the device tree. And thus, there is no way how to define a
list of compatible strings for it... (quite a long list, isn't it?)

The same (of_id) can be done for uio_dmem_genirq, however, there is no way how to
specify the amount of dynamic memory to be used for a specific device. For me, it
makes sense to use DT to obtain those two properties saying "those devices would
use this amount of memory and not more".

Perhaps, another module param is a way to go here. Something like of_dmem_count=2,
of_dmem_sizes=32k. Less flexible solution, however, if it is acceptable I'll rewrite
the current one.

Jan

> 
> Rob