Re: [PATCH v2] V4L: add media bus configuration subdev operations

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On Wed, 29 Jun 2011, Sakari Ailus wrote:

> Guennadi Liakhovetski wrote:
> > On Mon, 27 Jun 2011, Guennadi Liakhovetski wrote:
> > 
> > [snip]
> > 
> >>> If the structures are expected to be generic I somehow feel that a field of
> >>> flags isn't the best way to describe the configuration of CSI-2 or other
> >>> busses. Why not to just use a structure with bus type and an union for
> >>> bus-specific configuration parameters? It'd be easier to access and also to
> >>> change as needed than flags in an unsigned long field.
> >>
> >> Well, yes, a union can be a good idea, thanks.
> > 
> > ...on a second thought, we currently only have one field: flags, and it is 
> > common for all 3 bus types: parallel, reduced parallel (bt.656, etc.), and 
> > CSI-2. In the future, when we need more parameters for any of these busses 
> > we'll just add such a union, shouldn't be a problem.
> 
> What I meant above was that I would prefer to describe the capabilities
> in a structure which would contain appropriate data type for the field,
> not as flags or sets of flags in a bit field.
> 
> This would allow e.g. just testing for
> v4l2_mbus_config.u.parallel.hsync_active_low instead of
> v4l2_mbus_config.flags & V4L2_MBUS_HSYNC_ACTIVE_LOW. This way the flags
> used for the bus also express the bus explicitly rather than implicitly.

I don't think, using flags is any less explicit, than using struct 
members. As I already explained, I'd like to use the same interface for 
querying capabilities, the present configuration, and setting a 
configuration. This is easily achieved with flags. See the 
v4l2_mbus_config_compatible() function and try to rewrite all the bitwise 
operations in it, using your struct.

Thanks
Guennadi

> 
> Do you see downsides with this compared to using an integer field as
> flags? The other benefits of this are described in my earlier comment.
> 
> Regards,
> 
> -- 
> Sakari Ailus
> sakari.ailus@xxxxxx
> 

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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