linux-kernel - Re: [PATCH v4 1/5] counter: Internalize sysfs interface code

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Date:   Sat, 15 Aug 2020 12:33:11 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     David Lechner <david@...hnology.com>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        David.Laight@...LAB.COM
Subject: Re: [PATCH v4 1/5] counter: Internalize sysfs interface code

On Mon, Aug 10, 2020 at 05:48:07PM -0500, David Lechner wrote:
> 
> >>>>>     
> >>>>>     CPMAC ETHERNET DRIVER
> >>>>>     M:	Florian Fainelli <f.fainelli@...il.com>
> >>>>> diff --git a/drivers/counter/104-quad-8.c b/drivers/counter/104-quad-8.c
> >>>>> index 78766b6ec271..0f20920073d6 100644
> >>>>> --- a/drivers/counter/104-quad-8.c
> >>>>> +++ b/drivers/counter/104-quad-8.c
> >>>>> @@ -621,7 +621,7 @@ static const struct iio_chan_spec quad8_channels[] = {
> >>>>>     };
> >>>>>     
> >>>>>     static int quad8_signal_read(struct counter_device *counter,
> >>>>> -	struct counter_signal *signal, enum counter_signal_value *val)
> >>>>> +			     struct counter_signal *signal, u8 *val)
> >>>>
> >>>> I'm not a fan of replacing enum types with u8 everywhere in this patch.
> >>>> But if we have to for technical reasons (e.g. causes compiler error if
> >>>> we don't) then it would be helpful to add comments giving the enum type
> >>>> everywhere like this instance where u8 is actually an enum value.
> >>>>
> >>>> If we use u32 as the generic type for enums instead of u8, I think the
> >>>> compiler will happlily let us use enum type and u32 interchangeably and
> >>>> not complain.
> >>>
> >>> I switched to fixed-width types after the suggestion by David Laight:
> >>> https://lkml.org/lkml/2020/5/3/159. I'll CC David Laight just in case he
> >>> wants to chime in again.
> >>>
> >>> Enum types would be nice for making the valid values explicit, but there
> >>> is one benefit I have appreciated from the move to fixed-width types:
> >>> there has been a significant reduction of duplicate code; before, we had
> >>> a different read function for each different enum type, but now we use a
> >>> single function to handle them all.
> >>
> >> Yes, what I was trying to explain is that by using u32 instead of u8, I
> >> think we can actually do both.
> >>
> >> The function pointers in struct counter_device *counter would use u32 as a
> >> generic enum value in the declaration, but then the actual implementations
> >> could still use the proper enum type.
> > 
> > Oh, I see what you mean now. So for example:
> > 
> >      int (*signal_read)(struct counter_device *counter,
> >                         struct counter_signal *signal, u8 *val)
> > 
> > This will become instead:
> > 
> >      int (*signal_read)(struct counter_device *counter,
> >                         struct counter_signal *signal, u32 *val)
> > 
> > Then in the driver callback implementation we use the enum type we need:
> > 
> >      enum counter_signal_level signal_level = COUNTER_SIGNAL_HIGH;
> >      ...
> >      *val = signal_level;
> > 
> > Is that what you have in mind?
> > 
> 
> Yes.
> 
> Additionally, if we have...
> 
> 
>        int (*x_write)(struct counter_device *counter,
>                       ..., u32 val)
>   
> We should be able to define the implementation as:
> 
> static int my_driver_x_write(struct counter_device *counter,
>                               ..., enum some_type val)
> {
> 	...
> }
> 
> Not sure if it works if val is a pointer though. Little-
> endian systems would probably be fine, but maybe not big-
> endian combined with -fshort-enums compiler flag.
> 
> 
>        int (*x_read)(struct counter_device *counter,
>                      ..., u32 *val)
>   
> 
> static int my_driver_x_read(struct counter_device *counter,
>                              ..., enum some_type *val)
> {
> 	...
> }

Regardless of endianness for pointers, will targets that have
-fshort-enums enabled by default present a problem here? I imagine that
in these cases enum some_type will have a size of unsigned char because
that is the first type that can represent all the values:
https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html

What I'm worried about is whether we can gurantee u32 val can be swapped
out with enum some_type val -- or if this is not possible because some
architectures will be built with -fshort-enums enabled?

William Breathitt Gray

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