linux-kernel - Re: [PATCH 02/14] soundwire: Add SoundWire bus type

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Date:   Fri, 20 Oct 2017 17:50:57 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Vinod Koul <vinod.koul@...el.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        ALSA <alsa-devel@...a-project.org>, Mark <broonie@...nel.org>,
        Pierre <pierre-louis.bossart@...ux.intel.com>,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Shreyas NC <shreyas.nc@...el.com>, patches.audio@...el.com,
        alan@...ux.intel.com,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Sagar Dharia <sdharia@...eaurora.org>,
        srinivas.kandagatla@...aro.org, plai@...eaurora.org,
        Sudheer Papothi <spapothi@...eaurora.org>
Subject: Re: [PATCH 02/14] soundwire: Add SoundWire bus type

On Fri, 20 Oct 2017 17:46:49 +0200,
Vinod Koul wrote:
> 
> On Fri, Oct 20, 2017 at 08:59:42AM +0200, Takashi Iwai wrote:
> > On Fri, 20 Oct 2017 07:11:01 +0200,
> > Vinod Koul wrote:
> > > 
> > > On Thu, Oct 19, 2017 at 09:40:06AM +0200, Takashi Iwai wrote:
> > > > On Thu, 19 Oct 2017 05:03:18 +0200,
> > > > Vinod Koul wrote:
> > > > > +
> > > > > +config SOUNDWIRE_BUS
> > > > > +	tristate
> > > > > +	default SOUNDWIRE
> > > > > +
> > > > 
> > > > Does it make sense to be tristate?
> > > > Since CONFIG_SOUNDWIRE is a bool, the above would be also only either
> > > > Y or N.  If it's Y and others select M, it'll be still Y.
> > > 
> > > hmmm good point. I think would make sense to make SOUNDWIRE as tristate too,
> > > just like SOUND :)
> > 
> > It's one option.  Another would be to simply drop the "default" line.
> 
> Okay good suggestion
> 
> > > > > + * The match is done by comparing the mfg_id and part_id from the
> > > > > + * struct sdw_device_id. class_id is unused, as it is a placeholder
> > > > > + * in MIPI Spec.
> > > > > + */
> > > > > +static const struct sdw_device_id *
> > > > > +sdw_get_device_id(struct sdw_slave *slave, struct sdw_driver *drv)
> > > > > +{
> > > > > +	const struct sdw_device_id *id = drv->id_table;
> > > > > +
> > > > > +	while (id && id->mfg_id) {
> > > > > +		if (slave->id.mfg_id == id->mfg_id &&
> > > > > +				slave->id.part_id == id->part_id) {
> > > > 
> > > > Please indentation properly.
> > > 
> > > what do you advise?
> > > 
> > > 		if (slave->id.mfg_id == id->mfg_id &&
> > > 			slave->id.part_id == id->part_id) {
> > > 
> > > would mean below one is at same indent. Some people use:
> > > 
> > > 		if (slave->id.mfg_id == id->mfg_id &&
> > > 		   slave->id.part_id == id->part_id) {
> > > 
> > > Is it Documented anywhere...
> > 
> > This is a matter of taste.  The latter is the way Emacs or indent does
> > as default.
> 
> okay as vi user I will try to do above :)
> 
> > 
> > > > > --- a/include/linux/mod_devicetable.h
> > > > > +++ b/include/linux/mod_devicetable.h
> > > > > @@ -228,6 +228,13 @@ struct hda_device_id {
> > > > >  	unsigned long driver_data;
> > > > >  };
> > > > >  
> > > > > +struct sdw_device_id {
> > > > > +	__u16 mfg_id;
> > > > > +	__u16 part_id;
> > > > > +	__u8 class_id;
> > > > > +	kernel_ulong_t driver_data;
> > > > 
> > > > Better to think of alignment.
> > > 
> > > sorry not quite clear, do you mind elaborating which ones to align?
> > 
> > kernel_ulong_t may be aligned to 4 or 8 bytes, depending on
> > architecture, so there can be a hole between class_id and driver_data.
> > It's not an ABI, so we don't have to care too much, but it's still
> > something exposed, hence better to be conscious about alignment.
> 
> ah :) is that why hda is unsigned long :) Btw doesnt that cause compat
> issues, should we not do something like u64 here?

Oh, don't take the HD-audio case as a good reference, it's a bad guy
;)  In the case of hda, the definition isn't really exposed.

The alignment doesn't matter whether it's unsigned long or
kernel_ulong_t.  It's a generic issue when you define some struct and
expose it.  In a safer side, you can put the enough pad bytes so that
the long field is aligned in 8 bytes.  Or use packed struct.  Or you
can just ignore and let it be so, but aware of the possible holes in
your code.


Takashi