netdev - Re: [PATCH net-next v3 01/12] net: wwan: t7xx: Add control DMA interface

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Date:   Wed, 12 Jan 2022 20:16:38 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc:     "Martinez, Ricardo" <ricardo.martinez@...ux.intel.com>,
        Netdev <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org,
        kuba@...nel.org, davem@...emloft.net, johannes@...solutions.net,
        ryazanov.s.a@...il.com, loic.poulain@...aro.org,
        m.chetan.kumar@...el.com, chandrashekar.devegowda@...el.com,
        linuxwwan@...el.com, chiranjeevi.rapolu@...ux.intel.com,
        haijun.liu@...iatek.com, amir.hanania@...el.com,
        dinesh.sharma@...el.com, eliot.lee@...el.com,
        mika.westerberg@...ux.intel.com, moises.veleta@...el.com,
        pierre-louis.bossart@...el.com, muralidharan.sethuraman@...el.com,
        Soumya.Prakash.Mishra@...el.com, sreehari.kancharla@...el.com
Subject: Re: [PATCH net-next v3 01/12] net: wwan: t7xx: Add control DMA
 interface

On Wed, 12 Jan 2022, Andy Shevchenko wrote:

> On Wed, Jan 12, 2022 at 04:24:52PM +0200, Ilpo Järvinen wrote:
> > On Wed, 12 Jan 2022, Andy Shevchenko wrote:
> > > On Tue, Jan 11, 2022 at 08:55:58PM -0800, Martinez, Ricardo wrote:
> > > > On 12/16/2021 3:08 AM, Ilpo Järvinen wrote:
> > > > > On Mon, 6 Dec 2021, Ricardo Martinez wrote:
> > > 
> > > > > > +	if (req->entry.next == &ring->gpd_ring)
> > > > > > +		return list_first_entry(&ring->gpd_ring, struct cldma_request, entry);
> > > > > > +
> > > > > > +	return list_next_entry(req, entry);
> > > 
> > > ...
> > > 
> > > > > > +	if (req->entry.prev == &ring->gpd_ring)
> > > > > > +		return list_last_entry(&ring->gpd_ring, struct cldma_request, entry);
> > > > > > +
> > > > > > +	return list_prev_entry(req, entry);
> > > 
> > > ...
> > > 
> > > > > Wouldn't these two seems generic enough to warrant adding something like
> > > > > list_next/prev_entry_circular(...) to list.h?
> > > > 
> > > > Agree, in the upcoming version I'm planning to include something like this
> > > > to list.h as suggested:
> > > 
> > > I think you mean for next and prev, i.o.w. two helpers, correct?
> > > 
> > > > #define list_next_entry_circular(pos, ptr, member) \

One thing I missed earlier, the sigrature should instead of ptr have head:
#define list_next_entry_circular(pos, head, member)

> > > >     ((pos)->member.next == (ptr) ? \
> > > 
> > > I believe this is list_entry_is_head().
> > 
> > It takes .next so it's not the same as list_entry_is_head() and 
> > list_entry_is_last() doesn't exist.
> 
> But we have list_last_entry(). So, what about
> 
> list_last_entry() == pos ? first : next;
> 
> and counterpart
> 
> list_first_entry() == pos ? last : prev;
> 
> ?

Yes, although now that I think it more, using them implies the head 
element has to be always accessed. It might be marginally cache friendlier 
to use list_entry_is_head you originally suggested but get the next entry 
first:
({
	typeof(pos) next__ = list_next_entry(pos, member); \
	!list_entry_is_head(next__, head, member) ? next__ : list_next_entry(next__, member);
})
(This was written directly to email, entirely untested).

Here, the head element would only get accessed when we really need to walk 
through it.

> > > >     list_first_entry(ptr, typeof(*(pos)), member) : \
> > > >     list_next_entry(pos, member))

-- 
 i.