linux-kernel - Re: [PATCH] lib/scatterlist: introduce sg_nents_for

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Date:   Wed, 27 Feb 2019 16:15:37 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Ardelean, Alexandru" <Alex.Ardelean@...log.com>
Cc:     "vkoul@...nel.org" <vkoul@...nel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "bhole_prashant_q7@....ntt.co.jp" <bhole_prashant_q7@....ntt.co.jp>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "khandual@...ux.vnet.ibm.com" <khandual@...ux.vnet.ibm.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [PATCH] lib/scatterlist: introduce sg_nents_for_dma() helper

On Wed, Feb 27, 2019 at 2:26 PM Ardelean, Alexandru
<Alex.Ardelean@...log.com> wrote:
>
> On Wed, 2019-02-27 at 11:46 +0200, Andy Shevchenko wrote:
> > [External]
> >
> >
> > +Cc Vinod
> >
> > On Wed, Feb 27, 2019 at 11:45 AM Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > >
> > > On Wed, Feb 27, 2019 at 10:51 AM Alexandru Ardelean
> > > <alexandru.ardelean@...log.com> wrote:
> > > >
> > > > From: Andy Shevchenko <andy.shevchenko@...il.com>
> > > >
> > > > Sometimes the user needs to split each entry on the mapped scatter
> > > > list
> > > > due to DMA length constrains. This helper returns a number of
> > > > entities
> > > > assuming that each of them is not bigger than supplied maximum
> > > > length.
> > > >
> > > > Signed-off-by: Andy Shevchenko <andy.shevchenko@...il.com>
> > > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> > >
> > > Hmm... Usually we don't accept generic API without users.
> > > Do you have any use case in mind?
>
> Yep: this one:
> https://patchwork.kernel.org/patch/10814527/
>
> But, I can rework this patch to work without the helper.

Ah, okay, worth to mention this in comments (in addition to what you
put about the patch below).

> > > > Patch was sent in 2016 initially to the DMA engine sub-system.
> > > > Link:
> > > >   https://patchwork.kernel.org/patch/9389821/
> > > > This was part of a larger series:
> > > >
> > > > https://patchwork.kernel.org/project/linux-dmaengine/list/?q=sg_nents_for_dma&archive=both&series=&submitter=&delegate=&state=*
> > > >
> > > > I'm not sure if this is supposed to go into DMAEngine or
> > > > lib/scatterlist.
> > > > It doesn't look like lib/scatterlist is managed by DMAEngine, so (by
> > > > using
> > > > the `get_maintainers.pl` script) I'm sending this patch to this group
> > > > of
> > > > parties.
> > >
> > > The problem the patch tried to solve is much deeper and correct
> > > solution should be more generic, i.e.
> > > each channel should provide a set of parameters, such as DMA segment
> > > size, to the users (via DMA engine API) and users should prepare the
> > > SG list according to the limits of the channel.
> > > In that case we don't need to re-split/re-allocate given SG list at
> > > all, which would simplify DMA slave drivers and their users.
> > >
>
> I don't think I managed to follow [or find] the full length of that
> discussion. Or, the conclusion wasn't that obvious to me, from what I
> found.
> I assumed this may have been dropped/forgotten.
>
> In any case, I am fine with just reworking.

It's fine to apply it, my point that it help to solve a symptom in a
short-term (however this short-term can be easily a long one due to no
guarantee that all drivers using SG for DMA will be converted).
So, please, update the comments and resubmit as a new version.

-- 
With Best Regards,
Andy Shevchenko