linux-kernel - Re: [PATCH v2 01/13] scatterlist: introduce sg_nents_for

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Message-ID: <jea2owcqtjeomlbwkfopt3ujsnakn4p3xeyqhh7s4kowf7k7dr@deyg5pky5udo>
Date: Mon, 10 Nov 2025 09:21:18 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Stefan Wahren <wahrenst@....net>, Vinod Koul <vkoul@...nel.org>, 
	Thomas Andreatta <thomasandreatta2000@...il.com>, Caleb Sander Mateos <csander@...estorage.com>, 
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-rpi-kernel@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-arm-msm@...r.kernel.org, Olivier Dautricourt <olivierdautricourt@...il.com>, 
	Stefan Roese <sr@...x.de>, Florian Fainelli <florian.fainelli@...adcom.com>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, Ray Jui <rjui@...adcom.com>, 
	Scott Branden <sbranden@...adcom.com>, Lars-Peter Clausen <lars@...afoo.de>, 
	Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>, Daniel Mack <daniel@...que.org>, 
	Haojian Zhuang <haojian.zhuang@...il.com>, Robert Jarzmik <robert.jarzmik@...e.fr>, 
	Lizhi Hou <lizhi.hou@....com>, Brian Xu <brian.xu@....com>, 
	Raj Kumar Rampelli <raj.kumar.rampelli@....com>, Michal Simek <michal.simek@....com>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 01/13] scatterlist: introduce sg_nents_for_dma() helper

On Mon, Nov 10, 2025 at 09:05:26AM -0600, Bjorn Andersson wrote:
> On Mon, Nov 10, 2025 at 11:23:28AM +0100, Andy Shevchenko wrote:
> > 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 <andriy.shevchenko@...ux.intel.com>
> > ---
> >  include/linux/scatterlist.h |  2 ++
> >  lib/scatterlist.c           | 25 +++++++++++++++++++++++++
> >  2 files changed, 27 insertions(+)
> > 
> > diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> > index 29f6ceb98d74..6de1a2434299 100644
> > --- a/include/linux/scatterlist.h
> > +++ b/include/linux/scatterlist.h
> > @@ -441,6 +441,8 @@ static inline void sg_init_marker(struct scatterlist *sgl,
> >  
> >  int sg_nents(struct scatterlist *sg);
> >  int sg_nents_for_len(struct scatterlist *sg, u64 len);
> > +int sg_nents_for_dma(struct scatterlist *sgl, unsigned int sglen, size_t len);
> > +
> >  struct scatterlist *sg_last(struct scatterlist *s, unsigned int);
> >  void sg_init_table(struct scatterlist *, unsigned int);
> >  void sg_init_one(struct scatterlist *, const void *, unsigned int);
> > diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> > index 4af1c8b0775a..4d1a010f863c 100644
> > --- a/lib/scatterlist.c
> > +++ b/lib/scatterlist.c
> > @@ -64,6 +64,31 @@ int sg_nents_for_len(struct scatterlist *sg, u64 len)
> >  }
> >  EXPORT_SYMBOL(sg_nents_for_len);
> >  
> > +/**
> > + * sg_nents_for_dma - return the count of DMA-capable entries in scatterlist
> > + * @sgl:	The scatterlist
> > + * @sglen:	The current number of entries
> > + * @len:	The maximum length of DMA-capable block
> > + *
> > + * Description:
> > + * Determines the number of entries in @sgl which would be permitted in
> > + * DMA-capable transfer if list had been split accordingly, taking into
> > + * account chaining as well.
> > + *
> > + * Returns:
> > + *   the number of sgl entries needed
> > + *
> > + **/
> > +int sg_nents_for_dma(struct scatterlist *sgl, unsigned int sglen, size_t len)

All but two clients store the value in an unsigned int. Changing the
return type to unsigned int also signals that the function is just
returning a count (no errors).

Regards,
Bjorn

> > +{
> > +	struct scatterlist *sg;
> > +	int i, nents = 0;
> > +
> > +	for_each_sg(sgl, sg, sglen, i)
> > +		nents += DIV_ROUND_UP(sg_dma_len(sg), len);
> > +	return nents;
> > +}
> 
> We need an EXPORT_SYMBOL() here.
> 
> With that, this looks good to me.
> 
> Reviewed-by: Bjorn Andersson <andersson@...nel.org>
> 
> Regards,
> Bjorn
> 
> > +
> >  /**
> >   * sg_last - return the last scatterlist entry in a list
> >   * @sgl:	First entry in the scatterlist
> > -- 
> > 2.50.1
> > 
> > 
>