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

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Message-ID: <CAHp75VdgwM_ybfymh28iqxu2qOfzZo79VOF+XE+brj70Aji6YQ@mail.gmail.com>
Date:   Wed, 27 Feb 2019 11:46:11 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        bhole_prashant_q7@....ntt.co.jp, Kees Cook <keescook@...omium.org>,
        john.fastabend@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/scatterlist: introduce sg_nents_for_dma() helper

+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?
>
> > 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.
>
> We discussed this topic back in 2016 with Vinod on LPC, but seems it's
> not critical to solve. My case is to improve DMA performance for 8250
> UART.
>
> --
> With Best Regards,
> Andy Shevchenko



-- 
With Best Regards,
Andy Shevchenko