linux-kernel - Re: [PATCH v3 10/10] dmaengine: dw: Initialize max_sg

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Message-ID: <CAHp75Vd0ujdxzXtE3-XGxrbJHBJqj6suBvN4VUQbApPSNCstmg@mail.gmail.com>
Date:   Thu, 28 May 2020 23:31:09 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Vinod Koul <vkoul@...nel.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, linux-mips@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        dmaengine <dmaengine@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 10/10] dmaengine: dw: Initialize max_sg_nents with
 nollp flag

On Thu, May 28, 2020 at 6:52 PM Serge Semin
<Sergey.Semin@...kalelectronics.ru> wrote:
> On Thu, May 28, 2020 at 05:56:30PM +0300, Andy Shevchenko wrote:
> > On Wed, May 27, 2020 at 01:50:21AM +0300, Serge Semin wrote:

...

> > In principal I agree, one nit below.
> > If you are okay with it, feel free to add my Rb tag.

> > > +   /*
> > > +    * It might be crucial for some devices to have the hardware
> > > +    * accelerated multi-block transfers supported, aka LLPs in DW DMAC
> > > +    * notation. So if LLPs are supported then max_sg_nents is set to
> > > +    * zero which means unlimited number of SG entries can be handled in a
> > > +    * single DMA transaction, otherwise it's just one SG entry.
> > > +    */
> >
> > > +   caps->max_sg_nents = dwc->nollp;
> >
>
> > To be on the safer side I would explicitly do it like
> >
> >       if (dwc->nollp)
> >        /* your nice comment */
> >        = 1;
> >       else
> >        /* Unlimited */
> >        = 0;
> >
> > type or content of nollp theoretically can be changed and this will affect maximum segments.
>
> Agree. Though I don't like formatting you suggested. If I add my nice comment
> between if-statement and assignment the the former will be look detached from
> the if-statement, which seems a bit ugly. So I'd leave the comment above the
> whole if-else statement, especially seeing I've already mentioned there about
> the unlimited number of SG entries there.
>
>         /*
>          * It might be crucial for some devices to have the hardware
>          * accelerated multi-block transfers supported, aka LLPs in DW DMAC
>          * notation. So if LLPs are supported then max_sg_nents is set to
>          * zero which means unlimited number of SG entries can be handled in a
>          * single DMA transaction, otherwise it's just one SG entry.
>          */
>         if (dwc->nollp)
>                 caps->max_sg_nents = 1;
>         else
>                 caps->max_sg_nents = 0;

Fine with me, thanks!

-- 
With Best Regards,
Andy Shevchenko