linux-kernel - Re: [PATCH 6/8] dma-direct: turn ARCH_ZONE_DMA

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Date:   Thu, 01 Aug 2019 17:59:34 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     catalin.marinas@....com, wahrenst@....net, marc.zyngier@....com,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-mm@...ck.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        phill@...pberryi.org, f.fainelli@...il.com, will@...nel.org,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org, eric@...olt.net,
        mbrugger@...e.com, akpm@...ux-foundation.org,
        frowand.list@...il.com, linux-rpi-kernel@...ts.infradead.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 6/8] dma-direct: turn ARCH_ZONE_DMA_BITS into a variable

Hi Christoph, thanks for the review.

On Thu, 2019-08-01 at 16:04 +0200, Christoph Hellwig wrote:
> A few nitpicks, otherwise this looks great:
> 
> > @@ -201,7 +202,7 @@ static int __init mark_nonram_nosave(void)
> >   * everything else. GFP_DMA32 page allocations automatically fall back to
> >   * ZONE_DMA.
> >   *
> > - * By using 31-bit unconditionally, we can exploit ARCH_ZONE_DMA_BITS to
> > + * By using 31-bit unconditionally, we can exploit arch_zone_dma_bits to
> >   * inform the generic DMA mapping code.  32-bit only devices (if not
> > handled
> >   * by an IOMMU anyway) will take a first dip into ZONE_NORMAL and get
> >   * otherwise served by ZONE_DMA.
> > @@ -237,9 +238,18 @@ void __init paging_init(void)
> >  	printk(KERN_DEBUG "Memory hole size: %ldMB\n",
> >  	       (long int)((top_of_ram - total_ram) >> 20));
> >  
> > +	/*
> > +	 * Allow 30-bit DMA for very limited Broadcom wifi chips on many
> > +	 * powerbooks.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_PPC32))
> > +		arch_zone_dma_bits = 30;
> > +	else
> > +		arch_zone_dma_bits = 31;
> > +
> 
> So the above unconditionally comment obviously isn't true any more, and
> Ben also said for the recent ppc32 hack he'd prefer dynamic detection.
> 
> Maybe Ben and or other ppc folks can chime in an add a patch to the series
> to sort this out now that we have a dynamic ZONE_DMA threshold?

Noted, for now I'll remove the comment.

> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 59bdceea3737..40dfc9b4ee4c 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -19,9 +19,7 @@
> >   * Most architectures use ZONE_DMA for the first 16 Megabytes, but
> >   * some use it for entirely different regions:
> >   */
> > -#ifndef ARCH_ZONE_DMA_BITS
> > -#define ARCH_ZONE_DMA_BITS 24
> > -#endif
> > +unsigned int arch_zone_dma_bits __ro_after_init = 24;
> 
> I'd prefer to drop the arch_ prefix and just calls this zone_dma_bits.
> In the long run we really need to find a way to just automatically set
> this from the meminit code, but that is out of scope for this series.
> For now can you please just update the comment above to say something
> like:
> 
> /*
>  * Most architectures use ZONE_DMA for the first 16 Megabytes, but some use it
>  * it for entirely different regions.  In that case the arch code needs to
>  * override the variable below for dma-direct to work properly.
>  */

Ok perfect.


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