Re: [PATCH 6/8] dma-direct: turn ARCH_ZONE_DMA_BITS into a variable

From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To: Christoph Hellwig <hch@lst.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	eric@anholt.net, Paul Mackerras <paulus@samba.org>,
	will@kernel.org, phill@raspberryi.org,
	linux-s390@vger.kernel.org, f.fainelli@gmail.com,
	frowand.list@gmail.com, Michael Ellerman <mpe@ellerman.id.au>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	catalin.marinas@arm.com, devicetree@vger.kernel.org,
	Vasily Gorbik <gor@linux.ibm.com>,
	marc.zyngier@arm.com, robh+dt@kernel.org,
	linux-rpi-kernel@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org, mbrugger@suse.com,
	linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	wahrenst@gmx.net, akpm@linux-foundation.org,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 6/8] dma-direct: turn ARCH_ZONE_DMA_BITS into a variable
Date: Thu, 01 Aug 2019 17:59:34 +0200	[thread overview]
Message-ID: <ed5388412df78ad0a9ed69cdf3ac716eac075141.camel@suse.de> (raw)
In-Reply-To: <20190801140452.GB23435@lst.de>

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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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