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From: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
To: catalin.marinas@arm.com, hch@lst.de, wahrenst@gmx.net,
	marc.zyngier@arm.com, robh+dt@kernel.org,
	Robin Murphy <robin.murphy@arm.com>,
	linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
	linux-arch@vger.kernel.org, iommu@lists.linux-foundation.org,
	linux-mm@kvack.org, linux-riscv@lists.infradead.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@sifive.com>,
	Albert Ou <aou@eecs.berkeley.edu>
Cc: phill@raspberryi.org, f.fainelli@gmail.com,
	frowand.list@gmail.com, linux-kernel@vger.kernel.org,
	eric@anholt.net, mbrugger@suse.com,
	linux-rpi-kernel@lists.infradead.org, akpm@linux-foundation.org,
	will@kernel.org, nsaenzjulienne@suse.de
Subject: [PATCH v2 11/11] mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum zone_type'
Date: Tue, 20 Aug 2019 16:58:19 +0200
Message-ID: <20190820145821.27214-12-nsaenzjulienne@suse.de> (raw)
In-Reply-To: <20190820145821.27214-1-nsaenzjulienne@suse.de>

These zones usage has evolved with time and the comments were outdated.
This joins both ZONE_DMA and ZONE_DMA32 explanation and gives up to date
examples on how they are used on different architectures.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

---

Changes in v2:
- Try another approach merging both zones explanations into one
- Address Christoph's comments
- If this approach doesn't get much traction I'll just drop the patch
  from the series as it's not really essential

 include/linux/mmzone.h | 46 +++++++++++++++++++++++++-----------------
 1 file changed, 27 insertions(+), 19 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d77d717c620c..9c150223d41f 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -356,33 +356,41 @@ struct per_cpu_nodestat {
 #endif /* !__GENERATING_BOUNDS.H */
 
 enum zone_type {
-#ifdef CONFIG_ZONE_DMA
 	/*
-	 * ZONE_DMA is used when there are devices that are not able
-	 * to do DMA to all of addressable memory (ZONE_NORMAL). Then we
-	 * carve out the portion of memory that is needed for these devices.
-	 * The range is arch specific.
+	 * ZONE_DMA and ZONE_DMA32 are used when there are peripherals not able
+	 * to DMA to all of the addressable memory (ZONE_NORMAL).
+	 * On architectures where this area covers the whole 32 bit address
+	 * space ZONE_DMA32 is used. ZONE_DMA is left for the ones with smaller
+	 * DMA addressing constraints. This distinction is important as a 32bit
+	 * DMA mask is assumed when ZONE_DMA32 is defined. Some 64-bit
+	 * platforms may need both zones as they support peripherals with
+	 * different DMA addressing limitations.
+	 *
+	 * Some examples:
+	 *
+	 *  - i386 and x86_64 have a fixed 16M ZONE_DMA and ZONE_DMA32 for the
+	 *    rest of the lower 4G.
+	 *
+	 *  - arm only uses ZONE_DMA, the size, up to 4G, may vary depending on
+	 *    the specific device.
+	 *
+	 *  - powerpc only uses ZONE_DMA, the size, up to 2G, may vary
+	 *    depending on the specific device.
 	 *
-	 * Some examples
+	 *  - s390 uses ZONE_DMA fixed to the lower 2G.
 	 *
-	 * Architecture		Limit
-	 * ---------------------------
-	 * parisc, ia64, sparc	<4G
-	 * s390, powerpc	<2G
-	 * arm			Various
-	 * alpha		Unlimited or 0-16MB.
+	 *  - arm64 uses ZONE_DMA to mark the area addresable by all
+	 *    peripherals on the device, and ZONE_DMA32 for the rest of the
+	 *    lower 4G. ZONE_DMA32 might be left empty.
 	 *
-	 * i386, x86_64 and multiple other arches
-	 * 			<16M.
+	 *  - ia64 and riscv only use ZONE_DMA32.
+	 *
+	 *  - parisc uses neither.
 	 */
+#ifdef CONFIG_ZONE_DMA
 	ZONE_DMA,
 #endif
 #ifdef CONFIG_ZONE_DMA32
-	/*
-	 * x86_64 needs two ZONE_DMAs because it supports devices that are
-	 * only able to do DMA to the lower 16M but also 32 bit devices that
-	 * can only do DMA areas below 4G.
-	 */
 	ZONE_DMA32,
 #endif
 	/*
-- 
2.22.0

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  parent reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-20 14:58 [PATCH v2 00/11] Raspberry Pi 4 DMA addressing support Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 01/11] asm-generic: add dma_zone_size Nicolas Saenz Julienne
2019-08-26  7:09   ` Christoph Hellwig
2019-08-26 13:46     ` Nicolas Saenz Julienne
2019-08-28  9:44       ` Nicolas Saenz Julienne
2019-08-30 14:45       ` Catalin Marinas
2019-08-30 17:24         ` Nicolas Saenz Julienne
2019-09-02 13:01           ` Christoph Hellwig
2019-08-20 14:58 ` [PATCH v2 02/11] arm: use generic dma_zone_size Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 03/11] of/fdt: add of_fdt_machine_is_compatible function Nicolas Saenz Julienne
2019-08-20 17:16   ` Rob Herring
2019-08-20 17:27     ` Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 04/11] of/fdt: add early_init_dt_get_dma_zone_size() Nicolas Saenz Julienne
2019-08-20 17:14   ` Rob Herring
2019-08-20 14:58 ` [PATCH v2 05/11] arm64: mm: use arm64_dma_phys_limit instead of calling max_zone_dma_phys() Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 06/11] arm64: rename variables used to calculate ZONE_DMA32's size Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 07/11] arm64: re-introduce max_zone_dma_phys() Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 08/11] arm64: use both ZONE_DMA and ZONE_DMA32 Nicolas Saenz Julienne
2019-08-20 14:58 ` [PATCH v2 09/11] dma-direct: turn ARCH_ZONE_DMA_BITS into a variable Nicolas Saenz Julienne
2019-08-26  7:05   ` Christoph Hellwig
2019-08-26 11:33   ` Michael Ellerman
2019-08-20 14:58 ` [PATCH v2 10/11] arm64: edit zone_dma_bits to fine tune dma-direct min mask Nicolas Saenz Julienne
2019-08-26  7:06   ` Christoph Hellwig
2019-08-26 11:08     ` Nicolas Saenz Julienne
2019-08-27  7:03       ` Petr Tesarik
2019-08-20 14:58 ` Nicolas Saenz Julienne [this message]
2019-08-26  7:07   ` [PATCH v2 11/11] mm: refresh ZONE_DMA and ZONE_DMA32 comments in 'enum zone_type' Christoph Hellwig

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