From: Oskar Schirmer <os@emlix.com> To: Mike Frysinger <vapier.adi@gmail.com> Cc: "Oskar Schirmer" <os@emlix.com>, "Dmitry Torokhov" <dmitry.torokhov@gmail.com>, "Andrew Morton" <akpm@linux-foundation.org>, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, "Daniel Glöckner" <dg@emlix.com>, "Oliver Schneidewind" <osw@emlix.com>, "Johannes Weiner" <jw@emlix.com> Subject: Re: [PATCH] ad7877: keep dma rx buffers in seperate cache lines Date: Fri, 7 May 2010 12:15:44 +0200 [thread overview] Message-ID: <20100507101544.GB25342@emlix.com> (raw) In-Reply-To: <g2j8bd0f97a1005061146l2b01e45do15bf46162e38ce84@mail.gmail.com> On Thu, May 06, 2010 at 14:46:04 -0400, Mike Frysinger wrote: > On Thu, May 6, 2010 at 06:37, Oskar Schirmer wrote: > > struct ser_req { > > + u16 sample; > > + char __padalign[L1_CACHE_BYTES - sizeof(u16)]; > > + > > u16 reset; > > u16 ref_on; > > u16 command; > > - u16 sample; > > struct spi_message msg; > > struct spi_transfer xfer[6]; > > }; > > are you sure this is necessary ? ser_req is only ever used with > spi_sync() and it's allocated/released on the fly, so how could > anything be reading that memory between the start of the transmission > and the return to adi7877 ? msg is handed over to spi_sync, it contains the addresses which will be used to programme the DMA: the spi master transfer function will read these fields to start DMA. E.g., drivers/spi/atmel_spi.c, assures cache coherency in function atmel_spi_dma_map_xfer, one xfer at a time. Multiple transfers are worked on in a loop in atmel_spi_transfer, so when coherency for transfer #1 is ensured, addresses for transfer #2 are read from the msg and xfer structs, caching lines which have been just before invalidated. And, citing Documentation/DMA-API.txt, Part Id: "mapped region must begin exactly on a cache line boundary and end exactly on one", which is our case. > > > struct ad7877 { > > + u16 conversion_data[AD7877_NR_SENSE]; > > + char __padalign[L1_CACHE_BYTES > > + - AD7877_NR_SENSE * sizeof(u16)]; > > + > > struct input_dev *input; > > char phys[32]; > > > > @@ -182,8 +188,6 @@ struct ad7877 { > > u8 averaging; > > u8 pen_down_acc_interval; > > > > - u16 conversion_data[AD7877_NR_SENSE]; > > - > > struct spi_transfer xfer[AD7877_NR_SENSE + 2]; > > struct spi_message msg; > > i can see the spi_message inside of this struct being a problem > because the spi transfer is doing asynchronously with spi_async(). > however, i would add a comment right above these two fields with a > short explanation as to why they're at the start and why the pad > exists so someone down the line doesnt move it. The code says "pad to align according to L1 cache, and keep away other stuff by exactly the amount so it is off the line". I'ld guess a comment would repeat just this, so it is superfluous. But if opinions differ on this topic, we can have a comment added, sure. Oskar -- oskar schirmer, emlix gmbh, http://www.emlix.com fon +49 551 30664-0, fax -11, bahnhofsallee 1b, 37081 göttingen, germany sitz der gesellschaft: göttingen, amtsgericht göttingen hr b 3160 geschäftsführer: dr. uwe kracke, ust-idnr.: de 205 198 055 emlix - your embedded linux partner
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From: Oskar Schirmer <os@emlix.com> To: Mike Frysinger <vapier.adi@gmail.com> Cc: "Oskar Schirmer" <os@emlix.com>, "Dmitry Torokhov" <dmitry.torokhov@gmail.com>, "Andrew Morton" <akpm@linux-foundation.org>, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, "Daniel Glöckner" <dg@emlix.com>, "Oliver Schneidewind" <osw@emlix.com>, "Johannes Weiner" <jw@emlix.com> Subject: Re: [PATCH] ad7877: keep dma rx buffers in seperate cache lines Date: Fri, 7 May 2010 12:15:44 +0200 [thread overview] Message-ID: <20100507101544.GB25342@emlix.com> (raw) In-Reply-To: <g2j8bd0f97a1005061146l2b01e45do15bf46162e38ce84@mail.gmail.com> On Thu, May 06, 2010 at 14:46:04 -0400, Mike Frysinger wrote: > On Thu, May 6, 2010 at 06:37, Oskar Schirmer wrote: > > struct ser_req { > > + u16 sample; > > + char __padalign[L1_CACHE_BYTES - sizeof(u16)]; > > + > > u16 reset; > > u16 ref_on; > > u16 command; > > - u16 sample; > > struct spi_message msg; > > struct spi_transfer xfer[6]; > > }; > > are you sure this is necessary ? ser_req is only ever used with > spi_sync() and it's allocated/released on the fly, so how could > anything be reading that memory between the start of the transmission > and the return to adi7877 ? msg is handed over to spi_sync, it contains the addresses which will be used to programme the DMA: the spi master transfer function will read these fields to start DMA. E.g., drivers/spi/atmel_spi.c, assures cache coherency in function atmel_spi_dma_map_xfer, one xfer at a time. Multiple transfers are worked on in a loop in atmel_spi_transfer, so when coherency for transfer #1 is ensured, addresses for transfer #2 are read from the msg and xfer structs, caching lines which have been just before invalidated. And, citing Documentation/DMA-API.txt, Part Id: "mapped region must begin exactly on a cache line boundary and end exactly on one", which is our case. > > > struct ad7877 { > > + u16 conversion_data[AD7877_NR_SENSE]; > > + char __padalign[L1_CACHE_BYTES > > + - AD7877_NR_SENSE * sizeof(u16)]; > > + > > struct input_dev *input; > > char phys[32]; > > > > @@ -182,8 +188,6 @@ struct ad7877 { > > u8 averaging; > > u8 pen_down_acc_interval; > > > > - u16 conversion_data[AD7877_NR_SENSE]; > > - > > struct spi_transfer xfer[AD7877_NR_SENSE + 2]; > > struct spi_message msg; > > i can see the spi_message inside of this struct being a problem > because the spi transfer is doing asynchronously with spi_async(). > however, i would add a comment right above these two fields with a > short explanation as to why they're at the start and why the pad > exists so someone down the line doesnt move it. The code says "pad to align according to L1 cache, and keep away other stuff by exactly the amount so it is off the line". I'ld guess a comment would repeat just this, so it is superfluous. But if opinions differ on this topic, we can have a comment added, sure. Oskar -- oskar schirmer, emlix gmbh, http://www.emlix.com fon +49 551 30664-0, fax -11, bahnhofsallee 1b, 37081 göttingen, germany sitz der gesellschaft: göttingen, amtsgericht göttingen hr b 3160 geschäftsführer: dr. uwe kracke, ust-idnr.: de 205 198 055 emlix - your embedded linux partner -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
next prev parent reply other threads:[~2010-05-07 10:15 UTC|newest] Thread overview: 124+ messages / expand[flat|nested] mbox.gz Atom feed top 2010-05-06 10:37 [PATCH] ad7877: fix spi word size to 16 bit Oskar Schirmer 2010-05-06 10:37 ` Oskar Schirmer 2010-05-06 10:37 ` [PATCH] ad7877: keep dma rx buffers in seperate cache lines Oskar Schirmer 2010-05-06 18:46 ` Mike Frysinger 2010-05-06 18:46 ` Mike Frysinger 2010-05-07 10:15 ` Oskar Schirmer [this message] 2010-05-07 10:15 ` Oskar Schirmer 2010-05-07 18:28 ` Mike Frysinger 2010-05-07 18:28 ` Mike Frysinger 2010-05-08 22:32 ` Johannes Weiner 2010-05-08 22:32 ` Johannes Weiner 2010-05-09 4:45 ` Mike Frysinger 2010-05-09 4:45 ` Mike Frysinger 2010-05-09 8:50 ` Oskar Schirmer 2010-05-09 8:50 ` Oskar Schirmer 2010-05-07 12:07 ` Johannes Weiner 2010-05-10 10:34 ` [PATCH v2] " Oskar Schirmer 2010-05-10 10:34 ` Oskar Schirmer 2010-05-10 10:42 ` [PATCH v3] " Oskar Schirmer 2010-05-10 10:42 ` Oskar Schirmer 2010-05-10 16:39 ` Mike Frysinger 2010-05-10 20:54 ` Dmitry Torokhov 2010-05-10 21:22 ` Andrew Morton 2010-05-10 21:27 ` Mike Frysinger 2010-05-11 6:05 ` Dmitry Torokhov 2010-05-11 6:05 ` Dmitry Torokhov 2010-05-11 6:11 ` Mike Frysinger 2010-05-11 6:11 ` Mike Frysinger 2010-05-11 3:20 ` Andrew Morton 2010-05-11 6:21 ` Dmitry Torokhov 2010-05-11 6:23 ` Mike Frysinger 2010-05-11 6:23 ` Mike Frysinger 2010-05-11 6:33 ` Dmitry Torokhov 2010-05-11 6:33 ` Dmitry Torokhov 2010-05-11 6:37 ` Mike Frysinger 2010-05-11 6:42 ` Pekka Enberg 2010-05-11 13:57 ` Christoph Lameter 2010-05-11 16:52 ` Dmitry Torokhov 2010-05-11 17:31 ` Mike Frysinger 2010-05-11 18:59 ` Pekka Enberg 2010-05-11 20:03 ` Mike Frysinger 2010-05-11 20:07 ` Matt Mackall 2010-05-11 20:10 ` Mike Frysinger 2010-05-11 20:10 ` Mike Frysinger 2010-05-11 20:21 ` Christoph Lameter 2010-05-11 20:34 ` Mike Frysinger 2010-05-11 20:38 ` Pekka Enberg 2010-05-11 20:43 ` Mike Frysinger 2010-05-11 20:43 ` Mike Frysinger 2010-05-11 20:47 ` Pekka Enberg 2010-05-13 6:21 ` Paul Mundt 2010-05-11 20:46 ` Christoph Lameter 2010-05-11 20:54 ` Mike Frysinger 2010-05-11 20:54 ` Mike Frysinger 2010-05-11 21:01 ` Pekka Enberg 2010-05-11 21:11 ` Mike Frysinger 2010-05-11 21:11 ` Mike Frysinger 2010-05-12 1:58 ` FUJITA Tomonori [not found] ` <20100511214836.GH1726@emlix.com> 2010-05-11 21:53 ` Dmitry Torokhov 2010-05-11 22:39 ` Mike Frysinger 2010-05-11 22:39 ` Mike Frysinger 2010-05-12 2:07 ` [LKML] " Marc Gauthier 2010-05-12 3:03 ` Nick Piggin 2010-05-12 3:23 ` FUJITA Tomonori 2010-05-12 4:35 ` Mike Frysinger 2010-05-12 4:35 ` Mike Frysinger 2010-05-12 5:28 ` FUJITA Tomonori 2010-05-12 5:28 ` FUJITA Tomonori 2010-05-12 14:37 ` Mike Frysinger 2010-05-12 14:37 ` Mike Frysinger 2010-05-12 18:11 ` Dmitry Torokhov 2010-05-12 18:28 ` Mike Frysinger 2010-05-12 18:28 ` Mike Frysinger 2010-05-12 10:36 ` Johannes Weiner 2010-05-12 12:35 ` Marc Gauthier 2010-05-12 14:36 ` Mike Frysinger 2010-05-12 14:36 ` Mike Frysinger 2010-05-19 12:48 ` David Woodhouse 2010-05-19 13:07 ` Nick Piggin 2010-05-19 13:17 ` David Woodhouse 2010-05-19 13:36 ` Nick Piggin 2010-05-19 13:44 ` Johannes Weiner 2010-05-19 13:52 ` Nick Piggin 2010-05-19 14:38 ` FUJITA Tomonori 2010-05-19 14:58 ` David Woodhouse 2010-05-20 4:42 ` FUJITA Tomonori 2010-05-19 16:37 ` Dmitry Torokhov 2010-05-19 15:00 ` Johannes Weiner 2010-05-17 6:12 ` Michal Simek 2010-05-19 13:00 ` David Woodhouse 2010-05-11 22:37 ` Alan Cox 2010-05-11 20:22 ` Pekka Enberg 2010-05-11 14:12 ` Oskar Schirmer 2010-05-11 14:12 ` Oskar Schirmer 2010-05-06 11:18 ` [PATCH] ad7877: fix spi word size to 16 bit Hennerich, Michael 2010-05-06 11:18 ` Hennerich, Michael 2010-05-06 18:26 ` Mike Frysinger 2010-05-06 18:26 ` Mike Frysinger 2010-05-07 9:41 ` Daniel Glöckner 2010-05-07 9:41 ` Daniel Glöckner 2010-05-07 18:23 ` Mike Frysinger 2010-05-07 18:23 ` Mike Frysinger 2010-05-13 7:53 ` Dmitry Torokhov 2010-05-13 7:53 ` Dmitry Torokhov 2010-05-15 18:15 ` Oskar Schirmer 2010-05-15 18:15 ` Oskar Schirmer 2010-05-16 19:25 ` Mike Frysinger 2010-05-16 19:25 ` Mike Frysinger 2010-05-17 7:29 ` Oskar Schirmer 2010-05-17 7:29 ` Oskar Schirmer 2010-05-17 8:14 ` Hennerich, Michael 2010-05-17 8:14 ` Hennerich, Michael 2010-05-17 8:41 ` Oskar Schirmer 2010-05-17 8:41 ` Oskar Schirmer 2010-05-17 23:49 ` Mike Frysinger 2010-05-17 23:49 ` Mike Frysinger 2010-05-18 0:18 ` H Hartley Sweeten 2010-05-18 0:18 ` H Hartley Sweeten 2010-05-18 8:32 ` Oskar Schirmer 2010-05-18 8:32 ` Oskar Schirmer 2010-05-18 9:37 ` Daniel Glöckner 2010-05-18 9:37 ` Daniel Glöckner 2010-05-18 8:34 ` [PATCH v2] " Oskar Schirmer 2010-05-18 8:34 ` Oskar Schirmer
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