Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

[Mike Frysinger <vapier.adi@gmail.com>]

On Tue, May 11, 2010 at 02:21, Dmitry Torokhov wrote:
> On Tue, May 11, 2010 at 02:11:41AM -0400, Mike Frysinger wrote:
>> On Tue, May 11, 2010 at 02:05, Dmitry Torokhov wrote:
>> > On Mon, May 10, 2010 at 02:22:25PM -0700, Andrew Morton wrote:
>> >> On Mon, 10 May 2010 12:42:34 +0200 Oskar Schirmer wrote:
>> >> > With dma based spi transmission, data corruption
>> >> > is observed occasionally. With dma buffers located
>> >> > right next to msg and xfer fields, cache lines
>> >> > correctly flushed in preparation for dma usage
>> >> > may be polluted again when writing to fields
>> >> > in the same cache line.
>> >> >
>> >> > Make sure cache fields used with dma do not
>> >> > share cache lines with fields changed during
>> >> > dma handling. As both fields are part of a
>> >> > struct that is allocated via kzalloc, thus
>> >> > cache aligned, moving the fields to the 1st
>> >> > position and insert padding for alignment
>> >> > does the job.
>> >>
>> >> This sounds odd.  Doesn't it imply that some code somewhere is missing
>> >> some DMA synchronisation actions?
>> >>
>> >> >
>> >> > v2: add a comment to explain why alignment is needed
>> >> >
>> >> > v3: fix the typo in comment and layout (- to end of line)
>> >> >
>> >> > diff --git a/drivers/input/touchscreen/ad7877.c b/drivers/input/touchscreen/ad7877.c
>> >> > index 885354c..9ebb1b4 100644
>> >> > --- a/drivers/input/touchscreen/ad7877.c
>> >> > +++ b/drivers/input/touchscreen/ad7877.c
>> >> > @@ -153,15 +153,29 @@ enum {
>> >> >   */
>> >> >
>> >> >  struct ser_req {
>> >> > +   u16                     sample;
>> >> > +   /*
>> >> > +    * DMA (thus cache coherency maintenance) requires the
>> >> > +    * transfer buffers to live in their own cache lines.
>> >> > +    */
>> >> > +   char                    __padalign[L1_CACHE_BYTES - sizeof(u16)];
>> >>
>> >> It would be better to use __cacheline_aligned, rather than open-coding
>> >> things in this manner.
>> >>
>> >
>> > OK, then I have the following which I will apply unless someone shouts.
>> >
>> > --
>> > Dmitry
>> >
>> > Input: ad7877 - keep dma rx buffers in seperate cache lines
>> >
>> > From: Oskar Schirmer <os@emlix.com>
>> >
>> > With dma based spi transmission, data corruption is observed
>> > occasionally. With dma buffers located right next to msg and
>> > xfer fields, cache lines correctly flushed in preparation for
>> > dma usage may be polluted again when writing to fields in the
>> > same cache line.
>> >
>> > Make sure cache fields used with dma do not share cache lines
>> > with fields changed during dma handling. As both fields are part
>> > of a struct that is allocated via kzalloc, thus cache aligned,
>> > moving the fields to the 1st position and insert padding for
>> > alignment does the job.
>> >
>> > Signed-off-by: Oskar Schirmer <os@emlix.com>
>> > Signed-off-by: Daniel Glöckner <dg@emlix.com>
>> > Signed-off-by: Oliver Schneidewind <osw@emlix.com>
>> > Signed-off-by: Johannes Weiner <jw@emlix.com>
>> > Acked-by: Mike Frysinger <vapier@gentoo.org>
>> > [dtor@mail.ru - changed to use ___cacheline_aligned at suggestion
>> >  of akpm]
>> > Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
>> > ---
>> >
>> >  drivers/input/touchscreen/ad7877.c |   15 ++++++++++++---
>> >  1 files changed, 12 insertions(+), 3 deletions(-)
>> >
>> >
>> > diff --git a/drivers/input/touchscreen/ad7877.c b/drivers/input/touchscreen/ad7877.c
>> > index e019d53..0d2d7e5 100644
>> > --- a/drivers/input/touchscreen/ad7877.c
>> > +++ b/drivers/input/touchscreen/ad7877.c
>> > @@ -156,9 +156,14 @@ struct ser_req {
>> >        u16                     reset;
>> >        u16                     ref_on;
>> >        u16                     command;
>> > -       u16                     sample;
>> >        struct spi_message      msg;
>> >        struct spi_transfer     xfer[6];
>> > +
>> > +       /*
>> > +        * DMA (thus cache coherency maintenance) requires the
>> > +        * transfer buffers to live in their own cache lines.
>> > +        */
>> > +       u16 sample ____cacheline_aligned;
>> >  };
>> >
>> >  struct ad7877 {
>> > @@ -182,8 +187,6 @@ 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;
>> >
>> > @@ -195,6 +198,12 @@ struct ad7877 {
>> >        spinlock_t              lock;
>> >        struct timer_list       timer;          /* P: lock */
>> >        unsigned                pending:1;      /* P: lock */
>> > +
>> > +       /*
>> > +        * DMA (thus cache coherency maintenance) requires the
>> > +        * transfer buffers to live in their own cache lines.
>> > +        */
>> > +       u16 conversion_data[AD7877_NR_SENSE] ____cacheline_aligned;
>> >  };
>>
>> i'm not sure this is correct.  the cached_aligned attribute makes sure
>> it starts on a cache boundary, but it doesnt make sure it pads out to
>> one.  so it might work more of the time, but i dont think it's
>> guaranteed.
>
> The buffers are moved to the end of the structure - there is nothing
> else there.

what guarantee exactly do you have for that statement ?
-mike