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

[Mike Frysinger]

On Sat, May 8, 2010 at 18:32, Johannes Weiner wrote:
> On Fri, May 07, 2010 at 02:28:16PM -0400, Mike Frysinger wrote:
>> On Fri, May 7, 2010 at 06:15, Oskar Schirmer wrote:
>> > On Thu, May 06, 2010 at 14:46:04 -0400, Mike Frysinger wrote:
>> >> On Thu, May 6, 2010 at 06:37, Oskar Schirmer wrote:
>> >> > Âstruct ser_req {
>> >> > + Â Â Â u16 Â Â Â Â Â Â Â Â Â Â sample;
>> >> > +    char          Â__padalign[L1_CACHE_BYTES - sizeof(u16)];
>> >> > +
>> >> > Â Â Â Âu16 Â Â Â Â Â Â Â Â Â Â reset;
>> >> > Â Â Â Âu16 Â Â Â Â Â Â Â Â Â Â ref_on;
>> >> > Â Â Â Âu16 Â Â Â Â Â Â Â Â Â Â command;
>> >> > - Â Â Â u16 Â Â Â Â Â Â Â Â Â Â sample;
>> >> >    Âstruct spi_message   Âmsg;
>> >> >    Âstruct spi_transfer   xfer[6];
>> >> > Â};
>> >>
>> >> are you sure this is necessary ? Âser_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.
>>
>> so the issue is coming from the SPI master drivers and not the AD7877 driver
>
> No, the issue is coming from ad7877 placing a transmission buffer
> into the same cache line with memory locations that are accessed outside
> the driver's scope.

you missed the point of my comment.  as i clearly explained in the
other structure, the AD7877 driver was causing the cache desync.  here
it is the SPI master that is implicitly causing it.  i'm not talking
about the AD7877 being correct wrt to the implicit SPI/DMA
requirements, just what code exactly is triggering the cache issues.

> Â/*
> Â * DMA (thus cache coherency maintainance) requires the
> Â * transfer buffers to live in their own cache lines.
> Â */
>  char     __padalign[...];
>
> ? ÂIt might be obvious what the code does, but I agree with
> Mike that it might not be immediately apparent why it's needed.

comment looks fine once the spelling is fixed (maintenance).  thanks.
-mike
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