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

[Mike Frysinger]

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

>> > Âstruct ad7877 {
>> > + Â Â Â u16 Â Â Â Â Â Â Â Â Â Â conversion_data[AD7877_NR_SENSE];
>> > +    char          Â__padalign[L1_CACHE_BYTES
>> > + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â - AD7877_NR_SENSE * sizeof(u16)];
>> > +
>> >    Âstruct input_dev    Â*input;
>> >    Âchar          Âphys[32];
>> >
>> > @@ -182,8 +188,6 @@ struct ad7877 {
>> > Â Â Â Âu8 Â Â Â Â Â Â Â Â Â Â Âaveraging;
>> > Â Â Â Âu8 Â Â Â Â Â Â Â Â Â Â Âpen_down_acc_interval;
>> >
>> > - Â Â Â u16 Â Â Â Â Â Â Â Â Â Â conversion_data[AD7877_NR_SENSE];
>> > -
>> >    Âstruct spi_transfer   xfer[AD7877_NR_SENSE + 2];
>> >    Âstruct spi_message   Âmsg;
>>
>> 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.

not everyone knows to read every single piece of documentation that
may or may not be affected implicitly in the call stack.  a simple
comment here is not superfluous.

since the other struct is also going to be changed, a comment should
be placed there as well.
-mike
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/