On Fri, Jul 29, 2016 at 10:30:22AM -0700, Dmitry Torokhov wrote:
>On Fri, Jul 29, 2016 at 10:27 AM, Jason Gunthorpe
> wrote:
>
> On Thu, Jul 28, 2016 at 07:59:13PM -0700, Andrey Pronin wrote:
> > Annotate buffers used in spi transactions as cacheline_aligned
> > to use in DMA transfers.
> >
> > Signed-off-by: Andrey Pronin
> >Â drivers/char/tpm/st33zp24/spi.c | 4 ++--
> > drivers/char/tpm/tpm_tis_spi.c | 4 ++--
> >Â 2 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/char/tpm/st33zp24/spi.c
> b/drivers/char/tpm/st33zp24/spi.c
> > index 9f5a011..0e9aad9 100644
> > +++ b/drivers/char/tpm/st33zp24/spi.c
> > @@ -70,8 +70,8 @@
> >Â struct st33zp24_spi_phy {
> >Â Â Â Â struct spi_device *spi_device;
> >
> > -Â Â Â u8 tx_buf[ST33ZP24_SPI_BUFFER_SIZE];
> > -Â Â Â u8 rx_buf[ST33ZP24_SPI_BUFFER_SIZE];
> > +Â Â Â u8 tx_buf[ST33ZP24_SPI_BUFFER_SIZE] cacheline_aligned;
> > +Â Â Â u8 rx_buf[ST33ZP24_SPI_BUFFER_SIZE] cacheline_aligned;
> >
> >Â Â Â Â int io_lpcpd;
> >Â Â Â Â int latency;
>
> Hurm, this still looks wrong to me. Aligning the start of buffers is
> not enough, the DMA'able space must also end on a cache line as well.
>
> So, the buffers must also always be placed at the end of the struct.
>
> IMHO It would be cleaner and safer to always kmalloc the DMA buffer
> alone than to try and optimize like this.
>
>In this case moving them to the end of the structure and commenting why
>they have to be at the end might be less invasive change. More
>performance-efficient and resilient in low memory situations too.
kmallocs would be done in the driver initialization:
* you rarely are in low memory situation
* performance gain/loss is insignificant
I really don't see your point.
>Thanks,
>Dmitry
/Jarkko
