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SubjectRe: [PATCH 4.4 101/105] spi: Fix scatterlist elements size in spi_map_buf
On Fri, Jul 20, 2018 at 08:23:44PM +0100, Ben Hutchings wrote:
> On Sun, 2018-07-01 at 18:02 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Maxime Chevallier <maxime.chevallier@bootlin.com>
> >
> > commit ce99319a182fe766be67f96338386f3ec73e321c upstream.
> >
> > When SPI transfers can be offloaded using DMA, the SPI core need to
> > build a scatterlist to make sure that the buffer to be transferred is
> > dma-able.
> >
> > This patch fixes the scatterlist entry size computation in the case
> > where the maximum acceptable scatterlist entry supported by the DMA
> > controller is less than PAGE_SIZE, when the buffer is vmalloced.
> >
> > For each entry, the actual size is given by the minimum between the
> > desc_len (which is the max buffer size supported by the DMA controller)
> > and the remaining buffer length until we cross a page boundary.
> >
> > Fixes: 65598c13fd66 ("spi: Fix per-page mapping of unaligned vmalloc-ed buffer")
> > Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > ---
> >  drivers/spi/spi.c |   10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > --- a/drivers/spi/spi.c
> > +++ b/drivers/spi/spi.c
> > @@ -707,8 +707,14 @@ static int spi_map_buf(struct spi_master
> >   for (i = 0; i < sgs; i++) {
> >  
> >   if (vmalloced_buf) {
> > - min = min_t(size_t,
> > -     len, desc_len - offset_in_page(buf));
> > + /*
> > +  * Next scatterlist entry size is the minimum between
> > +  * the desc_len and the remaining buffer length that
> > +  * fits in a page.
> > +  */
> > + min = min_t(size_t, desc_len,
> > +     min_t(size_t, len,
> > +   PAGE_SIZE - offset_in_page(buf)));
> >   vm_page = vmalloc_to_page(buf);
> >   if (!vm_page) {
> >   sg_free_table(sgt);
>
> I think this makes sense upstream, but in 4.4 it seems to be a no-op
> since desc_len is always set to PAGE_SIZE when vmalloced_buf is true.
> This fix depends on (at least) commit df88e91bbfd1 "spi: respect the
> maximum segment size of DMA device" which went into 4.6.

Good catch. I'll leave it as-is for now though.

thanks,

greg k-h

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