[PATCH] spi: mediatek: Manually set dma_ops for spi_master device

[Daniel Kurtz]

Hi Robin,

On Thu, Jan 26, 2017 at 1:59 AM, Robin Murphy <robin.murphy at arm.com> wrote:
>
> On 25/01/17 10:24, Daniel Kurtz wrote:
> > Hi Robin,
> >
> > On Tue, Jan 24, 2017 at 11:14 PM, Robin Murphy <robin.murphy at arm.com> wrote:
> >> Hi Dan,
> >
> > [snip...]
> >
> >>> And I really don't know why we needed to set the coherent_dma_mask to 0 to
> >>> avoid SPI transaction errors.
> >>
> >> Following what I mentioned above, "git grep dma_alloc drivers/spi" makes
> >> it seem like coherent DMA isn't used anywhere relevant, which is rather
> >> puzzling. Unless of course somewhere along the line somebody's done the
> >> dev->dma_mask = &dev->dma_coherent_mask hack, with the result that
> >> writing one mask hits both, making *all* DMA fail and big transfers fall
> >> back to PIO.
> >
> > You mean this last line?
> >
> >>> @@ -575,6 +576,10 @@ static int mtk_spi_probe(struct platform_device *pdev)
> >>>               goto err_put_master;
> >>>       }
> >>>
> >>> +     /* Call of_dma_configure to set up spi_master's dma_ops */
> >>> +     of_dma_configure(&master->dev, master->dev.of_node);
> >>> +     /* But explicitly set the coherent_dma_mask to 0 */
> >>> +     master->dev.coherent_dma_mask = 0;
> >>>       if (!pdev->dev.dma_mask)
> >>>               pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
> >                                   ^^^^^
>
> Ha, I totally failed to spot that!
>
> > As predicted, setting the dma_mask = 0 causes "all DMA to fail".  In
> > particular, dma_capable() always returns false, so
> > swiotlb_map_sg_attrs() takes the map_single() path, instead of just
> > assigning:
> >
> >    sg->dma_address = dev_addr;
> >
> > swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl, int nelems,
> >                      enum dma_data_direction dir, unsigned long attrs)
> > {
> >         struct scatterlist *sg;
> >         int i;
> >
> >         BUG_ON(dir == DMA_NONE);
> >
> >         for_each_sg(sgl, sg, nelems, i) {
> >                 phys_addr_t paddr = sg_phys(sg);
> >                 dma_addr_t dev_addr = phys_to_dma(hwdev, paddr);
> >
> >                 if (swiotlb_force == SWIOTLB_FORCE ||
> >                     !dma_capable(hwdev, dev_addr, sg->length)) {
> >                         phys_addr_t map = map_single(hwdev, sg_phys(sg),
> >                                                      sg->length, dir, attrs);
> >                         ...
> >                         }
> >                         sg->dma_address = phys_to_dma(hwdev, map);
> >                 } else
> >                         sg->dma_address = dev_addr;
> >                 sg_dma_len(sg) = sg->length;
> >         }
> >         return nelems;
> > }
> >
> > So, I think this means that the only reason the MTK SPI driver ever
> > worked before was that it was tested on an older kernel, so the
> > spi_master was defaulting to swiotlb_dma_ops with a 0 dma_mask, and
> > therefore it was using SWIOTLB bounce buffers (via 'map_single'), and
> > not actually ever doing real DMA.
>
> Well, it's still "real DMA" if the device is able to slurp data out of
> the bounce buffer. That suggests there might be some mismatch between
> the default DMA mask it's getting given and the actual hardware
> capability (i.e. the bounce buffer happens to fall somewhere accessible,
> but other addresses may not be) - is crazy 33-bit mode involved here?

AFAICT, the Mediatek SPI does not have a 33-bit mode.  The Mediatek
SPI DMA registers are only 32-bits wide, so the default 0xffffffff
dma_mask is correct.  For larger addresses, swiotlb properly falls
back to using a bounce buffer.

However, I did discover what the real problem was.  The Mediatek SPI
DMA does not like un-aligned addresses.
Teaching the driver to use the FIFO even for large buffers, and
forcing un-aligned buffers to use the FIFO makes the SPI very
reliable:

https://patchwork.kernel.org/patch/9539735/

Thanks for your help!

-Dan