Re: [PATCH] dmaengine: sun6i: constrain descriptor addresses through the DMA mask

[James Hilliard]

On Thu, Oct 22, 2026 at 10:10 AM Frank Li <Frank.li@xxxxxxxxxxx> wrote:
>
> On Wed, Oct 21, 2026 at 03:11:29PM -0600, James Hilliard wrote:
> > Descriptor allocations pass GFP_DMA32 to dma_pool_alloc(). The DMA API
> > chooses the memory zone from the device address mask, rather than accepting
> > zone-selection flags from the driver. These flags can also reach the pool
> > metadata allocation and trigger a slab allocator warning, as observed on
> > H616 when preparing the first UART receive-DMA chain.
> >
> > Set a 32-bit coherent DMA mask before creating the descriptor pool and
> > remove GFP_DMA32 from its allocations. The driver does not encode the
> > extra descriptor-address bits supported by some variants, so descriptor
> > links must remain below 4 GiB. Keep the streaming mask unchanged for
> > variants with wider source and destination addresses.
>
> Spouse other dma address in src/dest in descriptor also limited to 32bit.

The src and dst fields contain the low 32 bits, but on variants with
has_high_addr, sun6i_dma_set_addr() stores address bits 33:32 in the
descriptor's para word: bits 17:16 for the source and bits 19:18 for the
destination. Those variants therefore support 34-bit payload addresses.
Variants without that capability retain 32-bit addressing.

> Do you means only descripor address have 32bit limiation, other have not
> such limiation?

For the current driver, yes: descriptor addresses must remain below
4 GiB, while payload addresses can use the wider range on supported
variants.

The descriptor restriction is not necessarily a hardware limitation.
Some variants also support extended descriptor addresses, but require
encoding their high bits into the otherwise-unused low address bits.
The driver does not implement that encoding: it writes the descriptor
address directly to DMA_CHAN_LLI_ADDR and stores the next descriptor
address directly in p_lli_next.

The patch preserves that existing restriction by setting the coherent
DMA mask used by dma_pool_alloc(), instead of passing GFP_DMA32.
The streaming mask and existing payload-address encoding remain
unchanged.

> Frank
> >
> > Fixes: ec31c5c59492 ("dmaengine: sun6i: Add support for 34-bit physical addresses")
> > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> > ---
> >  drivers/dma/sun6i-dma.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
> > index 7704b016aed8..0ef6ed1f936e 100644
> > --- a/drivers/dma/sun6i-dma.c
> > +++ b/drivers/dma/sun6i-dma.c
> > @@ -684,7 +684,7 @@ static struct dma_async_tx_descriptor *sun6i_dma_prep_dma_memcpy(
> >       if (!txd)
> >               return NULL;
> >
> > -     v_lli = dma_pool_alloc(sdev->pool, GFP_DMA32 | GFP_NOWAIT, &p_lli);
> > +     v_lli = dma_pool_alloc(sdev->pool, GFP_NOWAIT, &p_lli);
> >       if (!v_lli) {
> >               dev_err(sdev->slave.dev, "Failed to alloc lli memory\n");
> >               goto err_txd_free;
> > @@ -743,7 +743,7 @@ static struct dma_async_tx_descriptor *sun6i_dma_prep_slave_sg(
> >               return NULL;
> >
> >       for_each_sg(sgl, sg, sg_len, i) {
> > -             v_lli = dma_pool_alloc(sdev->pool, GFP_DMA32 | GFP_NOWAIT, &p_lli);
> > +             v_lli = dma_pool_alloc(sdev->pool, GFP_NOWAIT, &p_lli);
> >               if (!v_lli)
> >                       goto err_lli_free;
> >
> > @@ -826,7 +826,7 @@ static struct dma_async_tx_descriptor *sun6i_dma_prep_dma_cyclic(
> >               return NULL;
> >
> >       for (i = 0; i < periods; i++) {
> > -             v_lli = dma_pool_alloc(sdev->pool, GFP_DMA32 | GFP_NOWAIT, &p_lli);
> > +             v_lli = dma_pool_alloc(sdev->pool, GFP_NOWAIT, &p_lli);
> >               if (!v_lli) {
> >                       dev_err(sdev->slave.dev, "Failed to alloc lli memory\n");
> >                       goto err_lli_free;
> > @@ -1345,6 +1345,11 @@ static int sun6i_dma_probe(struct platform_device *pdev)
> >               return PTR_ERR(sdc->rstc);
> >       }
> >
> > +     /* This driver uses unextended 32-bit addresses for descriptor links. */
> > +     ret = dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32));
> > +     if (ret)
> > +             return ret;
> > +
> >       sdc->pool = dmam_pool_create(dev_name(&pdev->dev), &pdev->dev,
> >                                    sizeof(struct sun6i_dma_lli), 4, 0);
> >       if (!sdc->pool) {
> >
> > ---
> > base-commit: 93f51579e7df248780214094418f205253383cc5
> > change-id: 20261021-submit-sun6i-dma-descriptor-mask-v1-35bb381b932a
> >
> > Best regards,
> > --
> > James Hilliard <james.hilliard1@xxxxxxxxx>
> >