Re: [PATCH 03/25] dma-direct: take dma-ranges/offsets into account in resource mapping

Robin Murphy
2022-03-24T11:30:51Z

On 2022-03-24 01:48, Serge Semin wrote:
> A basic device-specific linear memory mapping was introduced back in
> commit ("dma: Take into account dma_pfn_offset") as a single-valued offset
> preserved in the device.dma_pfn_offset field, which was initialized for
> instance by means of the "dma-ranges" DT property. Afterwards the
> functionality was extended to support more than one device-specific region
> defined in the device.dma_range_map list of maps. But all of these
> improvements concerned a single pointer, page or sg DMA-mapping methods,
> while the system resource mapping function turned to miss the
> corresponding modification. Thus the dma_direct_map_resource() method now
> just casts the CPU physical address to the device DMA address with no
> dma-ranges-based mapping taking into account, which is obviously wrong.
> Let's fix it by using the phys_to_dma_direct() method to get the
> device-specific bus address from the passed memory resource for the case
> of the directly mapped DMA.

It may not have been well-documented at the time, but this was largely
intentional. The assumption based on known systems was that where
dma_pfn_offset existed, it would *not* apply to peer MMIO addresses.

For instance, DTs for TI Keystone 2 platforms only describe an offset
for RAM:

dma-ranges = <0x80000000 0x8 0x00000000 0x80000000>;

but a DMA controller might also want to access something in the MMIO
range 0x0-0x7fffffff, of which it still has an identical non-offset
view. If a driver was previously using dma_map_resource() for that, it
would now start getting DMA_MAPPING_ERROR because the dma_range_map
exists but doesn't describe the MMIO region. I agree that in hindsight
it's not an ideal situation, but it's how things have ended up, so at
this point I'm wary of making potentially-breaking changes.

May I ask what exactly your setup looks like, if you have a DMA
controller with an offset view of its "own" MMIO space?

Thanks,
Robin.

> Fixes: 25f1e1887088 ("dma: Take into account dma_pfn_offset")
> Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> ---
> kernel/dma/direct.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 50f48e9e4598..9ce8192b29ab 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -497,7 +497,7 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
> dma_addr_t dma_direct_map_resource(struct device *dev, phys_addr_t paddr,
> size_t size, enum dma_data_direction dir, unsigned long attrs)
> {
> - dma_addr_t dma_addr = paddr;
> + dma_addr_t dma_addr = phys_to_dma_direct(dev, paddr);
>
> if (unlikely(!dma_capable(dev, dma_addr, size, false))) {
> dev_err_once(dev,