On Mon, Apr 01, 2024 at 01:41:59PM -0400, Frank Li wrote:
> There are bunch of codes in driver like
>
> if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)))
> dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))
>
> Actually it is wrong because if dma_set_mask_and_coherent(64) fails,
> dma_set_mask_and_coherent(32) will fail for the same reason.
>
> And dma_set_mask_and_coherent(64) never returns failure.
>
> According to the definition of dma_set_mask(), it indicates the width of
> address that device DMA can access. If it can access 64-bit address, it
> must access 32-bit address inherently. So only need set biggest address
> width.
>
> See below code fragment:
>
> dma_set_mask(mask)
> {
> mask = (dma_addr_t)mask;
>
> if (!dev->dma_mask || !dma_supported(dev, mask))
> return -EIO;
>
> arch_dma_set_mask(dev, mask);
> *dev->dma_mask = mask;
> return 0;
> }
>
> dma_supported() will call dma_direct_supported or iommux's dma_supported
> call back function.
>
> int dma_direct_supported(struct device *dev, u64 mask)
> {
> u64 min_mask = (max_pfn - 1) << PAGE_SHIFT;
>
> /*
> * Because 32-bit DMA masks are so common we expect every architecture
> * to be able to satisfy them - either by not supporting more physical
> * memory, or by providing a ZONE_DMA32. If neither is the case, the
> * architecture needs to use an IOMMU instead of the direct mapping.
> */
> if (mask >= DMA_BIT_MASK(32))
> return 1;
>
> ...
> }
>
> The iommux's dma_supported() actually means iommu requires devices's
> minimized dma capability.
>
> An example:
>
> static int sba_dma_supported( struct device *dev, u64 mask)()
> {
> ...
> * check if mask is >= than the current max IO Virt Address
> * The max IO Virt address will *always* < 30 bits.
> */
> return((int)(mask >= (ioc->ibase - 1 +
> (ioc->pdir_size / sizeof(u64) * IOVP_SIZE) )));
> ...
> }
>
> 1 means supported. 0 means unsupported.
>
> Correct document to make it more clear and provide correct sample code.
>
> Signed-off-by: Frank Li <Frank.Li@xxxxxxx>
> ---
>
> Notes:
> Change from v1 to v2:
> - fixed typo, review by Randy Dunlap
>
> Documentation/core-api/dma-api-howto.rst | 24 ++++++++++++++++++++++--
> 1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/core-api/dma-api-howto.rst b/Documentation/core-api/dma-api-howto.rst
> index e8a55f9d61dbc..5f6a7d86b6bc2 100644
> --- a/Documentation/core-api/dma-api-howto.rst
> +++ b/Documentation/core-api/dma-api-howto.rst
> @@ -203,13 +203,33 @@ setting the DMA mask fails. In this manner, if a user of your driver reports
> that performance is bad or that the device is not even detected, you can ask
> them for the kernel messages to find out exactly why.
>
> -The standard 64-bit addressing device would do something like this::
> +The 24-bit addressing device would do something like this::
>
> - if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) {
> + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(24))) {
> dev_warn(dev, "mydev: No suitable DMA available\n");
> goto ignore_this_device;
> }
>
> +The standard 64-bit addressing device would do something like this::
> +
> + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))
> +
> +dma_set_mask_and_coherent() never return fail when DMA_BIT_MASK(64). Typical
> +error code like::
> +
> + /* Wrong code */
> + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)))
> + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))
> +
> +dma_set_mask_and_coherent() will never return failure when bigger then 32.
Nit:
s/then/than/
> +So typical code like::
> +
> + /* Recommended code */
> + if (support_64bit)
> + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64));
> + else
> + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> +
> If the device only supports 32-bit addressing for descriptors in the
> coherent allocations, but supports full 64-bits for streaming mappings
> it would look like this::
> --
> 2.34.1
>
