Re: [PATCH v9 25/27] PCI: dwc: Set coherent DMA-mask on MSI-address allocation

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jan 13, 2023 at 08:14:07PM +0300, Serge Semin wrote:
> The MSI target address requires to be reserved within the lowest 4GB
> memory in order to support the PCIe peripherals with no 64-bit MSI TLPs
> support. Since the allocation is done from the DMA-coherent memory let's
> modify the allocation procedure to setting the coherent DMA-mask only and
> avoiding the streaming DMA-mask modification. Thus at least the streaming
> DMA operations would work with no artificial limitations. It will be
> specifically useful for the eDMA-capable controllers so the corresponding
> DMA-engine clients would map the DMA buffers with no need in the SWIOTLB
> intervention for the buffers allocated above the 4GB memory region.
> 
> While at it let's add a brief comment about the reason of having the MSI
> target address allocated from the DMA-coherent memory limited with the 4GB
> upper bound.
> 
> Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> Reviewed-by: Robin Murphy <robin.murphy@arm.com>
> 
> ---
> 
> Changelog v8:
> - This is a new patch added on v8 stage of the series.
>   (@Robin, @Christoph)
> ---
>  drivers/pci/controller/dwc/pcie-designware-host.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> index 3ab6ae3712c4..e10608af39b4 100644
> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> @@ -366,7 +366,16 @@ static int dw_pcie_msi_host_init(struct dw_pcie_rp *pp)
>  						    dw_chained_msi_isr, pp);
>  	}
>  
> -	ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> +	/*
> +	 * Even though the iMSI-RX Module supports 64-bit addresses some
> +	 * peripheral PCIe devices may lack the 64-bit messages support. In
> +	 * order not to miss MSI TLPs from those devices the MSI target address
> +	 * has to be reserved within the lowest 4GB.
> +	 * Note until there is a better alternative found the reservation is
> +	 * done by allocating from the artificially limited DMA-coherent
> +	 * memory.
> +	 */
> +	ret = dma_set_coherent_mask(dev, DMA_BIT_MASK(32));

We used to restrict both streaming and coherent DMA masks to 32 bits.
Now we will only restrict coherent DMA to 32 bits.

So in essence this change removes a restriction on the streaming DMA
mask, right?

And I guess bt1 is the only driver where this will make a difference
(after the next patch) because no other dwc drivers set their own DMA
masks?

>  	if (ret)
>  		dev_warn(dev, "Failed to set DMA mask to 32-bit. Devices with only 32-bit MSI support may not work properly\n");
>  
> -- 
> 2.39.0
> 
>