Re: [PATCH] PCI: endpoint: Fix BAR backing DMA address alignment

[Frank Li <Frank.li@oss.nxp.com>]

On Fri, Aug 28, 2026 at 03:17:57AM +0900, Koichiro Den wrote:
> DWC BAR Match Mode requires the target address to be aligned to the BAR
> size. pci_epf_alloc_space() relies on dma_alloc_coherent() to provide
> that alignment. For CMA-backed allocations, the guaranteed physical
> alignment is capped at PAGE_SIZE << CONFIG_CMA_ALIGNMENT. With a direct
> DMA mapping, 4 KiB pages, and CONFIG_CMA_ALIGNMENT=8, a 2 MiB BAR may
> therefore receive a DMA address aligned to only 1 MiB.
> dw_pcie_prog_ep_inbound_atu() rejects such a mapping with -EINVAL.
>
> If the initial allocation is misaligned, free it and retry with
> mem_size - 1 bytes of padding. Select an aligned subrange and record its
> offset so pci_epf_free_space() can pass the original CPU address, DMA
> handle, and size to dma_free_coherent().

I understand this situation, but it mays waste memory, if 16MiB, it max
waste 16M-1 space.

As workaround, this is good enough.

add dt memory node for pcie-ep

my_dma_pool: dma@42000000 {
    compatible = "shared-dma-pool";
    reg = <0x42000000 0x02000000>;
    reusable;          /* <-- CMA: system CAN use these pages when idle */
};

pcie-ep {
	...
	memory-region = <&my_dma_pool>;
}

but if use "reusable", still meet CONFIG_CMA_ALIGNMENT. if use "no-map",
dma_alloc_from_dev_coherent() will bypass CONFIG_CMA_ALIGNMENT. but it will
waste memory if not use pcie-ep devices.

Maybe we try

 static int __init rmem_cma_setup(unsigned long node, struct reserved_mem *rmem)
 {
     ...
     ret = cma_init_reserved_mem(rmem->base, rmem->size, 0, rmem->name, &cma);
     if (ret) { ... }

+    /* linux,cma-force-align: bypass CONFIG_CMA_ALIGNMENT cap, align = get_order(size) */
+    if (of_get_flat_dt_prop(node, "linux,cma-force-align", NULL))
+        cma->force_align = true;

     if (default_cma)
         dma_contiguous_default_area = cma;
     ...
 }


 static struct page *cma_alloc_aligned(struct cma *cma, size_t size, gfp_t gfp)
 {
-    unsigned int align = min(get_order(size), CONFIG_CMA_ALIGNMENT);
+    unsigned int align;
+
+    if (cma && cma->force_align)
+        align = get_order(size);    /* no cap: align = natural power-of-2 for size */
+    else
+        align = min(get_order(size), CONFIG_CMA_ALIGNMENT);

     return cma_alloc(cma, size >> PAGE_SHIFT, align, gfp & __GFP_NOWARN);
 }

I am not sure if DT team accept add "linux,cma-force-align", which desgin
for specic device CMA region only, not for default cmd. No impact to other
system.

Frank

>
> Fixes: 5e8cb4033807 ("PCI: endpoint: Add EP core layer to enable EP controller and EP functions")
> Signed-off-by: Koichiro Den <den@valinux.co.jp>
> ---
>  drivers/pci/endpoint/pci-epf-core.c | 39 +++++++++++++++++++++++------
>  include/linux/pci-epf.h             |  7 ++++--
>  2 files changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/pci/endpoint/pci-epf-core.c b/drivers/pci/endpoint/pci-epf-core.c
> index 6987343c9e61..e8c3709d85c4 100644
> --- a/drivers/pci/endpoint/pci-epf-core.c
> +++ b/drivers/pci/endpoint/pci-epf-core.c
> @@ -8,6 +8,7 @@
>
>  #include <linux/device.h>
>  #include <linux/dma-mapping.h>
> +#include <linux/overflow.h>
>  #include <linux/slab.h>
>  #include <linux/module.h>
>
> @@ -278,13 +279,15 @@ void pci_epf_free_space(struct pci_epf *epf, void *addr, enum pci_barno bar,
>  	}
>
>  	dev = epc->dev.parent;
> -	dma_free_coherent(dev, epf_bar[bar].mem_size, addr,
> -			  epf_bar[bar].phys_addr);
> +	dma_free_coherent(dev, epf_bar[bar].mem_size,
> +			  (u8 *)addr - epf_bar[bar].alloc_offset,
> +			  epf_bar[bar].phys_addr - epf_bar[bar].alloc_offset);
>
>  	epf_bar[bar].phys_addr = 0;
>  	epf_bar[bar].addr = NULL;
>  	epf_bar[bar].size = 0;
>  	epf_bar[bar].mem_size = 0;
> +	epf_bar[bar].alloc_offset = 0;
>  	epf_bar[bar].barno = 0;
>  	epf_bar[bar].flags = 0;
>  }
> @@ -308,8 +311,10 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  {
>  	struct pci_epf_bar *epf_bar;
>  	dma_addr_t phys_addr;
> +	size_t alloc_offset = 0;
>  	struct pci_epc *epc;
>  	struct device *dev;
> +	size_t alloc_size;
>  	size_t mem_size;
>  	void *space;
>
> @@ -326,16 +331,31 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  	}
>
>  	dev = epc->dev.parent;
> -	space = dma_alloc_coherent(dev, mem_size, &phys_addr, GFP_KERNEL);
> -	if (!space) {
> -		dev_err(dev, "failed to allocate mem space\n");
> -		return NULL;
> +	alloc_size = mem_size;
> +	space = dma_alloc_coherent(dev, alloc_size, &phys_addr, GFP_KERNEL);
> +	if (!space)
> +		goto err_alloc;
> +	if (!IS_ALIGNED(phys_addr, mem_size)) {
> +		dma_free_coherent(dev, alloc_size, space, phys_addr);
> +
> +		if (check_add_overflow(mem_size, mem_size - 1, &alloc_size))
> +			return NULL;
> +
> +		space = dma_alloc_coherent(dev, alloc_size, &phys_addr,
> +					   GFP_KERNEL);
> +		if (!space)
> +			goto err_alloc;
> +
> +		alloc_offset = ALIGN(phys_addr, mem_size) - phys_addr;
> +		space = (u8 *)space + alloc_offset;
> +		phys_addr += alloc_offset;
>  	}
>
>  	epf_bar[bar].phys_addr = phys_addr;
>  	epf_bar[bar].addr = space;
>  	epf_bar[bar].size = size;
> -	epf_bar[bar].mem_size = mem_size;
> +	epf_bar[bar].mem_size = alloc_size;
> +	epf_bar[bar].alloc_offset = alloc_offset;
>  	epf_bar[bar].barno = bar;
>  	if (upper_32_bits(size) || epc_features->bar[bar].only_64bit)
>  		epf_bar[bar].flags |= PCI_BASE_ADDRESS_MEM_TYPE_64;
> @@ -343,6 +363,10 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  		epf_bar[bar].flags |= PCI_BASE_ADDRESS_MEM_TYPE_32;
>
>  	return space;
> +
> +err_alloc:
> +	dev_err(dev, "failed to allocate mem space\n");
> +	return NULL;
>  }
>  EXPORT_SYMBOL_GPL(pci_epf_alloc_space);
>
> @@ -413,6 +437,7 @@ int pci_epf_assign_bar_space(struct pci_epf *epf, size_t size,
>  	epf_bar[bar].addr = NULL;
>  	epf_bar[bar].size = bar_size;
>  	epf_bar[bar].mem_size = aligned_mem_size;
> +	epf_bar[bar].alloc_offset = 0;
>  	epf_bar[bar].barno = bar;
>  	if (upper_32_bits(size) || epc_features->bar[bar].only_64bit)
>  		epf_bar[bar].flags |= PCI_BASE_ADDRESS_MEM_TYPE_64;
> diff --git a/include/linux/pci-epf.h b/include/linux/pci-epf.h
> index 8a6c64a35890..749c6611d7b9 100644
> --- a/include/linux/pci-epf.h
> +++ b/include/linux/pci-epf.h
> @@ -131,8 +131,10 @@ struct pci_epf_bar_submap {
>   * @phys_addr: physical address that should be mapped to the BAR
>   * @addr: virtual address corresponding to the @phys_addr
>   * @size: the size of the address space present in BAR
> - * @mem_size: the size actually allocated to accommodate the iATU alignment
> - *            requirement
> + * @mem_size: size of the coherent allocation, including alignment padding, or
> + *            the required aligned backing size for assigned BAR space
> + * @alloc_offset: offset from the coherent allocation's CPU and DMA base
> + *                addresses to @addr and @phys_addr
>   * @barno: BAR number
>   * @flags: flags that are set for the BAR
>   * @num_submap: number of entries in @submap
> @@ -144,6 +146,7 @@ struct pci_epf_bar {
>  	void		*addr;
>  	size_t		size;
>  	size_t		mem_size;
> +	size_t		alloc_offset;
>  	enum pci_barno	barno;
>  	int		flags;
>
>
> base-commit: 0a6f72eda328c773324555491e1ad7f0b0153155
> --
> 2.51.0
>